qmake: fix file id mapping lifetime management

turns out that flushing the ids together with the ProFile cache was an
abysmal idea, as the latter expires after a few seconds after loading
the project, while references to the ProFiles (and thus the underlying
file ids) are kept for as long as the project stays loaded. the early
flush would cause re-use of the file ids, which would wreak all kinds of
havoc when re-loading projects.

instead, ref-count the vfs class based on its instances.

amends 60245e55d7.

Task-number: QTCREATORBUG-20113
Change-Id: I7382a71af4d6a991156c05719deae9fb1e4fb278
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/qtsupport/profilereader.cpp

@@ -166,7 +166,6 @@ void ProFileCacheManager::clear()
     // loop is concerned. Use a shared pointer once this is not true anymore.
     delete m_cache;
     m_cache = 0;
-    QMakeVfs::clearIds();
 }
 
 void ProFileCacheManager::discardFiles(const QString &prefix, QMakeVfs *vfs)

src/shared/proparser/qmakevfs.cpp

@@ -49,11 +49,29 @@ QMakeVfs::QMakeVfs()
 #ifndef QT_NO_TEXTCODEC
     m_textCodec = 0;
 #endif
+#ifdef PROEVALUATOR_THREAD_SAFE
+    QMutexLocker locker(&s_mutex);
+#endif
+    ++s_refCount;
 }
 
+QMakeVfs::~QMakeVfs()
+{
+#ifdef PROEVALUATOR_THREAD_SAFE
+    QMutexLocker locker(&s_mutex);
+#endif
+    if (!--s_refCount) {
+        s_fileIdCounter = 0;
+        s_fileIdMap.clear();
+        s_idFileMap.clear();
+    }
+}
+
+
 #ifdef PROPARSER_THREAD_SAFE
 QMutex QMakeVfs::s_mutex;
 #endif
+int QMakeVfs::s_refCount;
 QAtomicInt QMakeVfs::s_fileIdCounter;
 QHash<QString, int> QMakeVfs::s_fileIdMap;
 QHash<int, QString> QMakeVfs::s_idFileMap;
@@ -111,16 +129,6 @@ QString QMakeVfs::fileNameForId(int id)
     return s_idFileMap.value(id);
 }
 
-void QMakeVfs::clearIds()
-{
-#ifdef PROEVALUATOR_THREAD_SAFE
-    QMutexLocker locker(&s_mutex);
-#endif
-    s_fileIdCounter = 0;
-    s_fileIdMap.clear();
-    s_idFileMap.clear();
-}
-
 bool QMakeVfs::writeFile(int id, QIODevice::OpenMode mode, VfsFlags flags,
                          const QString &contents, QString *errStr)
 {

src/shared/proparser/qmakevfs.h

@@ -72,10 +72,10 @@ public:
     Q_DECLARE_FLAGS(VfsFlags, VfsFlag)
 
     QMakeVfs();
+    ~QMakeVfs();
 
     int idForFileName(const QString &fn, VfsFlags flags);
     QString fileNameForId(int id);
-    static void clearIds();
     bool writeFile(int id, QIODevice::OpenMode mode, VfsFlags flags, const QString &contents, QString *errStr);
     ReadResult readFile(int id, QString *contents, QString *errStr);
     bool exists(const QString &fn, QMakeVfs::VfsFlags flags);
@@ -93,6 +93,7 @@ private:
 #ifdef PROEVALUATOR_THREAD_SAFE
     static QMutex s_mutex;
 #endif
+    static int s_refCount;
     static QAtomicInt s_fileIdCounter;
     // Qt Creator's ProFile cache is a singleton to maximize its cross-project
     // effectiveness (shared prf files from QtVersions).