macOS: Detect use of heap-allocated QMacAutoReleasePool

QMacAutoReleasePool is backed by an NSAutoreleasePool, which documents that
"you should always drain an autorelease pool in the same context (invocation
of a method or function, or body of a loop) that it was created".

This means allocating QMacAutoReleasePool on the heap is not a supported
use-case, but unfortunately we can't detect it on construction time.

Instead we detect whether or not the associated NSAutoreleasePool has been
drained, and prevent a double-drain of the pool.

Change-Id: Ifd7380a06152e9e742d2e199476ed3adab326d9c
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

2 files changed, 69 insertions, 1 deletions

diff --git a/src/corelib/kernel/qcore_mac_objc.mm b/src/corelib/kernel/qcore_mac_objc.mm
index db7e55f4b1..24d73fa8be 100644
--- a/src/corelib/kernel/qcore_mac_objc.mm
+++ b/src/corelib/kernel/qcore_mac_objc.mm
@@ -84,18 +84,82 @@ QT_FOR_EACH_MUTABLE_CORE_GRAPHICS_TYPE(QT_DECLARE_WEAK_QDEBUG_OPERATOR_FOR_CF_TY
 
 // -------------------------------------------------------------------------
 
+QT_END_NAMESPACE
+QT_USE_NAMESPACE
+@interface QT_MANGLE_NAMESPACE(QMacAutoReleasePoolTracker) : NSObject
+{
+    NSAutoreleasePool **m_pool;
+}
+-(id)initWithPool:(NSAutoreleasePool**)pool;
+@end
+@implementation QT_MANGLE_NAMESPACE(QMacAutoReleasePoolTracker)
+-(id)initWithPool:(NSAutoreleasePool**)pool
+{
+    if (self = [super init])
+        m_pool = pool;
+    return self;
+}
+-(void)dealloc
+{
+    if (*m_pool) {
+        // The pool is still valid, which means we're not being drained from
+        // the corresponding QMacAutoReleasePool (see below).
+
+        // QMacAutoReleasePool has only a single member, the NSAutoreleasePool*
+        // so the address of that member is also the QMacAutoReleasePool itself.
+        QMacAutoReleasePool *pool = reinterpret_cast<QMacAutoReleasePool *>(m_pool);
+        qWarning() << "Premature drain of" << pool << "This can happen if you've allocated"
+            << "the pool on the heap, or as a member of a heap-allocated object. This is not a"
+            << "supported use of QMacAutoReleasePool, and might result in crashes when objects"
+            << "in the pool are deallocated and then used later on under the assumption they"
+            << "will be valid until" << pool << "has been drained.";
+
+        // Reset the pool so that it's not drained again later on
+        *m_pool = nullptr;
+    }
+
+    [super dealloc];
+}
+@end
+QT_NAMESPACE_ALIAS_OBJC_CLASS(QMacAutoReleasePoolTracker);
+QT_BEGIN_NAMESPACE
+
 QMacAutoReleasePool::QMacAutoReleasePool()
     : pool([[NSAutoreleasePool alloc] init])
 {
+    [[[QMacAutoReleasePoolTracker alloc] initWithPool:
+        reinterpret_cast<NSAutoreleasePool **>(&pool)] autorelease];
 }
 
 QMacAutoReleasePool::~QMacAutoReleasePool()
 {
+    if (!pool) {
+        qWarning() << "Prematurely drained pool" << this << "finally drained. Any objects belonging"
+            << "to this pool have already been released, and have potentially been invalid since the"
+            << "premature drain earlier on.";
+        return;
+    }
+
+    // Save and reset pool before draining, so that the pool tracker can know
+    // that it's being drained by its owning pool.
+    NSAutoreleasePool *savedPool = static_cast<NSAutoreleasePool*>(pool);
+    pool = nullptr;
+
     // Drain behaves the same as release, with the advantage that
     // if we're ever used in a garbage-collected environment, the
     // drain acts as a hint to the garbage collector to collect.
-    [static_cast<NSAutoreleasePool*>(pool) drain];
+    [savedPool drain];
+}
+
+#ifndef QT_NO_DEBUG_STREAM
+QDebug operator<<(QDebug debug, const QMacAutoReleasePool *pool)
+{
+    QDebugStateSaver saver(debug);
+    debug.nospace();
+    debug << "QMacAutoReleasePool(" << (const void *)pool << ')';
+    return debug;
 }
+#endif // !QT_NO_DEBUG_STREAM
 
 #ifdef Q_OS_MACOS
 /*
diff --git a/src/corelib/kernel/qcore_mac_p.h b/src/corelib/kernel/qcore_mac_p.h
index 5522a617b3..13143a08bb 100644
--- a/src/corelib/kernel/qcore_mac_p.h
+++ b/src/corelib/kernel/qcore_mac_p.h
@@ -153,6 +153,10 @@ Q_CORE_EXPORT QChar qt_mac_qtKey2CocoaKey(Qt::Key key);
 Q_CORE_EXPORT Qt::Key qt_mac_cocoaKey2QtKey(QChar keyCode);
 #endif
 
+#ifndef QT_NO_DEBUG_STREAM
+QDebug operator<<(QDebug debug, const QMacAutoReleasePool *pool);
+#endif
+
 Q_CORE_EXPORT void qt_apple_check_os_version();
 
 QT_END_NAMESPACE