BUG#30992 (Wrong implementation of pthread_mutex_trylock()):

Adding support for correct handling of pthread_mutex_trylock() on Win32
systems as well as when using the safe mutexes. 


include/my_pthread.h:
  Adding win_pthread_mutex_trylock() function as wrapper function for
  Windows implementation of pthread_mutex_trylock().
  
  Adding flag to safe_mutex_lock() that is shall not abort if the mutex
  is already locked and instead return EBUSY since this is the POSIX
  behaviour.
mysys/my_winthread.c:
  Added Win32 wrappper function to handle pthread_mutex_trylock().
mysys/thr_mutex.c:
  Added parameter 'try_lock' to safe_mutex_lock() to allow it to do double-
  duty as a pthread_mutex_trylock() in addition to working as
  pthread_mutex_lock().
  
  The code needs to return EBUSY instead of throwing an error in the event
  that the mutex is re-locked (we do not have recursive mutexes), but it
  also requires some special handling to avoid race conditions when
  calling the real pthread_mutex_{lock,trylock}().

include/my_pthread.h

@@ -135,6 +135,7 @@ struct timespec {
 
 void win_pthread_init(void);
 int win_pthread_setspecific(void *A,void *B,uint length);
+int win_pthread_mutex_trylock(pthread_mutex_t *mutex);
 int pthread_create(pthread_t *,pthread_attr_t *,pthread_handler,void *);
 int pthread_cond_init(pthread_cond_t *cond, const pthread_condattr_t *attr);
 int pthread_cond_wait(pthread_cond_t *cond, pthread_mutex_t *mutex);
@@ -195,7 +196,7 @@ extern int pthread_mutex_destroy (pthread_mutex_t *);
 #else
 #define pthread_mutex_init(A,B)  (InitializeCriticalSection(A),0)
 #define pthread_mutex_lock(A)	 (EnterCriticalSection(A),0)
-#define pthread_mutex_trylock(A) (WaitForSingleObject((A), 0) == WAIT_TIMEOUT)
+#define pthread_mutex_trylock(A) win_pthread_mutex_trylock((A))
 #define pthread_mutex_unlock(A)  LeaveCriticalSection(A)
 #define pthread_mutex_destroy(A) DeleteCriticalSection(A)
 #define my_pthread_setprio(A,B)  SetThreadPriority(GetCurrentThread(), (B))
@@ -593,7 +594,7 @@ typedef struct st_safe_mutex_info_t
 
 int safe_mutex_init(safe_mutex_t *mp, const pthread_mutexattr_t *attr,
                     const char *file, uint line);
-int safe_mutex_lock(safe_mutex_t *mp,const char *file, uint line);
+int safe_mutex_lock(safe_mutex_t *mp, my_bool try_lock, const char *file, uint line);
 int safe_mutex_unlock(safe_mutex_t *mp,const char *file, uint line);
 int safe_mutex_destroy(safe_mutex_t *mp,const char *file, uint line);
 int safe_cond_wait(pthread_cond_t *cond, safe_mutex_t *mp,const char *file,
@@ -616,12 +617,12 @@ void safe_mutex_end(FILE *file);
 #undef pthread_cond_timedwait
 #undef pthread_mutex_trylock
 #define pthread_mutex_init(A,B) safe_mutex_init((A),(B),__FILE__,__LINE__)
-#define pthread_mutex_lock(A) safe_mutex_lock((A),__FILE__,__LINE__)
+#define pthread_mutex_lock(A) safe_mutex_lock((A), FALSE, __FILE__, __LINE__)
 #define pthread_mutex_unlock(A) safe_mutex_unlock((A),__FILE__,__LINE__)
 #define pthread_mutex_destroy(A) safe_mutex_destroy((A),__FILE__,__LINE__)
 #define pthread_cond_wait(A,B) safe_cond_wait((A),(B),__FILE__,__LINE__)
 #define pthread_cond_timedwait(A,B,C) safe_cond_timedwait((A),(B),(C),__FILE__,__LINE__)
-#define pthread_mutex_trylock(A) pthread_mutex_lock(A)
+#define pthread_mutex_trylock(A) safe_mutex_lock((A), TRUE, __FILE__, __LINE__)
 #define pthread_mutex_t safe_mutex_t
 #define safe_mutex_assert_owner(mp) \
           DBUG_ASSERT((mp)->count > 0 && \

mysys/my_winthread.c

@@ -40,6 +40,31 @@ void win_pthread_init(void)
   pthread_mutex_init(&THR_LOCK_thread,MY_MUTEX_INIT_FAST);
 }
 
+/**
+   Adapter to @c pthread_mutex_trylock()
+
+   @retval 0      Mutex was acquired
+   @retval EBUSY  Mutex was already locked by a thread
+   @retval EINVAL Mutex could not be acquired due to other error
+ */
+int
+win_pthread_mutex_trylock(pthread_mutex_t *mutex)
+{
+  switch (WaitForSingleObject(mutex, 0)) {
+  case WAIT_TIMEOUT:
+    return EBUSY;
+
+  default:
+  case WAIT_FAILURE:
+    return EINVAL;
+
+  case WAIT_OBJECT_0:
+  case WAIT_ABANDONED: /* The mutex was acquired because it was
+                        * abandoned */
+    return 0;
+  }
+}
+
 /*
 ** We have tried to use '_beginthreadex' instead of '_beginthread' here
 ** but in this case the program leaks about 512 characters for each

mysys/thr_mutex.c

@@ -91,7 +91,7 @@ int safe_mutex_init(safe_mutex_t *mp,
 }
 
 
-int safe_mutex_lock(safe_mutex_t *mp,const char *file, uint line)
+int safe_mutex_lock(safe_mutex_t *mp, my_bool try_lock, const char *file, uint line)
 {
   int error;
   if (!mp->file)
@@ -104,15 +104,50 @@ int safe_mutex_lock(safe_mutex_t *mp,const char *file, uint line)
   }
 
   pthread_mutex_lock(&mp->global);
-  if (mp->count > 0 && pthread_equal(pthread_self(),mp->thread))
+  if (mp->count > 0)
   {
-    fprintf(stderr,"safe_mutex: Trying to lock mutex at %s, line %d, when the mutex was already locked at %s, line %d in thread %s\n",
+    if (try_lock)
+    {
+      pthread_mutex_unlock(&mp->global);
+      return EBUSY;
+    }
+    else if (pthread_equal(pthread_self(),mp->thread))
+    {
+      fprintf(stderr,
+              "safe_mutex: Trying to lock mutex at %s, line %d, when the"
+              " mutex was already locked at %s, line %d in thread %s\n",
               file,line,mp->file, mp->line, my_thread_name());
       fflush(stderr);
       abort();
     }
+  }
   pthread_mutex_unlock(&mp->global);
-  error=pthread_mutex_lock(&mp->mutex);
+
+  /*
+    If we are imitating trylock(), we need to take special
+    precautions.
+
+    - We cannot use pthread_mutex_lock() only since another thread can
+      overtake this thread and take the lock before this thread
+      causing pthread_mutex_trylock() to hang. In this case, we should
+      just return EBUSY. Hence, we use pthread_mutex_trylock() to be
+      able to return immediately.
+
+    - We cannot just use trylock() and continue execution below, since
+      this would generate an error and abort execution if the thread
+      was overtaken and trylock() returned EBUSY . In this case, we
+      instead just return EBUSY, since this is the expected behaviour
+      of trylock().
+   */
+  if (try_lock)
+  {
+    error= pthread_mutex_trylock(&mp->mutex);
+    if (error == EBUSY)
+      return error;
+  }
+  else
+    error= pthread_mutex_lock(&mp->mutex);
+
   if (error || (error=pthread_mutex_lock(&mp->global)))
   {
     fprintf(stderr,"Got error %d when trying to lock mutex at %s, line %d\n",