fz_store: Reap passes.

A few commits back, we introduced the fz_key_storable concept
to allow us to cope with objects that were used both as values
within the store and as parts of keys within the store.

This commit worked, but showed up performance problems; when the
store has several million PDF objects in it, bulk changes (such
as dropping a display list or document) could trigger many passes
across the store.

We therefore introduce a mechanism to ameliorate this. These
passes, now known as "reap passes", can be batched together using
fz_defer_reap_start and fz_defer_reap_end.

We trigger this start/end around display list dropping, and around
PDF content stream processing. This should be fine, as deferral
will be interrupted if we ever run our of memory during mallocing.

1 files changed, 144 insertions, 7 deletions

diff --git a/source/fitz/store.c b/source/fitz/store.c
index d9674eb6..639e0662 100644
--- a/source/fitz/store.c
+++ b/source/fitz/store.c
@@ -29,6 +29,10 @@ struct fz_store_s
 	/* We keep track of the size of the store, and keep it below max. */
 	size_t max;
 	size_t size;
+
+	/* Protected by the reap lock */
+	int defer_reap_count;
+	int needs_reaping;
 };
 
 void
@@ -50,6 +54,8 @@ fz_new_store_context(fz_context *ctx, size_t max)
 	store->tail = NULL;
 	store->size = 0;
 	store->max = max;
+	store->defer_reap_count = 0;
+	store->needs_reaping = 0;
 	ctx->store = store;
 }
 
@@ -86,16 +92,93 @@ void *fz_keep_key_storable(fz_context *ctx, const fz_key_storable *sc)
 	return fz_keep_storable(ctx, &sc->storable);
 }
 
-int fz_drop_key_storable(fz_context *ctx, const fz_key_storable *sc)
+/*
+	Entered with FZ_LOCK_ALLOC and FZ_LOCK_REAP held.
+	Drops FZ_LOCK_ALLOC.
+*/
+static void
+do_reap(fz_context *ctx)
+{
+	fz_store *store = ctx->store;
+	fz_item *item, *prev, *remove;
+
+	if (store == NULL)
+	{
+		fz_unlock(ctx, FZ_LOCK_ALLOC);
+		return;
+	}
+
+	fz_assert_lock_held(ctx, FZ_LOCK_ALLOC);
+	fz_assert_lock_held(ctx, FZ_LOCK_REAP);
+
+	ctx->store->needs_reaping = 0;
+
+	/* Reap the items */
+	remove = NULL;
+	for (item = store->tail; item; item = prev)
+	{
+		prev = item->prev;
+
+		if (item->type->needs_reap == NULL || item->type->needs_reap(ctx, item->key) == 0)
+			continue;
+
+		/* We have to drop it */
+		store->size -= item->size;
+
+		/* Unlink from the linked list */
+		if (item->next)
+			item->next->prev = item->prev;
+		else
+			store->tail = item->prev;
+		if (item->prev)
+			item->prev->next = item->next;
+		else
+			store->head = item->next;
+
+		/* Remove from the hash table */
+		if (item->type->make_hash_key)
+		{
+			fz_store_hash hash = { NULL };
+			hash.drop = item->val->drop;
+			if (item->type->make_hash_key(ctx, &hash, item->key))
+				fz_hash_remove(ctx, store->hash, &hash);
+		}
+
+		/* Store whether to drop this value or not in 'prev' */
+		item->prev = (item->val->refs > 0 && --item->val->refs == 0) ? item : NULL;
+
+		/* Store it in our removal chain - just singly linked */
+		item->next = remove;
+		remove = item;
+	}
+	fz_unlock(ctx, FZ_LOCK_ALLOC);
+
+	/* Now drop the remove chain */
+	for (item = remove; item != NULL; item = remove)
+	{
+		remove = item->next;
+
+		/* Drop a reference to the value (freeing if required) */
+		if (item->prev)
+			item->val->drop(ctx, item->val);
+
+		/* Always drops the key and drop the item */
+		item->type->drop_key(ctx, item->key);
+		fz_free(ctx, item);
+	}
+
+}
+
+void fz_drop_key_storable(fz_context *ctx, const fz_key_storable *sc)
 {
 	/* Explicitly drop const to allow us to use const
 	 * sanely throughout the code. */
 	fz_key_storable *s = (fz_key_storable *)sc;
 	int drop;
-	int only_refs_are_store_key_refs = 0;
+	int unlock = 1;
 
 	if (s == NULL)
-		return 0;
+		return;
 
 	if (s->storable.refs > 0)
 		(void)Memento_dropRef(s);
@@ -104,11 +187,22 @@ int fz_drop_key_storable(fz_context *ctx, const fz_key_storable *sc)
 	{
 		drop = --s->storable.refs == 0;
 		if (!drop && s->storable.refs == s->store_key_refs)
-			only_refs_are_store_key_refs = 1;
+		{
+			fz_lock(ctx, FZ_LOCK_REAP);
+			if (ctx->store->defer_reap_count > 0)
+				ctx->store->needs_reaping = 1;
+			else
+			{
+				do_reap(ctx);
+				unlock = 0;
+			}
+			fz_unlock(ctx, FZ_LOCK_REAP);
+		}
 	}
 	else
 		drop = 0;
-	fz_unlock(ctx, FZ_LOCK_ALLOC);
+	if (unlock)
+		fz_unlock(ctx, FZ_LOCK_ALLOC);
 	/*
 		If we are dropping the last reference to an object, then
 		it cannot possibly be in the store (as the store always
@@ -118,7 +212,6 @@ int fz_drop_key_storable(fz_context *ctx, const fz_key_storable *sc)
 	 */
 	if (drop)
 		s->storable.drop(ctx, &s->storable);
-	return only_refs_are_store_key_refs;
 }
 
 void *fz_keep_key_storable_key(fz_context *ctx, const fz_key_storable *sc)
@@ -385,8 +478,25 @@ fz_store_item(fz_context *ctx, void *key, void *val_, size_t itemsize, fz_store_
 		size = store->size + itemsize;
 		while (size > store->max)
 		{
+			size_t saved;
+			int relock = 0;
+
+			/* First, do any outstanding reaping, even if defer_reap_count > 0 */
+			fz_lock(ctx, FZ_LOCK_REAP);
+			if (store->needs_reaping)
+			{
+				do_reap(ctx); /* Drops alloc lock */
+				relock = 1;
+			}
+			fz_unlock(ctx, FZ_LOCK_REAP);
+			if (relock)
+				fz_lock(ctx, FZ_LOCK_ALLOC);
+			size = store->size + itemsize;
+			if (size <= store->max)
+				break;
+
 			/* ensure_space may drop, then retake the lock */
-			size_t saved = ensure_space(ctx, size - store->max);
+			saved = ensure_space(ctx, size - store->max);
 			size -= saved;
 			if (saved == 0)
 			{
@@ -794,5 +904,32 @@ void fz_filter_store(fz_context *ctx, fz_store_filter_fn *fn, void *arg, fz_stor
 		item->type->drop_key(ctx, item->key);
 		fz_free(ctx, item);
 	}
+}
+
+void fz_defer_reap_start(fz_context *ctx)
+{
+	if (ctx == NULL || ctx->store == NULL)
+		return;
+
+	fz_lock(ctx, FZ_LOCK_REAP);
+	ctx->store->defer_reap_count++;
+	fz_unlock(ctx, FZ_LOCK_REAP);
+}
+
+void fz_defer_reap_end(fz_context *ctx)
+{
+	int reap;
+
+	if (ctx == NULL || ctx->store == NULL)
+		return;
 
+	fz_lock(ctx, FZ_LOCK_ALLOC);
+	fz_lock(ctx, FZ_LOCK_REAP);
+	--ctx->store->defer_reap_count;
+	reap = ctx->store->defer_reap_count == 0 && ctx->store->needs_reaping;
+	if (reap)
+		do_reap(ctx); /* Drops FZ_LOCK_ALLOC */
+	fz_unlock(ctx, FZ_LOCK_REAP);
+	if (!reap)
+		fz_unlock(ctx, FZ_LOCK_ALLOC);
 }