Inspect lock file to determine busy state.

QSPI was relying on QLockFile.isLocked() to determine if the port
was in use. This was due to a misunderstanding of the QLockFile
class design.

Ideally the code in QSPI.isBusy() should be moved into a new method,
QLockFile.available(), since it contains implementation details of
that class.

Note that isBusy() is for information purposes only, since checking
for a lock without obtaining it is always racy.

Change-Id: I675bd4033e4228e6bf864bf2e7a00a2520ccc50e
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>
Reviewed-by: David Faure <david.faure@kdab.com>
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>

2 files changed, 24 insertions, 12 deletions

diff --git a/src/serialport/qserialport_unix.cpp b/src/serialport/qserialport_unix.cpp
index a00d0db..1ba23dd 100644
--- a/src/serialport/qserialport_unix.cpp
+++ b/src/serialport/qserialport_unix.cpp
@@ -199,9 +199,8 @@ bool QSerialPortPrivate::open(QIODevice::OpenMode mode)
     }
 
     QScopedPointer<QLockFile> newLockFileScopedPointer(new QLockFile(lockFilePath));
-    lockFileScopedPointer.swap(newLockFileScopedPointer);
 
-    if (lockFileScopedPointer->isLocked()) {
+    if (!newLockFileScopedPointer->tryLock()) {
         q->setError(QSerialPort::PermissionError);
         return false;
     }
@@ -227,12 +226,6 @@ bool QSerialPortPrivate::open(QIODevice::OpenMode mode)
         return false;
     }
 
-    lockFileScopedPointer->lock();
-    if (!lockFileScopedPointer->isLocked()) {
-        q->setError(QSerialPort::PermissionError);
-        return false;
-    }
-
 #ifdef TIOCEXCL
     if (::ioctl(descriptor, TIOCEXCL) == -1)
         q->setError(decodeSystemError());
@@ -268,6 +261,8 @@ bool QSerialPortPrivate::open(QIODevice::OpenMode mode)
     if ((flags & O_WRONLY) == 0)
         setReadNotificationEnabled(true);
 
+    lockFileScopedPointer.swap(newLockFileScopedPointer);
+
     return true;
 }
 
@@ -306,8 +301,7 @@ void QSerialPortPrivate::close()
     if (qt_safe_close(descriptor) == -1)
         q->setError(decodeSystemError());
 
-    if (lockFileScopedPointer->isLocked())
-        lockFileScopedPointer->unlock();
+    lockFileScopedPointer.reset(0);
 
     descriptor = -1;
     pendingBytesWritten = 0;
diff --git a/src/serialport/qserialportinfo_unix.cpp b/src/serialport/qserialportinfo_unix.cpp
index 3440ad2..783105e 100644
--- a/src/serialport/qserialportinfo_unix.cpp
+++ b/src/serialport/qserialportinfo_unix.cpp
@@ -50,6 +50,11 @@
 #include <QtCore/qdir.h>
 #include <QtCore/qscopedpointer.h>
 
+
+#include <errno.h>
+#include <sys/types.h> // kill
+#include <signal.h>    // kill
+
 #ifndef Q_OS_MAC
 
 #include "qtudev_p.h"
@@ -344,8 +349,21 @@ bool QSerialPortInfo::isBusy() const
     if (lockFilePath.isEmpty())
         return false;
 
-    QLockFile lockFile(lockFilePath);
-    return lockFile.isLocked();
+    QFile reader(lockFilePath);
+    if (!reader.open(QIODevice::ReadOnly))
+        return false;
+
+    QByteArray pidLine = reader.readLine();
+    pidLine.chop(1);
+    if (pidLine.isEmpty())
+        return false;
+
+    qint64 pid = pidLine.toLongLong();
+
+    if (pid && (::kill(pid, 0) == -1) && (errno == ESRCH))
+        return false; // PID doesn't exist anymore
+
+    return true;
 }
 
 bool QSerialPortInfo::isValid() const