Fix thread hanging problem (mostly seen with material preview, but that sneaky f*er could strike any time).

Story time: Once upon a time, in the green valley of fileselect, BLI_end_threads would get called on an empty threadbase, depending on the result of a previous call to readdir(). The function would then gladly decrement thread_level to -1 which would cause all kinds of fun havoc. THE END.

Made sure thread_level is only incremented and decremented when needed. The caller should never have to make sure of that, especially since it already lets you call with a null threadbase.

Please report any further hang (and how to reproduce, if possible).

1 files changed, 11 insertions, 8 deletions

diff --git a/source/blender/blenlib/intern/threads.c b/source/blender/blenlib/intern/threads.c
index 2812f17d58f..ed3e07b7f7e 100644
--- a/source/blender/blenlib/intern/threads.c
+++ b/source/blender/blenlib/intern/threads.c
@@ -142,10 +142,10 @@ void BLI_init_threads(ListBase *threadbase, void *(*do_thread)(void *), int tot)
 			tslot->do_thread= do_thread;
 			tslot->avail= 1;
 		}
+		
+		MEM_set_lock_callback(BLI_lock_malloc_thread, BLI_unlock_malloc_thread);
+		thread_levels++;
 	}
-
-	MEM_set_lock_callback(BLI_lock_malloc_thread, BLI_unlock_malloc_thread);
-	thread_levels++;
 }
 
 /* amount of available threads */
@@ -235,18 +235,21 @@ void BLI_end_threads(ListBase *threadbase)
 {
 	ThreadSlot *tslot;
 	
-	if (threadbase) {
+	/* only needed if there's actually some stuff to end
+	 * this way we don't end up decrementing thread_levels on an empty threadbase 
+	 * */
+	if (threadbase && threadbase->first != NULL) {
 		for(tslot= threadbase->first; tslot; tslot= tslot->next) {
 			if(tslot->avail==0) {
 				pthread_join(tslot->pthread, NULL);
 			}
 		}
 		BLI_freelistN(threadbase);
+
+		thread_levels--;
+		if(thread_levels==0)
+			MEM_set_lock_callback(NULL, NULL);
 	}
-	
-	thread_levels--;
-	if(thread_levels==0)
-		MEM_set_lock_callback(NULL, NULL);
 }
 
 void BLI_lock_thread(int type)