Fix crash when re-creating QThreadData after initially destroying it

We destroy the thread data for the main thread when the QCoreApplication
is destructed, and then delete the pthread key for the thread data in
the global static destructor function 'destroy_current_thread_data_key'.

The user may have its own Q_DESTRUCTOR_FUNCTION though, which may or may
not run after we've destroyed the key. If it runs after we've destroyed
the key, we'll end up trying to re-create the tread-data, as expected,
but set_thread_data() will fail to persist it, as pthread_setspecific
is called with an invalid key. The result is an infinite recursion:

...
6  in QThreadData::current () at qthread_unix.cpp:216
7  in QObject::QObject (this=0x48e1b30, dd=@0x48e1b40, parent=0x0) at qobject.cpp:703
8  in QThread::QThread (this=0x48e1b30, dd=@0x48e1b40, parent=0x0) at qthread.cpp:396
9  in QAdoptedThread::QAdoptedThread (this=0x48e1b30, data=0x48e1af0) at qthread.cpp:120
10 in QAdoptedThread::QAdoptedThread (this=0x48e1b30, data=0x48e1af0) at qthread.cpp:130
11 in QThreadData::current () at qthread_unix.cpp:219
12 in QObject::QObject (this=0x48e1a20, dd=@0x48e1a30, parent=0x0) at qobject.cpp:703
...

To solve this, we reset current_thread_data_once when destroying the key,
so that subsequent calls to pthread_once to potentially create the key
will call create_current_thread_data_key once more. This means we'll leak
the key for this particular use-case, since we don't end up calling
pthread_key_delete a second time, but this leak is small and happens
typically only for a short duration during application shutdown.

Change-Id: I580484a3239849e891172e24e7f77b75afd2c51b
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/src/corelib/thread/qthread_unix.cpp b/src/corelib/thread/qthread_unix.cpp
index f123e1813b..be9b946990 100644
--- a/src/corelib/thread/qthread_unix.cpp
+++ b/src/corelib/thread/qthread_unix.cpp
@@ -172,6 +172,12 @@ static void destroy_current_thread_data_key()
 {
     pthread_once(&current_thread_data_once, create_current_thread_data_key);
     pthread_key_delete(current_thread_data_key);
+
+    // Reset current_thread_data_once in case we end up recreating
+    // the thread-data in the rare case of QObject construction
+    // after destroying the QThreadData.
+    pthread_once_t pthread_once_init = PTHREAD_ONCE_INIT;
+    current_thread_data_once = pthread_once_init;
 }
 Q_DESTRUCTOR_FUNCTION(destroy_current_thread_data_key)