QLogging: Avoid infinite loop in error case

If the user code installs a message handler that uses
any API that prints logs (handled by qt_message_print)
then this will lead to an infinite loop.

This patch adds a thread local storage to check
if the message handler is done before the thread
is calling it again.

Note: This check is performed only if the compiler
supports thread_local (__declspec(thread) for MSVC).

Change-Id: I43b0460b8f39b26a18de48a5652a5e55f0b336f8
Reviewed-by: Rafael Roquetto <rafael.roquetto@kdab.com>

1 files changed, 38 insertions, 5 deletions

diff --git a/src/corelib/global/qlogging.cpp b/src/corelib/global/qlogging.cpp
index 5b7a674ddb..cff8846cdc 100644
--- a/src/corelib/global/qlogging.cpp
+++ b/src/corelib/global/qlogging.cpp
@@ -925,6 +925,32 @@ static void qDefaultMsgHandler(QtMsgType type, const char *buf)
     qDefaultMessageHandler(type, emptyContext, QString::fromLocal8Bit(buf));
 }
 
+#if defined(Q_COMPILER_THREAD_LOCAL) || (defined(Q_CC_MSVC) && !defined(Q_OS_WINCE))
+#if defined(Q_CC_MSVC)
+static __declspec(thread) bool msgHandlerGrabbed = false;
+#else
+static thread_local bool msgHandlerGrabbed = false;
+#endif
+
+static bool grabMessageHandler()
+{
+    if (msgHandlerGrabbed)
+        return false;
+
+    msgHandlerGrabbed = true;
+    return true;
+}
+
+static void ungrabMessageHandler()
+{
+    msgHandlerGrabbed = false;
+}
+
+#else
+static bool grabMessageHandler() { return true; }
+static void ungrabMessageHandler() { }
+#endif // (Q_COMPILER_THREAD_LOCAL) || ((Q_CC_MSVC) && !(Q_OS_WINCE))
+
 static void qt_message_print(QtMsgType msgType, const QMessageLogContext &context, const QString &message)
 {
 #ifndef QT_BOOTSTRAPPED
@@ -942,12 +968,19 @@ static void qt_message_print(QtMsgType msgType, const QMessageLogContext &contex
     if (!messageHandler)
         messageHandler = qDefaultMessageHandler;
 
-    // prefer new message handler over the old one
-    if (msgHandler == qDefaultMsgHandler
-            || messageHandler != qDefaultMessageHandler) {
-        (*messageHandler)(msgType, context, message);
+    // prevent recursion in case the message handler generates messages
+    // itself, e.g. by using Qt API
+    if (grabMessageHandler()) {
+        // prefer new message handler over the old one
+        if (msgHandler == qDefaultMsgHandler
+                || messageHandler != qDefaultMessageHandler) {
+            (*messageHandler)(msgType, context, message);
+        } else {
+            (*msgHandler)(msgType, message.toLocal8Bit().constData());
+        }
+        ungrabMessageHandler();
     } else {
-        (*msgHandler)(msgType, message.toLocal8Bit().constData());
+        fprintf(stderr, "%s", message.toLocal8Bit().constData());
     }
 }