[core] Implement an eviction policy for OfflineDatabase

When inserting an cached resource, or removing a region, remove least-recently used resources and tiles, not used by offline regions, until the used database size, as calculated by multiplying the number of in-use pages by the page size, is less than the maximum cache size minus 5 times the page size.

In addition, OfflineDatabase may be configured to ignore cache puts of individual resources larger than a certain size.

This policy is similar but not identical to the former SQLiteCache policy:

* It accounts for offline, by exempting resources required by offline regions from eviction.
* It must delete from two tables (resources and tiles), rather than one. Currently the strategy is naive: evict 50 rows at a time from each table.
* It makes maximumCacheSize and maximumCacheEntrySize completely independent. The SQLiteCache implementation evicted when `usedSize > maximumCacheSize - 2 * maximumCacheEntrySize`. This evicts when `usedSize > maximumCacheSize - 5 * pageSize`.
* It uses a non-unlimited default value for maximumCacheSize: 50 MB. We should have always had a limit in place; "a cache without an eviction policy is a resource leak".

2 files changed, 12 insertions, 20 deletions

diff --git a/include/mbgl/storage/default_file_source.hpp b/include/mbgl/storage/default_file_source.hpp
index e7ffe125b1..0cee7de328 100644
--- a/include/mbgl/storage/default_file_source.hpp
+++ b/include/mbgl/storage/default_file_source.hpp
@@ -3,6 +3,7 @@
 
 #include <mbgl/storage/file_source.hpp>
 #include <mbgl/storage/offline.hpp>
+#include <mbgl/util/constants.hpp>
 
 #include <vector>
 
@@ -14,15 +15,15 @@ template <typename T> class Thread;
 
 class DefaultFileSource : public FileSource {
 public:
-    DefaultFileSource(const std::string& cachePath, const std::string& assetRoot);
+    DefaultFileSource(const std::string& cachePath,
+                      const std::string& assetRoot,
+                      uint64_t maximumCacheSize      = util::DEFAULT_MAX_CACHE_SIZE,
+                      uint64_t maximumCacheEntrySize = util::DEFAULT_MAX_CACHE_ENTRY_SIZE);
     ~DefaultFileSource() override;
 
     void setAccessToken(const std::string&);
     std::string getAccessToken() const;
 
-    void setMaximumCacheSize(uint64_t size);
-    void setMaximumCacheEntrySize(uint64_t size);
-
     std::unique_ptr<FileRequest> request(const Resource&, Callback) override;
 
     /*
@@ -71,13 +72,11 @@ public:
                                                                     optional<OfflineRegionStatus>)>) const;
 
     /*
-     * Initiate the removal of offline region from the database.
+     * Remove an offline region from the database and perform any resources evictions
+     * necessary as a result.
      *
-     * All resources required by the region, but not also required by other regions, will
-     * become eligible for removal for space-optimization. Because the offline database is
-     * also used for ambient usage-based caching, and offline resources may still be useful
-     * for ambient usage, they are not immediately removed. To immediately remove resources
-     * not used by any extant region, call removeUnusedOfflineResources().
+     * Eviction works by removing the least-recently requested resources not also required
+     * by other regions, until the database shrinks below a certain size.
      *
      * Note that this method takes ownership of the input, reflecting the fact that once
      * region deletion is initiated, it is not legal to perform further actions with the
@@ -89,16 +88,6 @@ public:
      */
     void deleteOfflineRegion(OfflineRegion&&, std::function<void (std::exception_ptr)>);
 
-    /*
-     * Remove all resources in the database that are not required by any region, thus
-     * optimizing the disk space used by the offline database.
-     *
-     * When the operation is complete or encounters an error, the given callback will be
-     * executed on the database thread; it is the responsibility of the SDK bindings
-     * to re-execute a user-provided callback on the main thread.
-     */
-    void removeUnusedOfflineResources(std::function<void (std::exception_ptr)>);
-
     // For testing only.
     void put(const Resource&, const Response&);
     void goOffline();
diff --git a/include/mbgl/util/constants.hpp b/include/mbgl/util/constants.hpp
index 24f4b5ee72..667e2f8497 100644
--- a/include/mbgl/util/constants.hpp
+++ b/include/mbgl/util/constants.hpp
@@ -21,6 +21,9 @@ extern const double PITCH_MAX;
 extern const double MIN_ZOOM;
 extern const double MAX_ZOOM;
 
+extern const uint64_t DEFAULT_MAX_CACHE_SIZE;
+extern const uint64_t DEFAULT_MAX_CACHE_ENTRY_SIZE;
+
 } // namespace util
 
 namespace debug {