Changeset 99448 in vbox

Timestamp:

Apr 19, 2023 12:49:35 AM (20 months ago)

Author:

vboxsync

Message:

FE/Qt: Explanation of the QPointer heap issue when using a debug Qt build on windows.

File:

• trunk/src/VBox/Frontends/VirtualBox/src/globals/UIMessageCenter.cpp (modified) (1 diff)

trunk/src/VBox/Frontends/VirtualBox/src/globals/UIMessageCenter.cpp

@@ -2371 +2371 @@
 #if 1 /* With a debug Qt 5.15.2 (r175) build and paged heap on windows, the ~QPointer destructor will trigger heap corruption
          (VERIFIER STOP 0000000000000010: pid 0x253C: corrupted start stamp).  Clearing the QPointer prior to deletion works
-         around the issue (calling clear() after delete just triggers the problem a few lines earlier). */
+         around the issue (calling clear() after delete just triggers the problem a few lines earlier).
+
+         Update: The issue is using different allocator and free heap routines.   When using a debug Qt build on windows,
+                 it is linked against the debug runtime DLL (ucrtbased.dll), while this code is still using the release
+                 runtime DLL (ucrtbase.dll).  The QtSharedPointer::ExternalRefCountData instance is allocated by
+                 Qt5CoreVBoxd!QtSharedPointer::ExternalRefCountData::getAndRef() using the debug heap allocator, which adds
+                 an additional 48 byte header on the object.  When we delete and the clear/destroy the QPointer instance
+                 here, we'll be doing the freeing, but since we don't link with ucrtbased.dll, we'll use the regular
+                 heap routines that doesn't expect a 48 byte header and they probably think we're handing them a garbage
+                 pointer.
+
+                 The workaround here, moves the deleting of the QtSharedPointer::ExternalRefCountData structure from
+                 this module to the QObject destructor, which lives in the same module as where it was allocated and
+                 will be using the same allocator.  So, either we subclass the QPointer<> template and formalize this
+                 pattern a bit better, so a workaround can be done in one place, or we do some build time / makefile
+                 checks for this and forces the use of the debug CRT for the whole of VBox as well (not all that tempting)... */
     QIMessageBox *pSafe = pMessageBox;
     pMessageBox.clear();