fscache: load directories only once

If multiple threads access a directory that is not yet in the cache, the
directory will be loaded by each thread. Only one of the results is added
to the cache, all others are leaked. This wastes performance and memory.

On cache miss, add a future object to the cache to indicate that the
directory is currently being loaded. Subsequent threads register themselves
with the future object and wait. When the first thread has loaded the
directory, it replaces the future object with the result and notifies
waiting threads.

Signed-off-by: Karsten Blees <blees@dcon.de>

compat/win32/fscache.c

@@ -33,6 +33,8 @@ struct fsentry {
 	union {
 		/* Reference count of the directory listing. */
 		volatile long refcnt;
+		/* Handle to wait on the loading thread. */
+		HANDLE hwait;
 		struct {
 			/* More stat members (only used for file entries). */
 			off64_t st_size;
@@ -245,16 +247,43 @@ static inline int fscache_enabled(const char *path)
 	return enabled > 0 && !is_absolute_path(path);
 }
 
+/*
+ * Looks up a cache entry, waits if its being loaded by another thread.
+ * The mutex must be owned by the calling thread.
+ */
+static struct fsentry *fscache_get_wait(struct fsentry *key)
+{
+	struct fsentry *fse = hashmap_get(&map, key, NULL);
+
+	/* return if its a 'real' entry (future entries have refcnt == 0) */
+	if (!fse || fse->list || fse->refcnt)
+		return fse;
+
+	/* create an event and link our key to the future entry */
+	key->hwait = CreateEvent(NULL, TRUE, FALSE, NULL);
+	key->next = fse->next;
+	fse->next = key;
+
+	/* wait for the loading thread to signal us */
+	LeaveCriticalSection(&mutex);
+	WaitForSingleObject(key->hwait, INFINITE);
+	CloseHandle(key->hwait);
+	EnterCriticalSection(&mutex);
+
+	/* repeat cache lookup */
+	return hashmap_get(&map, key, NULL);
+}
+
 /*
  * Looks up or creates a cache entry for the specified key.
  */
 static struct fsentry *fscache_get(struct fsentry *key)
 {
-	struct fsentry *fse;
+	struct fsentry *fse, *future, *waiter;
 
 	EnterCriticalSection(&mutex);
 	/* check if entry is in cache */
-	fse = hashmap_get(&map, key, NULL);
+	fse = fscache_get_wait(key);
 	if (fse) {
 		fsentry_addref(fse);
 		LeaveCriticalSection(&mutex);
@@ -262,7 +291,7 @@ static struct fsentry *fscache_get(struct fsentry *key)
 	}
 	/* if looking for a file, check if directory listing is in cache */
 	if (!fse && key->list) {
-		fse = hashmap_get(&map, key->list, NULL);
+		fse = fscache_get_wait(key->list);
 		if (fse) {
 			LeaveCriticalSection(&mutex);
 			/* dir entry without file entry -> file doesn't exist */
@@ -271,16 +300,34 @@ static struct fsentry *fscache_get(struct fsentry *key)
 		}
 	}
 
+	/* add future entry to indicate that we're loading it */
+	future = key->list ? key->list : key;
+	future->next = NULL;
+	future->refcnt = 0;
+	hashmap_add(&map, future);
+
 	/* create the directory listing (outside mutex!) */
 	LeaveCriticalSection(&mutex);
-	fse = fsentry_create_list(key->list ? key->list : key);
-	if (!fse)
-		return NULL;
-
+	fse = fsentry_create_list(future);
 	EnterCriticalSection(&mutex);
-	/* add directory listing if it hasn't been added by some other thread */
-	if (!hashmap_get(&map, key, NULL))
-		fscache_add(fse);
+
+	/* remove future entry and signal waiting threads */
+	hashmap_remove(&map, future, NULL);
+	waiter = future->next;
+	while (waiter) {
+		HANDLE h = waiter->hwait;
+		waiter = waiter->next;
+		SetEvent(h);
+	}
+
+	/* leave on error (errno set by fsentry_create_list) */
+	if (!fse) {
+		LeaveCriticalSection(&mutex);
+		return NULL;
+	}
+
+	/* add directory listing to the cache */
+	fscache_add(fse);
 
 	/* lookup file entry if requested (fse already points to directory) */
 	if (key->list)