Bug#30992 Wrong implementation of pthread_mutex_trylock()

It's not possible to use WaitForSingleObject to wait
on a CRITICAL_SECTION, instead use the TryEnterCriticalSection function.
 - if "mutex" was already taken => return EBUSY
 - if "mutex" was aquired => return 0


include/config-win.h:
  Make windows.h define TryEnterCriticalSection
mysys/my_winthread.c:
  Use the TryEnterCriticalSection function to implement pthread_mutex_trylock
  Prevent recursive behaviour by looking at the RecursionCount variable
  in the CRITICAL_SECTION struct and return EBUSY from function if
  the mutex was already locked once.

include/config-win.h

@@ -19,6 +19,9 @@
 /* We have to do this define before including windows.h to get the AWE API
 functions */
 #define _WIN32_WINNT     0x0500
+#else
+/* Get NT 4.0 functions */
+#define _WIN32_WINNT     0x0400
 #endif
 
 #if defined(_MSC_VER) && _MSC_VER >= 1400

mysys/my_winthread.c

@@ -40,31 +40,29 @@ 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:
+  if (TryEnterCriticalSection(mutex))
+  {
+    /* Don't allow recursive lock */
+    if (mutex->RecursionCount > 1){
+      LeaveCriticalSection(mutex);
       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;
   }
+  return EBUSY;
 }
 
+
 /*
 ** We have tried to use '_beginthreadex' instead of '_beginthread' here
 ** but in this case the program leaks about 512 characters for each