[vlc-devel] [PATCH] darwin/thread: Ignore vlc_mutex_unlock failures
epirat07 at gmail.com
Mon Oct 28 18:24:44 CET 2019
macOS pthread implementation for pthread_cond_wait is buggy, causing
sometimes the mutex to not be locked when running thread cancellation
This causes random failures of vlc_mutex_unlock, as the cancellation
cleanup handler does not hold a lock on the mutex in rare cases.
src/darwin/thread.c | 27 ++++++++++++++++++++++++++-
1 file changed, 26 insertions(+), 1 deletion(-)
diff --git a/src/darwin/thread.c b/src/darwin/thread.c
index 9dcffad28d..10bcae9e28 100644
@@ -172,7 +172,32 @@ int vlc_mutex_trylock (vlc_mutex_t *p_mutex)
void vlc_mutex_unlock (vlc_mutex_t *p_mutex)
int val = pthread_mutex_unlock( p_mutex );
- VLC_THREAD_ASSERT ("unlocking mutex");
+ /* FIXME: We can't check for the success of the unlock
+ * here as due to a bug in Apples pthread implementation.
+ * The `pthread_cond_wait` function does not behave like
+ * it should According to POSIX, pthread_cond_wait is a
+ * cancellation point and when a thread is cancelled while
+ * in a condition wait, the mutex is re-acquired before
+ * calling the first cancellation cleanup handler:
+ * > The effect is as if the thread were unblocked, allowed
+ * > to execute up to the point of returning from the call to
+ * > pthread_cond_timedwait() or pthread_cond_wait(), but at
+ * > that point notices the cancellation request and instead
+ * > of returning to the caller of pthread_cond_timedwait()
+ * > or pthread_cond_wait(), starts the thread cancellation
+ * > activities, which includes calling cancellation cleanup
+ * > handlers.
+ * Unfortunately the mutex is not locked sometimes, causing
+ * the call to `pthread_mutex_unlock` to fail.
+ * Until this is fixed, enabling this assertion would lead to
+ * spurious test failures and VLC crashes when compiling with
+ * debug enabled, which would make it nearly impossible to
+ * proeprly test with debug builds on macOS.
+ * This was reported to Apple as FB6152751.
+ // VLC_THREAD_ASSERT ("unlocking mutex");
2.21.0 (Apple Git-122)
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