QObject: use per-thread storage for qFlagLocation()

qFlagLocation() uses a global char* array to transport source location
information from the connect() side to the metaobject side. The size
of the array is 2 (two), which just about suffices for a single connect()
statement.

Obviously, if more than one thread makes a (_any_) connection at the same
time, the data is useless and, worse, there's a data race.

The non-reentrancy of qFlagLocations() cannot and need not be fixed,
but use a per-thread flagged_locations array in QThreadData so threads
don't disturb each other.

Task-number: QTBUG-3680

Change-Id: If1797c60751f551694def69afee6fbe295bbe2d2
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

src/corelib/kernel/qobject.cpp

@@ -2029,14 +2029,9 @@ void QObject::deleteLater()
  *****************************************************************************/
 
 
-const int flagged_locations_count = 2;
-static const char* flagged_locations[flagged_locations_count] = {0};
-
 const char *qFlagLocation(const char *method)
 {
-    static int idx = 0;
-    flagged_locations[idx] = method;
-    idx = (idx+1) % flagged_locations_count;
+    QThreadData::current()->flaggedSignatures.store(method);
     return method;
 }
 
@@ -2048,14 +2043,11 @@ static int extract_code(const char *member)
 
 static const char * extract_location(const char *member)
 {
-    for (int i = 0; i < flagged_locations_count; ++i) {
-        if (member == flagged_locations[i]) {
-            // signature includes location information after the first null-terminator
-            const char *location = member + qstrlen(member) + 1;
-            if (*location != '\0')
-                return location;
-            return 0;
-        }
+    if (QThreadData::current()->flaggedSignatures.contains(member)) {
+        // signature includes location information after the first null-terminator
+        const char *location = member + qstrlen(member) + 1;
+        if (*location != '\0')
+            return location;
     }
     return 0;
 }

src/corelib/thread/qthread_p.h

@@ -242,6 +242,26 @@ public:
         return canWait;
     }
 
+    // This class provides per-thread (by way of being a QThreadData
+    // member) storage for qFlagLocation()
+    class FlaggedDebugSignatures
+    {
+        static const uint Count = 2;
+
+        uint idx;
+        const char* locations[Count];
+
+    public:
+        FlaggedDebugSignatures() : idx(0)
+        { std::fill_n(locations, Count, static_cast<char*>(0)); }
+
+        void store(const char* method)
+        { locations[idx++ % Count] = method; }
+
+        bool contains(const char *method) const
+        { return std::find(locations, locations + Count, method) != locations + Count; }
+    };
+
     QThread *thread;
     Qt::HANDLE threadId;
     bool quitNow;
@@ -252,6 +272,7 @@ public:
     bool canWait;
     QVector<void *> tls;
     bool isAdopted;
+    FlaggedDebugSignatures flaggedSignatures;
 };
 
 class QScopedLoopLevelCounter