Fix usage of condition variable in TQThread which could lead to a thread being run multiple times in parallel.

Although highly unlikely, the following situation was possible:
- a thread is restarted while still running. The call to the second 'start()' blocks because the thread is still running
- the OS decides to randomly wake up the thread that called 'start()'
- a second thread is started and runs in parallel to the first thread, with obvious undefined behavior

Signed-off-by: Michele Calgaro <michele.calgaro@yahoo.it>
(cherry picked from commit 3143d2d40ab5af1053596cd9b46c7034f22c4624)

1 files changed, 5 insertions, 3 deletions

diff --git a/src/kernel/qthread_unix.cpp b/src/kernel/qthread_unix.cpp
index b68b5cd7a..723ca8bcf 100644
--- a/src/kernel/qthread_unix.cpp
+++ b/src/kernel/qthread_unix.cpp
@@ -107,7 +107,7 @@ void TQThreadInstance::init(unsigned int stackSize)
     orphan = FALSE;
     disableThreadPostedEvents = FALSE;
 
-    pthread_cond_init(&thread_done, NULL);
+    pthread_cond_init(&thread_done, nullptr);
     thread_id = 0;
 
     eventLoop = 0;
@@ -326,9 +326,11 @@ void TQThread::usleep( unsigned long usecs )
 void TQThread::start(Priority priority)
 {
     TQMutexLocker locker( d->mutex() );
+    while ( d->running )
+    {
+	pthread_cond_wait(&d->thread_done, &locker.mutex()->d->handle);
+    }
 
-    if ( d->running )
-        pthread_cond_wait(&d->thread_done, &locker.mutex()->d->handle);
     d->running = TRUE;
     d->finished = FALSE;