ClangPchManager: Introduce PchTaskQueue

With the PchTaskQueue the pipeline is almost complete.

Task-number: QTCREATORBUG-21346
Change-Id: I5f05d525db1679eb37dd1d462076c1ed42958099
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

34 files changed, 980 insertions, 144 deletions

diff --git a/src/libs/clangsupport/refactoringdatabaseinitializer.h b/src/libs/clangsupport/refactoringdatabaseinitializer.h
index cba56475c8..94665409fe 100644
--- a/src/libs/clangsupport/refactoringdatabaseinitializer.h
+++ b/src/libs/clangsupport/refactoringdatabaseinitializer.h
@@ -190,9 +190,10 @@ public:
         table.setUseIfNotExists(true);
         table.setName("precompiledHeaders");
         table.addColumn("projectPartId", Sqlite::ColumnType::Integer, Sqlite::Contraint::PrimaryKey);
-        table.addColumn("pchPath", Sqlite::ColumnType::Text);
-        table.addColumn("pchBuildTime", Sqlite::ColumnType::Integer);
-
+        table.addColumn("projectPchPath", Sqlite::ColumnType::Text);
+        table.addColumn("projectPchBuildTime", Sqlite::ColumnType::Integer);
+        table.addColumn("systemPchPath", Sqlite::ColumnType::Text);
+        table.addColumn("systemPchBuildTime", Sqlite::ColumnType::Integer);
         table.initialize(database);
     }
 
diff --git a/src/libs/sqlite/sqlitebasestatement.cpp b/src/libs/sqlite/sqlitebasestatement.cpp
index 01cd43facb..87aa3f68b3 100644
--- a/src/libs/sqlite/sqlitebasestatement.cpp
+++ b/src/libs/sqlite/sqlitebasestatement.cpp
@@ -133,10 +133,8 @@ bool BaseStatement::next() const
         return true;
     else if (resultCode == SQLITE_DONE)
         return false;
-    else
-        checkForStepError(resultCode);
 
-    return false;
+    checkForStepError(resultCode);
 }
 
 void BaseStatement::step() const
diff --git a/src/tools/clangpchmanagerbackend/source/clangpchmanagerbackend-source.pri b/src/tools/clangpchmanagerbackend/source/clangpchmanagerbackend-source.pri
index f5a8068b6b..693e110402 100644
--- a/src/tools/clangpchmanagerbackend/source/clangpchmanagerbackend-source.pri
+++ b/src/tools/clangpchmanagerbackend/source/clangpchmanagerbackend-source.pri
@@ -6,7 +6,8 @@ SOURCES += \
     $$PWD/projectparts.cpp \
     $$PWD/projectpartqueue.cpp \
     $$PWD/pchtaskgenerator.cpp \
-    $$PWD/pchtasksmerger.cpp
+    $$PWD/pchtasksmerger.cpp \
+    $$PWD/pchtaskqueue.cpp
 
 HEADERS += \
     $$PWD/pchmanagerserver.h \
@@ -38,7 +39,8 @@ HEADERS += \
     $$PWD/usedmacrofilter.h \
     $$PWD/pchtasksmergerinterface.h \
     $$PWD/pchtasksmerger.h \
-    $$PWD/pchtaskqueueinterface.h
+    $$PWD/pchtaskqueueinterface.h \
+    $$PWD/pchtaskqueue.h
 
 !isEmpty(LIBTOOLING_LIBS) {
 SOURCES += \
diff --git a/src/tools/clangpchmanagerbackend/source/pchcreator.cpp b/src/tools/clangpchmanagerbackend/source/pchcreator.cpp
index 0e58621792..79062b5e2d 100644
--- a/src/tools/clangpchmanagerbackend/source/pchcreator.cpp
+++ b/src/tools/clangpchmanagerbackend/source/pchcreator.cpp
@@ -326,11 +326,16 @@ IdPaths PchCreator::generateProjectPartPch(const V2::ProjectPartContainer &proje
     return {projectPart.projectPartId.clone(), allIncludeIds(includes)};
 }
 
-void PchCreator::generatePch(const V2::ProjectPartContainer &projectPart)
+void PchCreator::generatePchDeprecated(const V2::ProjectPartContainer &projectPart)
 {
     m_projectIncludeIds = generateProjectPartPch(projectPart);
 }
 
+void PchCreator::generatePch(const PchTask &pchTask)
+{
+
+}
+
 IdPaths PchCreator::takeProjectIncludes()
 {
     return std::move(m_projectIncludeIds);
diff --git a/src/tools/clangpchmanagerbackend/source/pchcreator.h b/src/tools/clangpchmanagerbackend/source/pchcreator.h
index 34461dc16b..34170e2bb5 100644
--- a/src/tools/clangpchmanagerbackend/source/pchcreator.h
+++ b/src/tools/clangpchmanagerbackend/source/pchcreator.h
@@ -69,7 +69,8 @@ public:
     {
     }
 
-    void generatePch(const V2::ProjectPartContainer &projectsPart) override;
+    void generatePchDeprecated(const V2::ProjectPartContainer &projectsPart) override;
+    void generatePch(const PchTask &pchTask) override;
     IdPaths takeProjectIncludes() override;
     const ProjectPartPch &projectPartPch() override;
     void setUnsavedFiles(const V2::FileContainers &fileContainers) override;
diff --git a/src/tools/clangpchmanagerbackend/source/pchcreatorinterface.h b/src/tools/clangpchmanagerbackend/source/pchcreatorinterface.h
index c3098577f7..caa2d67649 100644
--- a/src/tools/clangpchmanagerbackend/source/pchcreatorinterface.h
+++ b/src/tools/clangpchmanagerbackend/source/pchcreatorinterface.h
@@ -26,6 +26,7 @@
 #pragma once
 
 #include "idpaths.h"
+#include "pchtask.h"
 #include "projectpartpch.h"
 #include "processorinterface.h"
 
@@ -41,7 +42,8 @@ public:
     PchCreatorInterface(const PchCreatorInterface &) = delete;
     PchCreatorInterface &operator=(const PchCreatorInterface &) = delete;
 
-    virtual void generatePch(const V2::ProjectPartContainer &projectsPart) = 0;
+    virtual void generatePchDeprecated(const V2::ProjectPartContainer &projectsPart) = 0;
+    virtual void generatePch(const PchTask &pchTask) = 0;
     virtual IdPaths takeProjectIncludes() = 0;
     virtual const ProjectPartPch &projectPartPch() = 0;
 
diff --git a/src/tools/clangpchmanagerbackend/source/pchtask.h b/src/tools/clangpchmanagerbackend/source/pchtask.h
index e1439d2ec9..a78b199215 100644
--- a/src/tools/clangpchmanagerbackend/source/pchtask.h
+++ b/src/tools/clangpchmanagerbackend/source/pchtask.h
@@ -40,7 +40,17 @@ public:
             FilePathIds &&includes,
             CompilerMacros &&compilerMacros,
             UsedMacros &&usedMacros)
-        : projectPartId(projectPartId)
+        : projectPartIds({projectPartId})
+        , includes(includes)
+        , compilerMacros(compilerMacros)
+        , usedMacros(usedMacros)
+    {}
+
+    PchTask(Utils::SmallStringVector &&projectPartIds,
+            FilePathIds &&includes,
+            CompilerMacros &&compilerMacros,
+            UsedMacros &&usedMacros)
+        : projectPartIds(std::move(projectPartIds))
         , includes(includes)
         , compilerMacros(compilerMacros)
         , usedMacros(usedMacros)
@@ -48,15 +58,17 @@ public:
 
     friend bool operator==(const PchTask &first, const PchTask &second)
     {
-        return first.projectPartId == second.projectPartId
-               && first.dependentIds == second.dependentIds && first.includes == second.includes
+        return first.systemPchPath == second.systemPchPath
+               && first.projectPartIds == second.projectPartIds && first.includes == second.includes
                && first.compilerMacros == second.compilerMacros
                && first.usedMacros == second.usedMacros;
     }
 
+    Utils::SmallStringView projectPartId() const { return projectPartIds.front(); }
+
 public:
-    Utils::SmallString projectPartId;
-    Utils::SmallStringVector dependentIds;
+    Utils::PathString systemPchPath;
+    Utils::SmallStringVector projectPartIds;
     FilePathIds includes;
     CompilerMacros compilerMacros;
     UsedMacros usedMacros;
diff --git a/src/tools/clangpchmanagerbackend/source/pchtaskqueue.cpp b/src/tools/clangpchmanagerbackend/source/pchtaskqueue.cpp
new file mode 100644
index 0000000000..6b82290ef1
--- /dev/null
+++ b/src/tools/clangpchmanagerbackend/source/pchtaskqueue.cpp
@@ -0,0 +1,205 @@
+/****************************************************************************
+**
+** Copyright (C) 2018 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of Qt Creator.
+**
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 as published by the Free Software
+** Foundation with exceptions as appearing in the file LICENSE.GPL3-EXCEPT
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-3.0.html.
+**
+****************************************************************************/
+
+#include "pchtaskqueue.h"
+
+#include <pchcreatorinterface.h>
+#include <precompiledheaderstorageinterface.h>
+#include <progresscounter.h>
+#include <sqlitetransaction.h>
+
+namespace ClangBackEnd {
+
+void PchTaskQueue::addPchTasks(PchTasks &&newPchTasks, PchTasks &destination)
+{
+    auto compare = [](const PchTask &first, const PchTask &second) {
+        return first.projectPartIds < second.projectPartIds;
+    };
+
+    const std::size_t oldSize = destination.size();
+
+    PchTasks mergedPchTasks;
+    mergedPchTasks.reserve(destination.size() + newPchTasks.size());
+    std::set_union(std::make_move_iterator(newPchTasks.begin()),
+                   std::make_move_iterator(newPchTasks.end()),
+                   std::make_move_iterator(destination.begin()),
+                   std::make_move_iterator(destination.end()),
+                   std::back_inserter(mergedPchTasks),
+                   compare);
+
+    destination = std::move(mergedPchTasks);
+
+    m_progressCounter.addTotal(int(destination.size() - oldSize));
+
+    processEntries();
+}
+
+void PchTaskQueue::removePchTasksByProjectPartId(const Utils::SmallStringVector &projectsPartIds,
+                                                 PchTasks &destination)
+{
+    class CompareDifference
+    {
+    public:
+        bool operator()(const PchTask &first, const Utils::SmallString &second)
+        {
+            return first.projectPartId() < second;
+        }
+
+        bool operator()(const Utils::SmallString &first, const PchTask &second)
+        {
+            return first < second.projectPartId();
+        }
+    };
+
+    const std::size_t oldSize = destination.size();
+
+    PchTasks notToBeRemovedProjectParts;
+    notToBeRemovedProjectParts.reserve(destination.size());
+    std::set_difference(std::make_move_iterator(destination.begin()),
+                        std::make_move_iterator(destination.end()),
+                        projectsPartIds.begin(),
+                        projectsPartIds.end(),
+                        std::back_inserter(notToBeRemovedProjectParts),
+                        CompareDifference{});
+
+    destination = std::move(notToBeRemovedProjectParts);
+
+    m_progressCounter.removeTotal(int(oldSize - destination.size()));
+}
+
+void PchTaskQueue::addSystemPchTasks(PchTasks &&pchTasks)
+{
+    addPchTasks(std::move(pchTasks), m_systemPchTasks);
+}
+
+void PchTaskQueue::addProjectPchTasks(PchTasks &&pchTasks)
+{
+    addPchTasks(std::move(pchTasks), m_projectPchTasks);
+}
+
+void PchTaskQueue::removePchTasks(const Utils::SmallStringVector &projectsPartIds)
+{
+    removePchTasksByProjectPartId(projectsPartIds, m_projectPchTasks);
+}
+
+void PchTaskQueue::processProjectPchTasks()
+{
+    uint systemRunningTaskCount = m_systemPchTaskScheduler.slotUsage().used;
+
+    if (!systemRunningTaskCount) {
+        uint freeTaskCount = m_projectPchTaskScheduler.slotUsage().free;
+
+        auto newEnd = std::prev(m_projectPchTasks.end(),
+                                std::min<int>(int(freeTaskCount), int(m_projectPchTasks.size())));
+        m_projectPchTaskScheduler.addTasks(createProjectTasks(
+            {std::make_move_iterator(newEnd), std::make_move_iterator(m_projectPchTasks.end())}));
+        m_projectPchTasks.erase(newEnd, m_projectPchTasks.end());
+    }
+}
+
+void PchTaskQueue::processSystemPchTasks()
+{
+    uint freeTaskCount = m_systemPchTaskScheduler.slotUsage().free;
+
+    auto newEnd = std::prev(m_systemPchTasks.end(),
+                            std::min<int>(int(freeTaskCount), int(m_systemPchTasks.size())));
+    m_systemPchTaskScheduler.addTasks(createSystemTasks(
+        {std::make_move_iterator(newEnd), std::make_move_iterator(m_systemPchTasks.end())}));
+    m_systemPchTasks.erase(newEnd, m_systemPchTasks.end());
+}
+
+void PchTaskQueue::processEntries()
+{
+    processSystemPchTasks();
+    processProjectPchTasks();
+}
+
+std::vector<PchTaskQueue::Task> PchTaskQueue::createProjectTasks(PchTasks &&pchTasks) const
+{
+    std::vector<Task> tasks;
+    tasks.reserve(pchTasks.size());
+
+    auto convert = [this](auto &&pchTask) {
+        return [pchTask = std::move(pchTask), this](PchCreatorInterface &pchCreator) mutable {
+            Sqlite::DeferredTransaction readTransaction(m_transactionsInterface);
+            pchTask.systemPchPath = m_precompiledHeaderStorage.fetchSystemPrecompiledHeaderPath(
+                pchTask.projectPartId());
+            readTransaction.commit();
+            pchCreator.generatePch(pchTask);
+            const auto &projectPartPch = pchCreator.projectPartPch();
+            Sqlite::ImmediateTransaction writeTransaction(m_transactionsInterface);
+            if (projectPartPch.pchPath.empty()) {
+                m_precompiledHeaderStorage.deleteProjectPrecompiledHeader(pchTask.projectPartId());
+            } else {
+                m_precompiledHeaderStorage
+                    .insertProjectPrecompiledHeader(pchTask.projectPartId(),
+                                                    projectPartPch.pchPath,
+                                                    projectPartPch.lastModified);
+            }
+            writeTransaction.commit();
+        };
+    };
+
+    std::transform(std::make_move_iterator(pchTasks.begin()),
+                   std::make_move_iterator(pchTasks.end()),
+                   std::back_inserter(tasks),
+                   convert);
+
+    return tasks;
+}
+
+std::vector<PchTaskQueue::Task> PchTaskQueue::createSystemTasks(PchTasks &&pchTasks) const
+{
+    std::vector<Task> tasks;
+    tasks.reserve(pchTasks.size());
+
+    auto convert = [this](auto &&pchTask) {
+        return [pchTask = std::move(pchTask), this](PchCreatorInterface &pchCreator) {
+            pchCreator.generatePch(pchTask);
+            const auto &projectPartPch = pchCreator.projectPartPch();
+            Sqlite::ImmediateTransaction transaction(m_transactionsInterface);
+            for (Utils::SmallStringView projectPartId : pchTask.projectPartIds) {
+                if (projectPartPch.pchPath.empty()) {
+                    m_precompiledHeaderStorage.deleteSystemPrecompiledHeader(projectPartId);
+                } else {
+                    m_precompiledHeaderStorage
+                        .insertSystemPrecompiledHeader(projectPartId,
+                                                       projectPartPch.pchPath,
+                                                       projectPartPch.lastModified);
+                }
+            }
+            transaction.commit();
+        };
+    };
+
+    std::transform(std::make_move_iterator(pchTasks.begin()),
+                   std::make_move_iterator(pchTasks.end()),
+                   std::back_inserter(tasks),
+                   convert);
+
+    return tasks;
+}
+
+} // namespace ClangBackEnd
diff --git a/src/tools/clangpchmanagerbackend/source/pchtaskqueue.h b/src/tools/clangpchmanagerbackend/source/pchtaskqueue.h
new file mode 100644
index 0000000000..5d00419817
--- /dev/null
+++ b/src/tools/clangpchmanagerbackend/source/pchtaskqueue.h
@@ -0,0 +1,86 @@
+/****************************************************************************
+**
+** Copyright (C) 2018 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of Qt Creator.
+**
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 as published by the Free Software
+** Foundation with exceptions as appearing in the file LICENSE.GPL3-EXCEPT
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-3.0.html.
+**
+****************************************************************************/
+
+#pragma once
+
+#include "pchtaskqueueinterface.h"
+#include "taskschedulerinterface.h"
+
+namespace Sqlite {
+class TransactionInterface;
+}
+
+namespace ClangBackEnd {
+class PchCreatorInterface;
+class PrecompiledHeaderStorageInterface;
+class ProgressCounter;
+
+class PchTaskQueue final : public PchTaskQueueInterface
+{
+public:
+    using Task = std::function<void (PchCreatorInterface&)>;
+
+    PchTaskQueue(TaskSchedulerInterface<Task> &systemPchTaskScheduler,
+                 TaskSchedulerInterface<Task> &projectPchTaskScheduler,
+                 ProgressCounter &progressCounter,
+                 PrecompiledHeaderStorageInterface &precompiledHeaderStorage,
+                 Sqlite::TransactionInterface &transactionsInterface)
+        : m_systemPchTaskScheduler(systemPchTaskScheduler)
+        , m_projectPchTaskScheduler(projectPchTaskScheduler)
+        , m_precompiledHeaderStorage(precompiledHeaderStorage)
+        , m_transactionsInterface(transactionsInterface)
+        , m_progressCounter(progressCounter)
+    {}
+
+    void addSystemPchTasks(PchTasks &&pchTasks) override;
+    void addProjectPchTasks(PchTasks &&pchTasks) override;
+    void removePchTasks(const Utils::SmallStringVector &projectsPartIds) override;
+
+    void processEntries() override;
+
+    const PchTasks &systemPchTasks() const { return m_systemPchTasks; }
+    const PchTasks &projectPchTasks() const { return m_projectPchTasks; }
+
+    std::vector<Task> createProjectTasks(PchTasks &&pchTasks) const;
+    std::vector<Task> createSystemTasks(PchTasks &&pchTasks) const;
+
+private:
+    void addPchTasks(PchTasks &&pchTasks, PchTasks &destination);
+    void removePchTasksByProjectPartId(const Utils::SmallStringVector &projectsPartIds,
+                                       PchTasks &destination);
+    void processProjectPchTasks();
+    void processSystemPchTasks();
+
+private:
+    PchTasks m_systemPchTasks;
+    PchTasks m_projectPchTasks;
+    TaskSchedulerInterface<Task> &m_systemPchTaskScheduler;
+    TaskSchedulerInterface<Task> &m_projectPchTaskScheduler;
+    PrecompiledHeaderStorageInterface &m_precompiledHeaderStorage;
+    Sqlite::TransactionInterface &m_transactionsInterface;
+    ProgressCounter &m_progressCounter;
+};
+
+} // namespace ClangBackEnd
diff --git a/src/tools/clangpchmanagerbackend/source/precompiledheaderstorage.h b/src/tools/clangpchmanagerbackend/source/precompiledheaderstorage.h
index 94c454231e..67ee9b4fe6 100644
--- a/src/tools/clangpchmanagerbackend/source/precompiledheaderstorage.h
+++ b/src/tools/clangpchmanagerbackend/source/precompiledheaderstorage.h
@@ -47,7 +47,7 @@ public:
         m_transaction.commit();
     }
 
-    void insertPrecompiledHeader(Utils::SmallStringView projectPartName,
+    void insertProjectPrecompiledHeader(Utils::SmallStringView projectPartName,
                                  Utils::SmallStringView pchPath,
                                  long long pchBuildTime) override
     {
@@ -55,43 +55,104 @@ public:
             Sqlite::ImmediateTransaction transaction{m_database};
 
             m_insertProjectPartStatement.write(projectPartName);
-            m_insertPrecompiledHeaderStatement .write(projectPartName, pchPath, pchBuildTime);
+            m_insertProjectPrecompiledHeaderStatement.write(projectPartName, pchPath, pchBuildTime);
 
             transaction.commit();
         } catch (const Sqlite::StatementIsBusy) {
-            insertPrecompiledHeader(projectPartName, pchPath, pchBuildTime);
+            insertProjectPrecompiledHeader(projectPartName, pchPath, pchBuildTime);
         }
     }
 
-    void deletePrecompiledHeader(Utils::SmallStringView projectPartName) override
+    void deleteProjectPrecompiledHeader(Utils::SmallStringView projectPartName) override
     {
         try {
             Sqlite::ImmediateTransaction transaction{m_database};
 
-            m_deletePrecompiledHeaderStatement.write(projectPartName);
+            m_deleteProjectPrecompiledHeaderStatement.write(projectPartName);
 
             transaction.commit();
         } catch (const Sqlite::StatementIsBusy) {
-            deletePrecompiledHeader(projectPartName);
+            deleteProjectPrecompiledHeader(projectPartName);
         }
     }
 
+    void insertSystemPrecompiledHeader(Utils::SmallStringView projectPartName,
+                                       Utils::SmallStringView pchPath,
+                                       long long pchBuildTime) override
+    {
+        try {
+            Sqlite::ImmediateTransaction transaction{m_database};
+
+            m_insertProjectPartStatement.write(projectPartName);
+            m_insertSystemPrecompiledHeaderStatement.write(projectPartName, pchPath, pchBuildTime);
+
+            transaction.commit();
+        } catch (const Sqlite::StatementIsBusy) {
+            insertSystemPrecompiledHeader(projectPartName, pchPath, pchBuildTime);
+        }
+    }
+
+    void deleteSystemPrecompiledHeader(Utils::SmallStringView projectPartName) override
+    {
+        try {
+            Sqlite::ImmediateTransaction transaction{m_database};
+
+            m_deleteSystemPrecompiledHeaderStatement.write(projectPartName);
+
+            transaction.commit();
+        } catch (const Sqlite::StatementIsBusy) {
+            deleteSystemPrecompiledHeader(projectPartName);
+        }
+    }
+
+    Utils::PathString fetchSystemPrecompiledHeaderPath(Utils::SmallStringView projectPartName) override
+    {
+        try {
+            Sqlite::DeferredTransaction transaction{m_database};
+
+            auto value = m_fetchSystemPrecompiledHeaderPathStatement
+                             .template value<Utils::PathString>(projectPartName);
+
+            if (value)
+                return value.value();
+
+            transaction.commit();
+        } catch (const Sqlite::StatementIsBusy) {
+            return fetchSystemPrecompiledHeaderPath(projectPartName);
+        }
+
+        return Utils::SmallStringView("");
+    }
 
 public:
     Sqlite::ImmediateNonThrowingDestructorTransaction m_transaction;
     Database &m_database;
-    WriteStatement m_insertPrecompiledHeaderStatement {
-        "INSERT OR REPLACE INTO precompiledHeaders(projectPartId, pchPath, pchBuildTime) VALUES((SELECT projectPartId FROM projectParts WHERE projectPartName = ?),?,?)",
-        m_database
-    };
+    WriteStatement m_insertProjectPrecompiledHeaderStatement{
+        "INSERT INTO precompiledHeaders(projectPartId, projectPchPath, projectPchBuildTime) "
+        "VALUES((SELECT projectPartId FROM projectParts WHERE projectPartName = ?001),?002,?003) "
+        "ON CONFLICT (projectPartId) DO UPDATE SET projectPchPath=?002,projectPchBuildTime=?003",
+        m_database};
+    WriteStatement m_insertSystemPrecompiledHeaderStatement{
+        "INSERT INTO precompiledHeaders(projectPartId, systemPchPath, systemPchBuildTime) "
+        "VALUES((SELECT projectPartId FROM projectParts WHERE projectPartName = ?001),?002,?003) "
+        "ON CONFLICT (projectPartId) DO UPDATE SET systemPchPath=?002,systemPchBuildTime=?003",
+        m_database};
     WriteStatement m_insertProjectPartStatement{
-        "INSERT OR IGNORE INTO projectParts(projectPartName) VALUES (?)",
-        m_database
-    };
-    WriteStatement m_deletePrecompiledHeaderStatement{
-        "DELETE FROM precompiledHeaders WHERE projectPartId = (SELECT projectPartId FROM projectParts WHERE projectPartName = ?)",
-        m_database
-    };
+        "INSERT OR IGNORE INTO projectParts(projectPartName) VALUES (?)", m_database};
+    WriteStatement m_deleteProjectPrecompiledHeaderStatement{
+        "UPDATE OR IGNORE precompiledHeaders SET projectPchPath=NULL,projectPchBuildTime=NULL "
+        "WHERE projectPartId = (SELECT projectPartId FROM "
+        "projectParts WHERE projectPartName = ?)",
+        m_database};
+    WriteStatement m_deleteSystemPrecompiledHeaderStatement{
+        "UPDATE OR IGNORE precompiledHeaders SET systemPchPath=NULL,systemPchBuildTime=NULL "
+        "WHERE projectPartId = (SELECT projectPartId FROM "
+        "projectParts WHERE projectPartName = ?)",
+        m_database};
+    ReadStatement m_fetchSystemPrecompiledHeaderPathStatement{
+        "SELECT systemPchPath FROM precompiledHeaders WHERE projectPartId = (SELECT projectPartId "
+        "FROM projectParts WHERE projectPartName = ?)",
+        m_database};
 };
 
 }
diff --git a/src/tools/clangpchmanagerbackend/source/precompiledheaderstorageinterface.h b/src/tools/clangpchmanagerbackend/source/precompiledheaderstorageinterface.h
index 9ff6e1efe2..2ade4e885a 100644
--- a/src/tools/clangpchmanagerbackend/source/precompiledheaderstorageinterface.h
+++ b/src/tools/clangpchmanagerbackend/source/precompiledheaderstorageinterface.h
@@ -25,7 +25,7 @@
 
 #pragma once
 
-#include <utils/smallstringview.h>
+#include <utils/smallstring.h>
 
 namespace ClangBackEnd {
 
@@ -34,14 +34,21 @@ class PrecompiledHeaderStorageInterface
 public:
     PrecompiledHeaderStorageInterface() = default;
 
-    PrecompiledHeaderStorageInterface(const PrecompiledHeaderStorageInterface&) = delete;
-    PrecompiledHeaderStorageInterface &operator=(const PrecompiledHeaderStorageInterface&) = delete;
-
-    virtual void insertPrecompiledHeader(Utils::SmallStringView projectPartName,
-                                         Utils::SmallStringView pchPath,
-                                         long long pchBuildTime) = 0;
-
-    virtual void deletePrecompiledHeader(Utils::SmallStringView projectPartName) = 0;
+    PrecompiledHeaderStorageInterface(const PrecompiledHeaderStorageInterface &) = delete;
+    PrecompiledHeaderStorageInterface &operator=(const PrecompiledHeaderStorageInterface &) = delete;
+
+    virtual void insertProjectPrecompiledHeader(Utils::SmallStringView projectPartName,
+                                                Utils::SmallStringView pchPath,
+                                                long long pchBuildTime)
+        = 0;
+    virtual void deleteProjectPrecompiledHeader(Utils::SmallStringView projectPartName) = 0;
+    virtual void insertSystemPrecompiledHeader(Utils::SmallStringView projectPartName,
+                                               Utils::SmallStringView pchPath,
+                                               long long pchBuildTime)
+        = 0;
+    virtual void deleteSystemPrecompiledHeader(Utils::SmallStringView projectPartName) = 0;
+    virtual Utils::PathString fetchSystemPrecompiledHeaderPath(
+        Utils::SmallStringView projectPartName) = 0;
 
 protected:
     ~PrecompiledHeaderStorageInterface() = default;
diff --git a/src/tools/clangpchmanagerbackend/source/projectpartqueue.cpp b/src/tools/clangpchmanagerbackend/source/projectpartqueue.cpp
index 07229c23ee..48f133f7d7 100644
--- a/src/tools/clangpchmanagerbackend/source/projectpartqueue.cpp
+++ b/src/tools/clangpchmanagerbackend/source/projectpartqueue.cpp
@@ -90,7 +90,7 @@ void ProjectPartQueue::removeProjectParts(const Utils::SmallStringVector &projec
 
 void ProjectPartQueue::processEntries()
 {
-    uint taskCount = m_taskScheduler.freeSlots();
+    uint taskCount = m_taskScheduler.slotUsage().free;
 
     auto newEnd = std::prev(m_projectParts.end(), std::min<int>(int(taskCount), int(m_projectParts.size())));
     m_taskScheduler.addTasks(
@@ -112,13 +112,13 @@ std::vector<ProjectPartQueue::Task> ProjectPartQueue::createPchTasks(
 
     auto convert = [this] (auto &&projectPart) {
         return [projectPart=std::move(projectPart), this] (PchCreatorInterface &pchCreator) {
-            pchCreator.generatePch(projectPart);
+            pchCreator.generatePchDeprecated(projectPart);
             const auto &projectPartPch = pchCreator.projectPartPch();
             Sqlite::ImmediateTransaction transaction(m_transactionsInterface);
             if (projectPartPch.pchPath.empty()) {
-                m_precompiledHeaderStorage.deletePrecompiledHeader(projectPartPch.projectPartId);
+                m_precompiledHeaderStorage.deleteProjectPrecompiledHeader(projectPartPch.projectPartId);
             } else {
-                m_precompiledHeaderStorage.insertPrecompiledHeader(projectPartPch.projectPartId,
+                m_precompiledHeaderStorage.insertProjectPrecompiledHeader(projectPartPch.projectPartId,
                                                                    projectPartPch.pchPath,
                                                                    projectPartPch.lastModified);
             }
diff --git a/src/tools/clangpchmanagerbackend/source/taskscheduler.h b/src/tools/clangpchmanagerbackend/source/taskscheduler.h
index c58c867db5..e2891a1163 100644
--- a/src/tools/clangpchmanagerbackend/source/taskscheduler.h
+++ b/src/tools/clangpchmanagerbackend/source/taskscheduler.h
@@ -99,14 +99,15 @@ public:
         return m_futures;
     }
 
-    uint freeSlots()
+    SlotUsage slotUsage()
     {
         removeFinishedFutures();
 
         if (m_isDisabled)
-            return 0;
+            return {};
 
-        return uint(std::max(int(m_hardwareConcurrency) - int(m_futures.size()), 0));
+        return {uint(std::max(int(m_hardwareConcurrency) - int(m_futures.size()), 0)),
+                uint(m_futures.size())};
     }
 
     void syncTasks()
diff --git a/src/tools/clangpchmanagerbackend/source/taskschedulerinterface.h b/src/tools/clangpchmanagerbackend/source/taskschedulerinterface.h
index a79a3ed8d2..daff16cff8 100644
--- a/src/tools/clangpchmanagerbackend/source/taskschedulerinterface.h
+++ b/src/tools/clangpchmanagerbackend/source/taskschedulerinterface.h
@@ -31,6 +31,12 @@ namespace ClangBackEnd {
 
 using uint = unsigned int;
 
+struct SlotUsage
+{
+    uint free = 0;
+    uint used = 0;
+};
+
 template <typename Task>
 class TaskSchedulerInterface
 {
@@ -40,7 +46,7 @@ public:
     TaskSchedulerInterface &operator=(const TaskSchedulerInterface &) = delete;
 
     virtual void addTasks(std::vector<Task> &&tasks) = 0;
-    virtual uint freeSlots() = 0;
+    virtual SlotUsage slotUsage() = 0;
 
 protected:
     ~TaskSchedulerInterface() = default;
diff --git a/src/tools/clangrefactoringbackend/source/symbolindexertaskqueue.h b/src/tools/clangrefactoringbackend/source/symbolindexertaskqueue.h
index c0379b2c29..7ca3513284 100644
--- a/src/tools/clangrefactoringbackend/source/symbolindexertaskqueue.h
+++ b/src/tools/clangrefactoringbackend/source/symbolindexertaskqueue.h
@@ -94,7 +94,7 @@ public:
 
     void processEntries()
     {
-        uint taskCount = m_symbolIndexerScheduler.freeSlots();
+        uint taskCount = m_symbolIndexerScheduler.slotUsage().free;
 
         auto newEnd = std::prev(m_tasks.end(), std::min<int>(int(taskCount), int(m_tasks.size())));
         m_symbolIndexerScheduler.addTasks({std::make_move_iterator(newEnd),
diff --git a/src/tools/clangrefactoringbackend/source/symbolindexing.h b/src/tools/clangrefactoringbackend/source/symbolindexing.h
index d20142870e..10e1be0358 100644
--- a/src/tools/clangrefactoringbackend/source/symbolindexing.h
+++ b/src/tools/clangrefactoringbackend/source/symbolindexing.h
@@ -105,7 +105,7 @@ public:
         m_indexerScheduler.disable();
         while (!m_indexerScheduler.futures().empty()) {
             m_indexerScheduler.syncTasks();
-            m_indexerScheduler.freeSlots();
+            m_indexerScheduler.slotUsage();
         }
     }
 
diff --git a/src/tools/clangrefactoringbackend/source/symbolstorage.h b/src/tools/clangrefactoringbackend/source/symbolstorage.h
index 4aae17c599..02d52bfb50 100644
--- a/src/tools/clangrefactoringbackend/source/symbolstorage.h
+++ b/src/tools/clangrefactoringbackend/source/symbolstorage.h
@@ -332,7 +332,7 @@ public:
         m_database
     };
     mutable ReadStatement m_getPrecompiledHeader{
-        "SELECT pchPath, pchBuildTime FROM precompiledHeaders WHERE projectPartId = ?",
+        "SELECT projectPchPath, projectPchBuildTime FROM precompiledHeaders WHERE projectPartId = ?",
         m_database
     };
 };
diff --git a/tests/unit/unittest/filepathstorage-test.cpp b/tests/unit/unittest/filepathstorage-test.cpp
index f94f3dfad2..be689b44f3 100644
--- a/tests/unit/unittest/filepathstorage-test.cpp
+++ b/tests/unit/unittest/filepathstorage-test.cpp
@@ -90,7 +90,7 @@ protected:
                 .Times(AnyNumber());
         EXPECT_CALL(selectAllSources, valuesReturnStdVectorSource(_))
                 .Times(AnyNumber());
-        EXPECT_CALL(selectDirectoryPathFromDirectoriesByDirectoryId, valueReturnPathString(_))
+        EXPECT_CALL(selectDirectoryPathFromDirectoriesByDirectoryId, valueReturnPathString(An<int>()))
                 .Times(AnyNumber());
         EXPECT_CALL(selectSourceNameAndDirectoryIdFromSourcesBySourceId, valueReturnSourceNameAndDirectoryId(_))
                 .Times(AnyNumber());
diff --git a/tests/unit/unittest/gtest-creator-printing.cpp b/tests/unit/unittest/gtest-creator-printing.cpp
index 072455ab76..098b1b658d 100644
--- a/tests/unit/unittest/gtest-creator-printing.cpp
+++ b/tests/unit/unittest/gtest-creator-printing.cpp
@@ -1033,8 +1033,8 @@ std::ostream &operator<<(std::ostream &out, const PchCreatorIncludes &includes)
 }
 std::ostream &operator<<(std::ostream &out, const PchTask &task)
 {
-    return out << "(" << task.projectPartId << ", " << task.dependentIds << ", " << task.includes
-               << ", " << task.compilerMacros << ", " << task.usedMacros << ")";
+    return out << "(" << task.projectPartIds << ", " << task.includes << ", " << task.compilerMacros
+               << ", " << task.usedMacros << ")";
 }
 
 std::ostream &operator<<(std::ostream &out, const BuildDependency &dependency)
@@ -1048,6 +1048,11 @@ std::ostream &operator<<(std::ostream &out, const BuildDependency &dependency)
                << ")";
 }
 
+std::ostream &operator<<(std::ostream &out, const SlotUsage &slotUsage)
+{
+    return out << "(" << slotUsage.free << ", " << slotUsage.used << ")";
+}
+
 const char *sourceTypeString(SourceType sourceType)
 {
     using ClangBackEnd::SymbolTag;
diff --git a/tests/unit/unittest/gtest-creator-printing.h b/tests/unit/unittest/gtest-creator-printing.h
index 8dd07c4795..db52d584d9 100644
--- a/tests/unit/unittest/gtest-creator-printing.h
+++ b/tests/unit/unittest/gtest-creator-printing.h
@@ -175,6 +175,7 @@ class PchTask;
 class BuildDependency;
 class SourceEntry;
 class FilePathCaching;
+class SlotUsage;
 
 std::ostream &operator<<(std::ostream &out, const SourceLocationEntry &entry);
 std::ostream &operator<<(std::ostream &out, const IdPaths &idPaths);
@@ -259,6 +260,7 @@ std::ostream &operator<<(std::ostream &out, const PchCreatorIncludes &includes);
 std::ostream &operator<<(std::ostream &out, const PchTask &task);
 std::ostream &operator<<(std::ostream &out, const BuildDependency &dependency);
 std::ostream &operator<<(std::ostream &out, const SourceEntry &entry);
+std::ostream &operator<<(std::ostream &out, const SlotUsage &slotUsage);
 
 void PrintTo(const FilePath &filePath, ::std::ostream *os);
 void PrintTo(const FilePathView &filePathView, ::std::ostream *os);
diff --git a/tests/unit/unittest/mockpchcreator.h b/tests/unit/unittest/mockpchcreator.h
index 4d1fd24707..992976d4de 100644
--- a/tests/unit/unittest/mockpchcreator.h
+++ b/tests/unit/unittest/mockpchcreator.h
@@ -35,21 +35,13 @@
 class MockPchCreator : public ClangBackEnd::PchCreatorInterface
 {
 public:
-    MOCK_METHOD1(generatePch,
-                 void(const ClangBackEnd::V2::ProjectPartContainer &projectPart));
-    MOCK_METHOD0(takeProjectIncludes,
-                 ClangBackEnd::IdPaths());
-    MOCK_METHOD0(projectPartPch,
-                 const ClangBackEnd::ProjectPartPch &());
-    MOCK_METHOD1(setUnsavedFiles,
-                 void (const ClangBackEnd::V2::FileContainers &fileContainers));
-    MOCK_METHOD0(clear,
-                 void());
-    MOCK_METHOD0(doInMainThreadAfterFinished,
-                 void());
-    MOCK_CONST_METHOD0(isUsed,
-                       bool());
-    MOCK_METHOD1(setIsUsed,
-                 void(bool));
-
+    MOCK_METHOD1(generatePchDeprecated, void(const ClangBackEnd::V2::ProjectPartContainer &projectPart));
+    MOCK_METHOD1(generatePch, void(const ClangBackEnd::PchTask &pchTask));
+    MOCK_METHOD0(takeProjectIncludes, ClangBackEnd::IdPaths());
+    MOCK_METHOD0(projectPartPch, const ClangBackEnd::ProjectPartPch &());
+    MOCK_METHOD1(setUnsavedFiles, void(const ClangBackEnd::V2::FileContainers &fileContainers));
+    MOCK_METHOD0(clear, void());
+    MOCK_METHOD0(doInMainThreadAfterFinished, void());
+    MOCK_CONST_METHOD0(isUsed, bool());
+    MOCK_METHOD1(setIsUsed, void(bool));
 };
diff --git a/tests/unit/unittest/mockprecompiledheaderstorage.h b/tests/unit/unittest/mockprecompiledheaderstorage.h
index 11493a5560..c267ecb535 100644
--- a/tests/unit/unittest/mockprecompiledheaderstorage.h
+++ b/tests/unit/unittest/mockprecompiledheaderstorage.h
@@ -32,10 +32,16 @@
 class MockPrecompiledHeaderStorage : public ClangBackEnd::PrecompiledHeaderStorageInterface
 {
 public:
-    MOCK_METHOD3(insertPrecompiledHeader,
-                 void (Utils::SmallStringView projectPartName,
-                       Utils::SmallStringView pchPath,
-                       long long pchBuildTime));
-
-    MOCK_METHOD1(deletePrecompiledHeader, void (Utils::SmallStringView projectPartName));
+    MOCK_METHOD3(insertProjectPrecompiledHeader,
+                 void(Utils::SmallStringView projectPartName,
+                      Utils::SmallStringView pchPath,
+                      long long pchBuildTime));
+    MOCK_METHOD1(deleteProjectPrecompiledHeader, void(Utils::SmallStringView projectPartName));
+    MOCK_METHOD3(insertSystemPrecompiledHeader,
+                 void(Utils::SmallStringView projectPartName,
+                      Utils::SmallStringView pchPath,
+                      long long pchBuildTime));
+    MOCK_METHOD1(deleteSystemPrecompiledHeader, void(Utils::SmallStringView projectPartName));
+    MOCK_METHOD1(fetchSystemPrecompiledHeaderPath,
+                 Utils::PathString(Utils::SmallStringView projectPartName));
 };
diff --git a/tests/unit/unittest/mocksqlitereadstatement.cpp b/tests/unit/unittest/mocksqlitereadstatement.cpp
index 66406f09fa..128b3d899f 100644
--- a/tests/unit/unittest/mocksqlitereadstatement.cpp
+++ b/tests/unit/unittest/mocksqlitereadstatement.cpp
@@ -138,6 +138,13 @@ MockSqliteReadStatement::value<Utils::PathString>(const int &directoryId)
 }
 
 template <>
+Utils::optional<Utils::PathString>
+MockSqliteReadStatement::value<Utils::PathString>(const Utils::SmallStringView &path)
+{
+    return valueReturnPathString(path);
+}
+
+template <>
 Utils::optional<ClangBackEnd::ProjectPartArtefact>
 MockSqliteReadStatement::value<ClangBackEnd::ProjectPartArtefact, 4>(const int& sourceId)
 {
diff --git a/tests/unit/unittest/mocksqlitereadstatement.h b/tests/unit/unittest/mocksqlitereadstatement.h
index e5926d2d17..4fbdda2a6d 100644
--- a/tests/unit/unittest/mocksqlitereadstatement.h
+++ b/tests/unit/unittest/mocksqlitereadstatement.h
@@ -96,6 +96,9 @@ public:
     MOCK_METHOD1(valueReturnPathString,
                  Utils::optional<Utils::PathString>(int));
 
+    MOCK_METHOD1(valueReturnPathString,
+                 Utils::optional<Utils::PathString>(Utils::SmallStringView));
+
     MOCK_METHOD1(valueReturnSmallString,
                  Utils::optional<Utils::SmallString>(int));
 
@@ -222,6 +225,10 @@ Utils::optional<Utils::PathString>
 MockSqliteReadStatement::value<Utils::PathString>(const int&);
 
 template <>
+Utils::optional<Utils::PathString>
+MockSqliteReadStatement::value<Utils::PathString>(const Utils::SmallStringView&);
+
+template <>
 Utils::optional<ClangBackEnd::ProjectPartArtefact>
 MockSqliteReadStatement::value<ClangBackEnd::ProjectPartArtefact, 4>(const int&);
 
diff --git a/tests/unit/unittest/mocktaskscheduler.h b/tests/unit/unittest/mocktaskscheduler.h
index 647e8aefc0..dde6211828 100644
--- a/tests/unit/unittest/mocktaskscheduler.h
+++ b/tests/unit/unittest/mocktaskscheduler.h
@@ -36,8 +36,8 @@ class MockTaskScheduler : public ClangBackEnd::TaskSchedulerInterface<Task>
 public:
     MOCK_METHOD1_T(addTasks,
                  void (const std::vector<Task> &));
-    MOCK_METHOD0(freeSlots,
-                 uint ());
+    MOCK_METHOD0(slotUsage,
+                 ClangBackEnd::SlotUsage ());
 
     void addTasks(std::vector<Task> &&tasks)
     {
diff --git a/tests/unit/unittest/pchcreator-test.cpp b/tests/unit/unittest/pchcreator-test.cpp
index 0b4685891c..8b556af6b7 100644
--- a/tests/unit/unittest/pchcreator-test.cpp
+++ b/tests/unit/unittest/pchcreator-test.cpp
@@ -223,14 +223,14 @@ TEST_F(PchCreator, CreateProjectPartClangCompilerArguments)
 
 TEST_F(PchCreatorVerySlowTest, IncludesForCreatePchsForProjectParts)
 {
-    creator.generatePch(projectPart1);
+    creator.generatePchDeprecated(projectPart1);
 
     ASSERT_THAT(creator.takeProjectIncludes().id, "project1");
 }
 
 TEST_F(PchCreatorVerySlowTest, ProjectPartPchsSendToPchManagerClient)
 {
-    creator.generatePch(projectPart1);
+    creator.generatePchDeprecated(projectPart1);
 
     EXPECT_CALL(mockPchManagerClient,
                 precompiledHeadersUpdated(
@@ -242,7 +242,7 @@ TEST_F(PchCreatorVerySlowTest, ProjectPartPchsSendToPchManagerClient)
 
 TEST_F(PchCreatorVerySlowTest, UpdateFileWatcherIncludes)
 {
-    creator.generatePch(projectPart1);
+    creator.generatePchDeprecated(projectPart1);
 
     EXPECT_CALL(mockClangPathWatcher,
                 updateIdPaths(ElementsAre(creator.projectIncludes())));
@@ -252,7 +252,7 @@ TEST_F(PchCreatorVerySlowTest, UpdateFileWatcherIncludes)
 
 TEST_F(PchCreatorVerySlowTest, IdPathsForCreatePchsForProjectParts)
 {
-    creator.generatePch(projectPart1);
+    creator.generatePchDeprecated(projectPart1);
 
     ASSERT_THAT(creator.takeProjectIncludes(),
                 AllOf(Field(&IdPaths::id, "project1"),
@@ -263,7 +263,7 @@ TEST_F(PchCreatorVerySlowTest, IdPathsForCreatePchsForProjectParts)
 
 TEST_F(PchCreatorVerySlowTest, ProjectPartPchForCreatesPchForProjectPart)
 {
-    creator.generatePch(projectPart1);
+    creator.generatePchDeprecated(projectPart1);
 
     ASSERT_THAT(creator.projectPartPch(),
                 AllOf(Field(&ProjectPartPch::projectPartId, Eq("project1")),
@@ -273,7 +273,7 @@ TEST_F(PchCreatorVerySlowTest, ProjectPartPchForCreatesPchForProjectPart)
 
 TEST_F(PchCreatorVerySlowTest, ProjectPartPchCleared)
 {
-    creator.generatePch(projectPart1);
+    creator.generatePchDeprecated(projectPart1);
 
     creator.clear();
 
@@ -283,7 +283,7 @@ TEST_F(PchCreatorVerySlowTest, ProjectPartPchCleared)
 
 TEST_F(PchCreatorVerySlowTest, ProjectIncludesCleared)
 {
-    creator.generatePch(projectPart1);
+    creator.generatePchDeprecated(projectPart1);
 
     creator.clear();
 
@@ -299,7 +299,7 @@ TEST_F(PchCreatorVerySlowTest, FaultyProjectPartPchForCreatesNoPchForProjectPart
                                            {},
                                            {id(TESTDATA_DIR "/builddependencycollector/project/faulty.cpp")}};
 
-    creator.generatePch(faultyProjectPart);
+    creator.generatePchDeprecated(faultyProjectPart);
 
     ASSERT_THAT(creator.projectPartPch(),
                 AllOf(Field(&ProjectPartPch::projectPartId, Eq("faultyProject")),
diff --git a/tests/unit/unittest/pchtaskgenerator-test.cpp b/tests/unit/unittest/pchtaskgenerator-test.cpp
index 9fb9038012..0cb4c93583 100644
--- a/tests/unit/unittest/pchtaskgenerator-test.cpp
+++ b/tests/unit/unittest/pchtaskgenerator-test.cpp
@@ -80,7 +80,7 @@ TEST_F(PchTaskGenerator, Create)
     EXPECT_CALL(mockPchTaskMerger,
                 mergeTasks(ElementsAre(
                     AllOf(Field(&PchTaskSet::system,
-                                AllOf(Field(&PchTask::projectPartId, Eq("ProjectPart1")),
+                                AllOf(Field(&PchTask::projectPartIds, ElementsAre("ProjectPart1")),
                                       Field(&PchTask::includes, ElementsAre(4, 5)),
                                       Field(&PchTask::compilerMacros,
                                             ElementsAre(CompilerMacro{"SE", "4", 4},
@@ -88,7 +88,8 @@ TEST_F(PchTaskGenerator, Create)
                                       Field(&PchTask::usedMacros,
                                             ElementsAre(UsedMacro{"SE", 4}, UsedMacro{"WU", 5})))),
                           AllOf(Field(&PchTaskSet::project,
-                                      AllOf(Field(&PchTask::projectPartId, Eq("ProjectPart1")),
+                                      AllOf(Field(&PchTask::projectPartIds,
+                                                  ElementsAre("ProjectPart1")),
                                             Field(&PchTask::includes, ElementsAre(1, 3)),
                                             Field(&PchTask::compilerMacros,
                                                   ElementsAre(CompilerMacro{"YI", "1", 1},
diff --git a/tests/unit/unittest/pchtaskqueue-test.cpp b/tests/unit/unittest/pchtaskqueue-test.cpp
new file mode 100644
index 0000000000..2ee1bf3cad
--- /dev/null
+++ b/tests/unit/unittest/pchtaskqueue-test.cpp
@@ -0,0 +1,343 @@
+/****************************************************************************
+**
+** Copyright (C) 2018 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of Qt Creator.
+**
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 as published by the Free Software
+** Foundation with exceptions as appearing in the file LICENSE.GPL3-EXCEPT
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-3.0.html.
+**
+****************************************************************************/
+
+#include "googletest.h"
+
+#include "mockpchcreator.h"
+#include "mockprecompiledheaderstorage.h"
+#include "mocksqlitetransactionbackend.h"
+#include "mocktaskscheduler.h"
+
+#include <pchtaskqueue.h>
+#include <progresscounter.h>
+
+namespace {
+
+using ClangBackEnd::PchTask;
+using ClangBackEnd::SlotUsage;
+
+class PchTaskQueue : public testing::Test
+{
+protected:
+    NiceMock<MockTaskScheduler<ClangBackEnd::PchTaskQueue::Task>> mockSytemPchTaskScheduler;
+    NiceMock<MockTaskScheduler<ClangBackEnd::PchTaskQueue::Task>> mockProjectPchTaskScheduler;
+    MockPrecompiledHeaderStorage mockPrecompiledHeaderStorage;
+    MockSqliteTransactionBackend mockSqliteTransactionBackend;
+    NiceMock<MockFunction<void(int, int)>> mockSetProgressCallback;
+    ClangBackEnd::ProgressCounter progressCounter{mockSetProgressCallback.AsStdFunction()};
+    ClangBackEnd::PchTaskQueue queue{mockSytemPchTaskScheduler,
+                                     mockProjectPchTaskScheduler,
+                                     progressCounter,
+                                     mockPrecompiledHeaderStorage,
+                                     mockSqliteTransactionBackend};
+    PchTask systemTask1{"ProjectPart1",
+                        {1, 2},
+                        {{"YI", "1", 1}, {"SAN", "3", 3}},
+                        {{"LIANG", 0}, {"YI", 1}}};
+    PchTask systemTask2{"ProjectPart2",
+                        {1, 2},
+                        {{"YI", "1", 1}, {"SAN", "3", 3}},
+                        {{"LIANG", 0}, {"YI", 1}}};
+    PchTask systemTask2b{"ProjectPart2",
+                         {3, 4},
+                         {{"YI", "1", 1}, {"SAN", "3", 3}},
+                         {{"LIANG", 0}, {"YI", 1}}};
+    PchTask systemTask3{"ProjectPart3",
+                        {1, 2},
+                        {{"YI", "1", 1}, {"SAN", "3", 3}},
+                        {{"LIANG", 0}, {"YI", 1}}};
+    PchTask projectTask1{"ProjectPart1",
+                         {11, 12},
+                         {{"SE", "4", 4}, {"WU", "5", 5}},
+                         {{"ER", 2}, {"SAN", 3}}};
+    PchTask projectTask2{"ProjectPart2",
+                         {11, 12},
+                         {{"SE", "4", 4}, {"WU", "5", 5}},
+                         {{"ER", 2}, {"SAN", 3}}};
+    PchTask projectTask2b{"ProjectPart2",
+                          {21, 22},
+                          {{"SE", "4", 4}, {"WU", "5", 5}},
+                          {{"ER", 2}, {"SAN", 3}}};
+    PchTask projectTask3{"ProjectPart3",
+                         {21, 22},
+                         {{"SE", "4", 4}, {"WU", "5", 5}},
+                         {{"ER", 2}, {"SAN", 3}}};
+    PchTask systemTask4{Utils::SmallStringVector{"ProjectPart1", "ProjectPart3"},
+                        {1, 2},
+                        {{"YI", "1", 1}, {"SAN", "3", 3}},
+                        {{"LIANG", 0}, {"YI", 1}}};
+};
+
+TEST_F(PchTaskQueue, AddProjectPchTask)
+{
+    queue.addProjectPchTasks({projectTask1});
+
+    queue.addProjectPchTasks({projectTask2});
+
+    ASSERT_THAT(queue.projectPchTasks(), ElementsAre(projectTask1, projectTask2));
+}
+
+TEST_F(PchTaskQueue, AddSystemPchTask)
+{
+    queue.addSystemPchTasks({systemTask1});
+
+    queue.addSystemPchTasks({systemTask2});
+
+    ASSERT_THAT(queue.systemPchTasks(), ElementsAre(systemTask1, systemTask2));
+}
+
+TEST_F(PchTaskQueue, AddProjectPchTasksCallsProcessEntriesForSystemTaskSchedulerIsNotBusy)
+{
+    InSequence s;
+
+    EXPECT_CALL(mockSytemPchTaskScheduler, slotUsage()).WillRepeatedly(Return(SlotUsage{2, 0}));
+    EXPECT_CALL(mockProjectPchTaskScheduler, slotUsage()).WillRepeatedly(Return(SlotUsage{2, 0}));
+    EXPECT_CALL(mockProjectPchTaskScheduler, addTasks(SizeIs(2)));
+
+    queue.addProjectPchTasks({projectTask1, projectTask2});
+}
+
+TEST_F(PchTaskQueue, AddProjectPchTasksCallsProcessEntriesForSystemTaskSchedulerIsBusy)
+{
+    InSequence s;
+
+    EXPECT_CALL(mockSytemPchTaskScheduler, slotUsage()).WillRepeatedly(Return(SlotUsage{2, 1}));
+    EXPECT_CALL(mockProjectPchTaskScheduler, slotUsage()).Times(0);
+    EXPECT_CALL(mockProjectPchTaskScheduler, addTasks(_)).Times(0);
+
+    queue.addProjectPchTasks({projectTask1, projectTask2});
+}
+
+TEST_F(PchTaskQueue, AddSystemPchTasksCallsProcessEntries)
+{
+    InSequence s;
+
+    EXPECT_CALL(mockSytemPchTaskScheduler, slotUsage()).WillRepeatedly(Return(SlotUsage{2, 0}));
+    EXPECT_CALL(mockSytemPchTaskScheduler, addTasks(SizeIs(2)));
+    EXPECT_CALL(mockSytemPchTaskScheduler, slotUsage()).WillRepeatedly(Return(SlotUsage{2, 1}));
+    EXPECT_CALL(mockProjectPchTaskScheduler, slotUsage()).Times(0);
+    EXPECT_CALL(mockProjectPchTaskScheduler, addTasks(_)).Times(0);
+
+    queue.addSystemPchTasks({projectTask1, projectTask2});
+}
+
+TEST_F(PchTaskQueue, AddProjectPchTasksCallsProgressCounter)
+{
+    queue.addProjectPchTasks({projectTask1, projectTask2});
+
+    EXPECT_CALL(mockSetProgressCallback, Call(0, 3));
+
+    queue.addProjectPchTasks({projectTask2b, projectTask3});
+}
+
+TEST_F(PchTaskQueue, AddSystemPchTasksCallsProgressCounter)
+{
+    queue.addSystemPchTasks({systemTask1, systemTask2});
+
+    EXPECT_CALL(mockSetProgressCallback, Call(0, 3));
+
+    queue.addSystemPchTasks({systemTask2b, systemTask3});
+}
+
+TEST_F(PchTaskQueue, AddPchCallsProgressCounter)
+{
+    queue.addSystemPchTasks({systemTask1, systemTask2});
+
+    EXPECT_CALL(mockSetProgressCallback, Call(0, 3));
+
+    queue.addSystemPchTasks({systemTask2b, systemTask3});
+}
+
+TEST_F(PchTaskQueue, ReplaceIdenticalProjectPchTasks)
+{
+    queue.addProjectPchTasks({projectTask1, projectTask2});
+
+    queue.addProjectPchTasks({projectTask1, projectTask2});
+
+    ASSERT_THAT(queue.projectPchTasks(), ElementsAre(projectTask1, projectTask2));
+}
+
+TEST_F(PchTaskQueue, ReplaceIdenticalSystemPchTasks)
+{
+    queue.addSystemPchTasks({systemTask1, systemTask2});
+
+    queue.addSystemPchTasks({systemTask1, systemTask2});
+
+    ASSERT_THAT(queue.systemPchTasks(), ElementsAre(systemTask1, systemTask2));
+}
+
+TEST_F(PchTaskQueue, ReplaceProjectPchTasksWithSameId)
+{
+    queue.addProjectPchTasks({projectTask1, projectTask2});
+
+    queue.addProjectPchTasks({projectTask1, projectTask2b, projectTask3});
+
+    ASSERT_THAT(queue.projectPchTasks(), ElementsAre(projectTask1, projectTask2b, projectTask3));
+}
+
+TEST_F(PchTaskQueue, ReplaceSystemPchTasksWithSameId)
+{
+    queue.addSystemPchTasks({systemTask1, systemTask2});
+
+    queue.addSystemPchTasks({systemTask1, systemTask2b, systemTask3});
+
+    ASSERT_THAT(queue.systemPchTasks(), ElementsAre(systemTask1, systemTask2b, systemTask3));
+}
+
+TEST_F(PchTaskQueue, RemoveProjectPchTasksByProjectPartId)
+{
+    queue.addProjectPchTasks({projectTask1, projectTask2, projectTask3});
+
+    queue.removePchTasks(projectTask2.projectPartIds);
+
+    ASSERT_THAT(queue.projectPchTasks(), ElementsAre(projectTask1, projectTask3));
+}
+
+TEST_F(PchTaskQueue, DontRemoveSystemPchTasksByProjectPartId)
+{
+    queue.addSystemPchTasks({systemTask1, systemTask2, systemTask3});
+
+    queue.removePchTasks(systemTask2.projectPartIds);
+
+    ASSERT_THAT(queue.systemPchTasks(), ElementsAre(systemTask1, systemTask2, systemTask3));
+}
+
+TEST_F(PchTaskQueue, RemovePchTasksCallsProgressCounter)
+{
+    queue.addSystemPchTasks({systemTask1, systemTask2, systemTask3});
+    queue.addProjectPchTasks({projectTask1, projectTask2, projectTask3});
+
+    EXPECT_CALL(mockSetProgressCallback, Call(0, 5));
+
+    queue.removePchTasks(systemTask2.projectPartIds);
+}
+
+TEST_F(PchTaskQueue, CreateProjectTasksSizeEqualsInputSize)
+{
+    auto tasks = queue.createProjectTasks({projectTask1, projectTask1});
+
+    ASSERT_THAT(tasks, SizeIs(2));
+}
+
+TEST_F(PchTaskQueue, CreateProjectTaskFromPchTask)
+{
+    InSequence s;
+    MockPchCreator mockPchCreator;
+    ClangBackEnd::ProjectPartPch projectPartPch{"", "/path/to/pch", 99};
+    auto tasks = queue.createProjectTasks({projectTask1});
+    auto projectTask = projectTask1;
+    projectTask.systemPchPath = "/path/to/pch";
+
+    EXPECT_CALL(mockSqliteTransactionBackend, lock());
+    EXPECT_CALL(mockSqliteTransactionBackend, deferredBegin());
+    EXPECT_CALL(mockPrecompiledHeaderStorage, fetchSystemPrecompiledHeaderPath(Eq("ProjectPart1")))
+        .WillOnce(Return(Utils::PathString{"/path/to/pch"}));
+    EXPECT_CALL(mockSqliteTransactionBackend, commit());
+    EXPECT_CALL(mockSqliteTransactionBackend, unlock());
+    EXPECT_CALL(mockPchCreator, generatePch(Eq(projectTask)));
+    EXPECT_CALL(mockPchCreator, projectPartPch()).WillOnce(ReturnRef(projectPartPch));
+    EXPECT_CALL(mockSqliteTransactionBackend, lock());
+    EXPECT_CALL(mockSqliteTransactionBackend, immediateBegin());
+    EXPECT_CALL(mockPrecompiledHeaderStorage,
+                insertProjectPrecompiledHeader(Eq("ProjectPart1"), Eq("/path/to/pch"), 99));
+    EXPECT_CALL(mockSqliteTransactionBackend, commit());
+    EXPECT_CALL(mockSqliteTransactionBackend, unlock());
+
+    tasks.front()(mockPchCreator);
+}
+
+TEST_F(PchTaskQueue, DeleteProjectPchEntryInDatabaseIfNoPchIsGenerated)
+{
+    InSequence s;
+    MockPchCreator mockPchCreator;
+    ClangBackEnd::ProjectPartPch projectPartPch{"", "", 0};
+    auto tasks = queue.createProjectTasks({projectTask1});
+    auto projectTask = projectTask1;
+    projectTask.systemPchPath = "/path/to/pch";
+
+    EXPECT_CALL(mockSqliteTransactionBackend, lock());
+    EXPECT_CALL(mockSqliteTransactionBackend, deferredBegin());
+    EXPECT_CALL(mockPrecompiledHeaderStorage, fetchSystemPrecompiledHeaderPath(Eq("ProjectPart1")))
+        .WillOnce(Return(Utils::PathString{"/path/to/pch"}));
+    EXPECT_CALL(mockSqliteTransactionBackend, commit());
+    EXPECT_CALL(mockSqliteTransactionBackend, unlock());
+    EXPECT_CALL(mockPchCreator, generatePch(Eq(projectTask)));
+    EXPECT_CALL(mockPchCreator, projectPartPch()).WillOnce(ReturnRef(projectPartPch));
+    EXPECT_CALL(mockSqliteTransactionBackend, lock());
+    EXPECT_CALL(mockSqliteTransactionBackend, immediateBegin());
+    EXPECT_CALL(mockPrecompiledHeaderStorage, deleteProjectPrecompiledHeader(Eq("ProjectPart1")));
+    EXPECT_CALL(mockSqliteTransactionBackend, commit());
+    EXPECT_CALL(mockSqliteTransactionBackend, unlock());
+
+    tasks.front()(mockPchCreator);
+}
+
+TEST_F(PchTaskQueue, CreateSystemTasksSizeEqualsInputSize)
+{
+    auto tasks = queue.createSystemTasks({systemTask1, systemTask2});
+
+    ASSERT_THAT(tasks, SizeIs(2));
+}
+
+TEST_F(PchTaskQueue, CreateSystemTaskFromPchTask)
+{
+    InSequence s;
+    MockPchCreator mockPchCreator;
+    ClangBackEnd::ProjectPartPch projectPartPch{"", "/path/to/pch", 99};
+    auto tasks = queue.createSystemTasks({systemTask4});
+
+    EXPECT_CALL(mockPchCreator, generatePch(Eq(systemTask4)));
+    EXPECT_CALL(mockPchCreator, projectPartPch()).WillOnce(ReturnRef(projectPartPch));
+    EXPECT_CALL(mockSqliteTransactionBackend, lock());
+    EXPECT_CALL(mockSqliteTransactionBackend, immediateBegin());
+    EXPECT_CALL(mockPrecompiledHeaderStorage,
+                insertSystemPrecompiledHeader(Eq("ProjectPart1"), Eq("/path/to/pch"), 99));
+    EXPECT_CALL(mockPrecompiledHeaderStorage,
+                insertSystemPrecompiledHeader(Eq("ProjectPart3"), Eq("/path/to/pch"), 99));
+    EXPECT_CALL(mockSqliteTransactionBackend, commit());
+    EXPECT_CALL(mockSqliteTransactionBackend, unlock());
+
+    tasks.front()(mockPchCreator);
+}
+
+TEST_F(PchTaskQueue, DeleteSystemPchEntryInDatabaseIfNoPchIsGenerated)
+{
+    InSequence s;
+    MockPchCreator mockPchCreator;
+    ClangBackEnd::ProjectPartPch projectPartPch{"", "", 0};
+    auto tasks = queue.createSystemTasks({systemTask4});
+
+    EXPECT_CALL(mockPchCreator, generatePch(Eq(systemTask4)));
+    EXPECT_CALL(mockPchCreator, projectPartPch()).WillOnce(ReturnRef(projectPartPch));
+    EXPECT_CALL(mockSqliteTransactionBackend, lock());
+    EXPECT_CALL(mockSqliteTransactionBackend, immediateBegin());
+    EXPECT_CALL(mockPrecompiledHeaderStorage, deleteSystemPrecompiledHeader(Eq("ProjectPart1")));
+    EXPECT_CALL(mockPrecompiledHeaderStorage, deleteSystemPrecompiledHeader(Eq("ProjectPart3")));
+    EXPECT_CALL(mockSqliteTransactionBackend, commit());
+    EXPECT_CALL(mockSqliteTransactionBackend, unlock());
+
+    tasks.front()(mockPchCreator);
+}
+} // namespace
diff --git a/tests/unit/unittest/precompiledheaderstorage-test.cpp b/tests/unit/unittest/precompiledheaderstorage-test.cpp
index 3910b9a370..61fdfb5a88 100644
--- a/tests/unit/unittest/precompiledheaderstorage-test.cpp
+++ b/tests/unit/unittest/precompiledheaderstorage-test.cpp
@@ -29,6 +29,7 @@
 #include <precompiledheaderstorage.h>
 #include <refactoringdatabaseinitializer.h>
 #include <sqlitedatabase.h>
+#include <sqlitereadstatement.h>
 #include <sqlitewritestatement.h>
 #include <sqlitetransaction.h>
 
@@ -41,9 +42,17 @@ class PrecompiledHeaderStorage : public testing::Test
 protected:
     NiceMock<MockSqliteDatabase> database;
     Storage storage{database};
-    MockSqliteWriteStatement &insertPrecompiledHeaderStatement  = storage.m_insertPrecompiledHeaderStatement;
     MockSqliteWriteStatement &insertProjectPartStatement = storage.m_insertProjectPartStatement;
-    MockSqliteWriteStatement &deletePrecompiledHeaderStatement  = storage.m_deletePrecompiledHeaderStatement;
+    MockSqliteWriteStatement &insertProjectPrecompiledHeaderStatement
+        = storage.m_insertProjectPrecompiledHeaderStatement;
+    MockSqliteWriteStatement &deleteProjectPrecompiledHeaderStatement
+        = storage.m_deleteProjectPrecompiledHeaderStatement;
+    MockSqliteWriteStatement &insertSystemPrecompiledHeaderStatement
+        = storage.m_insertSystemPrecompiledHeaderStatement;
+    MockSqliteWriteStatement &deleteSystemPrecompiledHeaderStatement
+        = storage.m_deleteSystemPrecompiledHeaderStatement;
+    MockSqliteReadStatement &fetchSystemPrecompiledHeaderPathStatement
+        = storage.m_fetchSystemPrecompiledHeaderPathStatement;
 };
 
 TEST_F(PrecompiledHeaderStorage, UseTransaction)
@@ -56,76 +65,112 @@ TEST_F(PrecompiledHeaderStorage, UseTransaction)
     Storage storage{database};
 }
 
-TEST_F(PrecompiledHeaderStorage, InsertPrecompiledHeader)
+TEST_F(PrecompiledHeaderStorage, InsertProjectPrecompiledHeader)
 {
     InSequence s;
 
     EXPECT_CALL(database, immediateBegin());
     EXPECT_CALL(insertProjectPartStatement, write(TypedEq<Utils::SmallStringView>("project1")));
-    EXPECT_CALL(insertPrecompiledHeaderStatement,
+    EXPECT_CALL(insertProjectPrecompiledHeaderStatement,
                 write(TypedEq<Utils::SmallStringView>("project1"),
                       TypedEq<Utils::SmallStringView>("/path/to/pch"),
                       TypedEq<long long>(22)));
     EXPECT_CALL(database, commit());
 
-    storage.insertPrecompiledHeader("project1", "/path/to/pch", 22);
+    storage.insertProjectPrecompiledHeader("project1", "/path/to/pch", 22);
 }
 
-TEST_F(PrecompiledHeaderStorage, InsertPrecompiledHeaderStatementIsBusy)
+TEST_F(PrecompiledHeaderStorage, InsertProjectPrecompiledHeaderStatementIsBusy)
 {
     InSequence s;
 
     EXPECT_CALL(database, immediateBegin()).WillOnce(Throw(Sqlite::StatementIsBusy("busy")));
     EXPECT_CALL(database, immediateBegin());
     EXPECT_CALL(insertProjectPartStatement, write(TypedEq<Utils::SmallStringView>("project1")));
-    EXPECT_CALL(insertPrecompiledHeaderStatement,
+    EXPECT_CALL(insertProjectPrecompiledHeaderStatement,
                 write(TypedEq<Utils::SmallStringView>("project1"),
                       TypedEq<Utils::SmallStringView>("/path/to/pch"),
                       TypedEq<long long>(22)));
     EXPECT_CALL(database, commit());
 
-    storage.insertPrecompiledHeader("project1", "/path/to/pch", 22);
+    storage.insertProjectPrecompiledHeader("project1", "/path/to/pch", 22);
 }
 
-TEST_F(PrecompiledHeaderStorage, DeletePrecompiledHeader)
+TEST_F(PrecompiledHeaderStorage, DeleteProjectPrecompiledHeader)
 {
     InSequence s;
 
     EXPECT_CALL(database, immediateBegin());
-    EXPECT_CALL(deletePrecompiledHeaderStatement, write(TypedEq<Utils::SmallStringView>("project1")));
+    EXPECT_CALL(deleteProjectPrecompiledHeaderStatement, write(TypedEq<Utils::SmallStringView>("project1")));
     EXPECT_CALL(database, commit());
 
-    storage.deletePrecompiledHeader("project1");
+    storage.deleteProjectPrecompiledHeader("project1");
 }
 
-TEST_F(PrecompiledHeaderStorage, DeletePrecompiledHeaderStatementIsBusy)
+TEST_F(PrecompiledHeaderStorage, DeleteProjectPrecompiledHeaderStatementIsBusy)
 {
     InSequence s;
 
     EXPECT_CALL(database, immediateBegin()).WillOnce(Throw(Sqlite::StatementIsBusy("busy")));
     EXPECT_CALL(database, immediateBegin());
-    EXPECT_CALL(deletePrecompiledHeaderStatement, write(TypedEq<Utils::SmallStringView>("project1")));
+    EXPECT_CALL(deleteProjectPrecompiledHeaderStatement, write(TypedEq<Utils::SmallStringView>("project1")));
     EXPECT_CALL(database, commit());
 
-    storage.deletePrecompiledHeader("project1");
+    storage.deleteProjectPrecompiledHeader("project1");
 }
 
-TEST_F(PrecompiledHeaderStorage, InsertPrecompiledHeaderStatement)
+TEST_F(PrecompiledHeaderStorage, InsertSystemPrecompiledHeader)
 {
-    ASSERT_THAT(insertPrecompiledHeaderStatement.sqlStatement,
-                Eq("INSERT OR REPLACE INTO precompiledHeaders(projectPartId, pchPath, pchBuildTime) VALUES((SELECT projectPartId FROM projectParts WHERE projectPartName = ?),?,?)"));
+    InSequence s;
+
+    EXPECT_CALL(database, immediateBegin());
+    EXPECT_CALL(insertProjectPartStatement, write(TypedEq<Utils::SmallStringView>("project1")));
+    EXPECT_CALL(insertSystemPrecompiledHeaderStatement,
+                write(TypedEq<Utils::SmallStringView>("project1"),
+                      TypedEq<Utils::SmallStringView>("/path/to/pch"),
+                      TypedEq<long long>(22)));
+    EXPECT_CALL(database, commit());
+
+    storage.insertSystemPrecompiledHeader("project1", "/path/to/pch", 22);
 }
 
-TEST_F(PrecompiledHeaderStorage, InsertProjectPartStatement)
+TEST_F(PrecompiledHeaderStorage, InsertSystemPrecompiledHeaderStatementIsBusy)
 {
-    ASSERT_THAT(insertProjectPartStatement.sqlStatement,
-                Eq("INSERT OR IGNORE INTO projectParts(projectPartName) VALUES (?)"));
+    InSequence s;
+
+    EXPECT_CALL(database, immediateBegin()).WillOnce(Throw(Sqlite::StatementIsBusy("busy")));
+    EXPECT_CALL(database, immediateBegin());
+    EXPECT_CALL(insertProjectPartStatement, write(TypedEq<Utils::SmallStringView>("project1")));
+    EXPECT_CALL(insertSystemPrecompiledHeaderStatement,
+                write(TypedEq<Utils::SmallStringView>("project1"),
+                      TypedEq<Utils::SmallStringView>("/path/to/pch"),
+                      TypedEq<long long>(22)));
+    EXPECT_CALL(database, commit());
+
+    storage.insertSystemPrecompiledHeader("project1", "/path/to/pch", 22);
+}
+
+TEST_F(PrecompiledHeaderStorage, DeleteSystemPrecompiledHeader)
+{
+    InSequence s;
+
+    EXPECT_CALL(database, immediateBegin());
+    EXPECT_CALL(deleteSystemPrecompiledHeaderStatement, write(TypedEq<Utils::SmallStringView>("project1")));
+    EXPECT_CALL(database, commit());
+
+    storage.deleteSystemPrecompiledHeader("project1");
 }
 
-TEST_F(PrecompiledHeaderStorage, DeletePrecompiledHeaderStatement)
+TEST_F(PrecompiledHeaderStorage, DeleteSystemPrecompiledHeaderStatementIsBusy)
 {
-    ASSERT_THAT(deletePrecompiledHeaderStatement.sqlStatement,
-                Eq("DELETE FROM precompiledHeaders WHERE projectPartId = (SELECT projectPartId FROM projectParts WHERE projectPartName = ?)"));
+    InSequence s;
+
+    EXPECT_CALL(database, immediateBegin()).WillOnce(Throw(Sqlite::StatementIsBusy("busy")));
+    EXPECT_CALL(database, immediateBegin());
+    EXPECT_CALL(deleteSystemPrecompiledHeaderStatement, write(TypedEq<Utils::SmallStringView>("project1")));
+    EXPECT_CALL(database, commit());
+
+    storage.deleteSystemPrecompiledHeader("project1");
 }
 
 TEST_F(PrecompiledHeaderStorage, CompilePrecompiledHeaderStatements)
@@ -136,4 +181,48 @@ TEST_F(PrecompiledHeaderStorage, CompilePrecompiledHeaderStatements)
     ASSERT_NO_THROW(ClangBackEnd::PrecompiledHeaderStorage<>{database});
 }
 
+TEST_F(PrecompiledHeaderStorage, FetchSystemPrecompiledHeaderCalls)
+{
+    InSequence s;
+
+    EXPECT_CALL(database, deferredBegin());
+    EXPECT_CALL(fetchSystemPrecompiledHeaderPathStatement,
+                valueReturnPathString(TypedEq<Utils::SmallStringView>("project1")));
+    EXPECT_CALL(database, commit());
+
+    storage.fetchSystemPrecompiledHeaderPath("project1");
+}
+
+TEST_F(PrecompiledHeaderStorage, FetchSystemPrecompiledHeader)
+{
+    EXPECT_CALL(fetchSystemPrecompiledHeaderPathStatement,
+                valueReturnPathString(TypedEq<Utils::SmallStringView>("project1")))
+        .WillOnce(Return(Utils::PathString{"/path/to/pch"}));
+
+    auto path = storage.fetchSystemPrecompiledHeaderPath("project1");
+
+    ASSERT_THAT(path, "/path/to/pch");
+}
+
+TEST_F(PrecompiledHeaderStorage, FetchSystemPrecompiledHeaderReturnsEmptyPath)
+{
+    EXPECT_CALL(fetchSystemPrecompiledHeaderPathStatement,
+                valueReturnPathString(TypedEq<Utils::SmallStringView>("project1")))
+        .WillOnce(Return(Utils::PathString{}));
+
+    auto path = storage.fetchSystemPrecompiledHeaderPath("project1");
+
+    ASSERT_THAT(path, IsEmpty());
+}
+
+TEST_F(PrecompiledHeaderStorage, FetchSystemPrecompiledHeaderReturnsNullOptional)
+{
+    EXPECT_CALL(fetchSystemPrecompiledHeaderPathStatement,
+                valueReturnPathString(TypedEq<Utils::SmallStringView>("project1")))
+        .WillOnce(Return(Utils::optional<Utils::PathString>{}));
+
+    auto path = storage.fetchSystemPrecompiledHeaderPath("project1");
+
+    ASSERT_THAT(path, IsEmpty());
+}
 }
diff --git a/tests/unit/unittest/projectpartqueue-test.cpp b/tests/unit/unittest/projectpartqueue-test.cpp
index 0c06a48cd6..65318a3e91 100644
--- a/tests/unit/unittest/projectpartqueue-test.cpp
+++ b/tests/unit/unittest/projectpartqueue-test.cpp
@@ -35,6 +35,9 @@
 
 namespace {
 
+using ClangBackEnd::SlotUsage;
+using ClangBackEnd::V2::ProjectPartContainer;
+
 class ProjectPartQueue : public testing::Test
 {
 protected:
@@ -83,7 +86,7 @@ TEST_F(ProjectPartQueue, AddProjectPartCallsProcessEntries)
 {
     InSequence s;
 
-    EXPECT_CALL(mockTaskScheduler, freeSlots()).WillRepeatedly(Return(2));
+    EXPECT_CALL(mockTaskScheduler, slotUsage()).WillRepeatedly(Return(SlotUsage{2, 0}));
     EXPECT_CALL(mockTaskScheduler, addTasks(SizeIs(2)));
 
     queue.addProjectParts({projectPart1, projectPart2});
@@ -148,11 +151,11 @@ TEST_F(ProjectPartQueue, CreateTaskFromProjectPart)
     ClangBackEnd::ProjectPartPch projectPartPch{"project1", "/path/to/pch", 99};
     auto tasks = queue.createPchTasks({projectPart1});
 
-    EXPECT_CALL(mockPchCreator, generatePch(Eq(projectPart1)));
+    EXPECT_CALL(mockPchCreator, generatePchDeprecated(Eq(projectPart1)));
     EXPECT_CALL(mockPchCreator, projectPartPch()).WillOnce(ReturnRef(projectPartPch));
     EXPECT_CALL(mockSqliteTransactionBackend, lock());
     EXPECT_CALL(mockSqliteTransactionBackend, immediateBegin());
-    EXPECT_CALL(mockPrecompiledHeaderStorage, insertPrecompiledHeader(Eq("project1"), Eq("/path/to/pch"), 99));
+    EXPECT_CALL(mockPrecompiledHeaderStorage, insertProjectPrecompiledHeader(Eq("project1"), Eq("/path/to/pch"), 99));
     EXPECT_CALL(mockSqliteTransactionBackend, commit());
     EXPECT_CALL(mockSqliteTransactionBackend, unlock());
 
@@ -166,24 +169,15 @@ TEST_F(ProjectPartQueue, DeletePchEntryInDatabaseIfNoPchIsGenerated)
     ClangBackEnd::ProjectPartPch projectPartPch{"project1", "", 0};
     auto tasks = queue.createPchTasks({projectPart1});
 
-    EXPECT_CALL(mockPchCreator, generatePch(Eq(projectPart1)));
+    EXPECT_CALL(mockPchCreator, generatePchDeprecated(Eq(projectPart1)));
     EXPECT_CALL(mockPchCreator, projectPartPch()).WillOnce(ReturnRef(projectPartPch));
     EXPECT_CALL(mockSqliteTransactionBackend, lock());
     EXPECT_CALL(mockSqliteTransactionBackend, immediateBegin());
-    EXPECT_CALL(mockPrecompiledHeaderStorage, deletePrecompiledHeader(Eq("project1")));
+    EXPECT_CALL(mockPrecompiledHeaderStorage, deleteProjectPrecompiledHeader(Eq("project1")));
     EXPECT_CALL(mockSqliteTransactionBackend, commit());
     EXPECT_CALL(mockSqliteTransactionBackend, unlock());
 
     tasks.front()(mockPchCreator);
 }
 
-
-//TEST_F(PchManagerClient, ProjectPartPchRemovedFromDatabase)
-//{
-//    EXPECT_CALL(mockPrecompiledHeaderStorage, deletePrecompiledHeader(TypedEq<Utils::SmallStringView>(projectPartId)));
-
-//    projectUpdater.removeProjectParts({QString(projectPartId)});
-//}
-
-
 }
diff --git a/tests/unit/unittest/refactoringdatabaseinitializer-test.cpp b/tests/unit/unittest/refactoringdatabaseinitializer-test.cpp
index ba7d62ddb9..555d742a60 100644
--- a/tests/unit/unittest/refactoringdatabaseinitializer-test.cpp
+++ b/tests/unit/unittest/refactoringdatabaseinitializer-test.cpp
@@ -139,7 +139,7 @@ TEST_F(RefactoringDatabaseInitializer, AddPrecompiledHeaderTable)
 {
     InSequence s;
 
-    EXPECT_CALL(mockDatabase, execute(Eq("CREATE TABLE IF NOT EXISTS precompiledHeaders(projectPartId INTEGER PRIMARY KEY, pchPath TEXT, pchBuildTime INTEGER)")));
+    EXPECT_CALL(mockDatabase, execute(Eq("CREATE TABLE IF NOT EXISTS precompiledHeaders(projectPartId INTEGER PRIMARY KEY, projectPchPath TEXT, projectPchBuildTime INTEGER, systemPchPath TEXT, systemPchBuildTime INTEGER)")));
 
     initializer.createPrecompiledHeadersTable();
 }
@@ -171,7 +171,7 @@ TEST_F(RefactoringDatabaseInitializer, CreateInTheContructor)
     EXPECT_CALL(mockDatabase, execute(Eq("CREATE TABLE IF NOT EXISTS fileStatuses(sourceId INTEGER PRIMARY KEY, size INTEGER, lastModified INTEGER, buildDependencyTimeStamp INTEGER, isInPrecompiledHeader INTEGER)")));
     EXPECT_CALL(mockDatabase, execute(Eq("CREATE TABLE IF NOT EXISTS sourceDependencies(sourceId INTEGER, dependencySourceId INTEGER)")));
     EXPECT_CALL(mockDatabase, execute(Eq("CREATE INDEX IF NOT EXISTS index_sourceDependencies_sourceId_dependencySourceId ON sourceDependencies(sourceId, dependencySourceId)")));
-    EXPECT_CALL(mockDatabase, execute(Eq("CREATE TABLE IF NOT EXISTS precompiledHeaders(projectPartId INTEGER PRIMARY KEY, pchPath TEXT, pchBuildTime INTEGER)")));
+    EXPECT_CALL(mockDatabase, execute(Eq("CREATE TABLE IF NOT EXISTS precompiledHeaders(projectPartId INTEGER PRIMARY KEY, projectPchPath TEXT, projectPchBuildTime INTEGER, systemPchPath TEXT, systemPchBuildTime INTEGER)")));
     EXPECT_CALL(mockDatabase, commit());
     EXPECT_CALL(mockDatabase, setIsInitialized(true));
 
diff --git a/tests/unit/unittest/symbolindexertaskqueue-test.cpp b/tests/unit/unittest/symbolindexertaskqueue-test.cpp
index 83d722cbad..2932310eb0 100644
--- a/tests/unit/unittest/symbolindexertaskqueue-test.cpp
+++ b/tests/unit/unittest/symbolindexertaskqueue-test.cpp
@@ -35,6 +35,7 @@ using ClangBackEnd::FilePathId;
 using ClangBackEnd::SymbolsCollectorInterface;
 using ClangBackEnd::SymbolIndexerTask;
 using ClangBackEnd::SymbolStorageInterface;
+using ClangBackEnd::SlotUsage;
 
 using Callable = ClangBackEnd::SymbolIndexerTask::Callable;
 
@@ -167,7 +168,7 @@ TEST_F(SymbolIndexerTaskQueue, ProcessTasksCallsFreeSlotsAndAddTasksInScheduler)
                             {3, 1, Callable{}},
                             {5, 1, Callable{}}});
 
-    EXPECT_CALL(mockTaskScheduler, freeSlots()).WillRepeatedly(Return(2));
+    EXPECT_CALL(mockTaskScheduler, slotUsage()).WillRepeatedly(Return(SlotUsage{2, 0}));
     EXPECT_CALL(mockTaskScheduler, addTasks(SizeIs(2)));
 
     queue.processEntries();
@@ -177,7 +178,7 @@ TEST_F(SymbolIndexerTaskQueue, ProcessTasksCallsFreeSlotsAndAddTasksWithNoTaskIn
 {
     InSequence s;
 
-    EXPECT_CALL(mockTaskScheduler, freeSlots()).WillRepeatedly(Return(2));
+    EXPECT_CALL(mockTaskScheduler, slotUsage()).WillRepeatedly(Return(SlotUsage{2, 0}));
     EXPECT_CALL(mockTaskScheduler, addTasks(IsEmpty()));
 
     queue.processEntries();
@@ -190,7 +191,7 @@ TEST_F(SymbolIndexerTaskQueue, ProcessTasksCallsFreeSlotsAndMoveAllTasksInSchedu
                             {3, 1, Callable{}},
                             {5, 1, Callable{}}});
 
-    EXPECT_CALL(mockTaskScheduler, freeSlots()).WillRepeatedly(Return(4));
+    EXPECT_CALL(mockTaskScheduler, slotUsage()).WillRepeatedly(Return(SlotUsage{4, 0}));
     EXPECT_CALL(mockTaskScheduler, addTasks(SizeIs(3)));
 
     queue.processEntries();
@@ -201,7 +202,7 @@ TEST_F(SymbolIndexerTaskQueue, ProcessTasksRemovesProcessedTasks)
     queue.addOrUpdateTasks({{1, 1, Callable{}},
                             {3, 1, Callable{}},
                             {5, 1, Callable{}}});
-    ON_CALL(mockTaskScheduler, freeSlots()).WillByDefault(Return(2));
+    ON_CALL(mockTaskScheduler, slotUsage()).WillByDefault(Return(SlotUsage{2, 0}));
 
     queue.processEntries();
 
diff --git a/tests/unit/unittest/taskscheduler-test.cpp b/tests/unit/unittest/taskscheduler-test.cpp
index 337436aeb8..9afc52a724 100644
--- a/tests/unit/unittest/taskscheduler-test.cpp
+++ b/tests/unit/unittest/taskscheduler-test.cpp
@@ -40,6 +40,7 @@ using Task = std::function<void(ClangBackEnd::ProcessorInterface&)>;
 using ClangBackEnd::ProcessorInterface;
 using ClangBackEnd::SymbolsCollectorInterface;
 using ClangBackEnd::SymbolStorageInterface;
+using ClangBackEnd::SlotUsage;
 using NiceMockProcessorManager = NiceMock<MockProcessorManager>;
 using Scheduler = ClangBackEnd::TaskScheduler<NiceMockProcessorManager, Task>;
 
@@ -95,18 +96,18 @@ TEST_F(TaskScheduler, FreeSlots)
 {
     deferredScheduler.addTasks({nocall, nocall});
 
-    auto count = deferredScheduler.freeSlots();
+    auto slotUsage = deferredScheduler.slotUsage();
 
-    ASSERT_THAT(count, 2);
+    ASSERT_THAT(slotUsage, AllOf(Field(&SlotUsage::free, 2), Field(&SlotUsage::used, 2)));
 }
 
 TEST_F(TaskScheduler, ReturnZeroFreeSlotsIfMoreCallsThanCores)
 {
     deferredScheduler.addTasks({nocall, nocall, nocall, nocall, nocall, nocall});
 
-    auto count = deferredScheduler.freeSlots();
+    auto slotUsage = deferredScheduler.slotUsage();
 
-    ASSERT_THAT(count, 0);
+    ASSERT_THAT(slotUsage, AllOf(Field(&SlotUsage::free, 0), Field(&SlotUsage::used, 6)));
 }
 
 TEST_F(TaskScheduler, FreeSlotsAfterFinishing)
@@ -114,9 +115,9 @@ TEST_F(TaskScheduler, FreeSlotsAfterFinishing)
     scheduler.addTasks({nocall, nocall});
     scheduler.syncTasks();
 
-    auto count = scheduler.freeSlots();
+    auto slotUsage = scheduler.slotUsage();
 
-    ASSERT_THAT(count, 4);
+    ASSERT_THAT(slotUsage, AllOf(Field(&SlotUsage::free, 4), Field(&SlotUsage::used, 0)));
 }
 
 TEST_F(TaskScheduler, NoFuturesAfterFreeSlots)
@@ -124,7 +125,7 @@ TEST_F(TaskScheduler, NoFuturesAfterFreeSlots)
     scheduler.addTasks({nocall, nocall});
     scheduler.syncTasks();
 
-    scheduler.freeSlots();
+    scheduler.slotUsage();
 
     ASSERT_THAT(scheduler.futures(), IsEmpty());
 }
@@ -142,7 +143,7 @@ TEST_F(TaskScheduler, FreeSlotsCallsCleanupMethodsAfterTheWorkIsDone)
     EXPECT_CALL(mockSymbolsCollector, setIsUsed(false));
     EXPECT_CALL(mockSymbolsCollector, clear());
 
-    scheduler.freeSlots();
+    scheduler.slotUsage();
 }
 
 TEST_F(TaskScheduler, FreeSlotsCallsProgressMethodsAfterTheWorkIsDone)
@@ -153,7 +154,7 @@ TEST_F(TaskScheduler, FreeSlotsCallsProgressMethodsAfterTheWorkIsDone)
 
     EXPECT_CALL(mockSetProgressCallback, Call(2, 100));
 
-    scheduler.freeSlots();
+    scheduler.slotUsage();
 }
 
 TEST_F(TaskScheduler, AddTaskCallSymbolsCollectorManagerUnusedSymbolsCollector)
diff --git a/tests/unit/unittest/unittest.pro b/tests/unit/unittest/unittest.pro
index 043c7ff0b2..969a8bc6df 100644
--- a/tests/unit/unittest/unittest.pro
+++ b/tests/unit/unittest/unittest.pro
@@ -109,7 +109,8 @@ SOURCES += \
     builddependenciesprovider-test.cpp \
     builddependenciesstorage-test.cpp \
     usedmacrofilter-test.cpp \
-    pchtasksmerger-test.cpp
+    pchtasksmerger-test.cpp \
+    pchtaskqueue-test.cpp
 
 !isEmpty(LIBCLANG_LIBS) {
 SOURCES += \