Fix ancient bug in ecpg's pthread_once() emulation for Windows.

tglsfdc · tglsfdc · commit f38b66ec0562 · 2020-10-24T13:12:41.000-04:00
We must not set the "done" flag until after we've executed the initialization function. Otherwise, other threads can fall through the initial unlocked test before initialization is really complete. This has been seen to cause rare failures of ecpg's thread/descriptor test, and it could presumably cause other sorts of misbehavior in threaded ECPG-using applications, since ecpglib relies on pthread_once() in several places. Diagnosis and patch by me, based on investigation by Alexander Lakhin. Back-patch to all supported branches (the bug dates to 2007). Discussion: https://postgr.es/m/16685-d6cd241872c101d3@postgresql.org
diff --git a/src/interfaces/ecpg/ecpglib/misc.c b/src/interfaces/ecpg/ecpglib/misc.c
@@ -475,8 +475,8 @@ win32_pthread_once(volatile pthread_once_t *once, void (*fn) (void))
 		pthread_mutex_lock(&win32_pthread_once_lock);
 		if (!*once)
 		{
-			*once = true;
 			fn();
+			*once = true;
 		}
 		pthread_mutex_unlock(&win32_pthread_once_lock);
 	}

Original file line number	Diff line number	Diff line change
`@@ -475,8 +475,8 @@ win32_pthread_once(volatile pthread_once_t once, void (fn) (void))`
`475`	`475`	`pthread_mutex_lock(&win32_pthread_once_lock);`
`476`	`476`	`if (!*once)`
`477`	`477`	`{`
`478`		`- *once = true;`
`479`	`478`	`fn();`
	`479`	`+ *once = true;`
`480`	`480`	`}`
`481`	`481`	`pthread_mutex_unlock(&win32_pthread_once_lock);`
`482`	`482`	`}`