[Tarantool-patches] [PATCH 3/4] fiber: keep reference to userdata if fiber created once

[Oleg Babin via Tarantool-patches <tarantool-patches@dev.tarantool.org>]

From: Oleg Babin <babinoleg@mail.ru>

This patch reworks approach to fiber management in Lua. Before
this patch each action that should return fiber led to new
userdata creation that was quite slow and made GC suffer. This
patch introduces new field in struct fiber to store a reference to
userdata that was created once for a fiber. It allows speedup
operations as fiber.self() and fiber.id().
Simple benchmark shows that access to fiber storage is faster in
two times, fiber.find() - 2-3 times and fiber.new/create functions
don't have any changes.
---
 src/lib/core/fiber.h |   5 ++
 src/lua/fiber.c      | 110 +++++++++++++++----------------------------
 2 files changed, 42 insertions(+), 73 deletions(-)

diff --git a/src/lib/core/fiber.h b/src/lib/core/fiber.h
index 593847df7..3e9663a1a 100644
--- a/src/lib/core/fiber.h
+++ b/src/lib/core/fiber.h
@@ -623,6 +623,11 @@ struct fiber {
 			 * Should not be used in other fibers.
 			 */
 			struct lua_State *stack;
+			/**
+			 * Optional reference to userdata
+			 * represented current fiber in Lua.
+			 */
+			int ref;
 			/**
 			 * Optional fiber.storage Lua reference.
 			 */
diff --git a/src/lua/fiber.c b/src/lua/fiber.c
index 5575f2079..268ddf9cc 100644
--- a/src/lua/fiber.c
+++ b/src/lua/fiber.c
@@ -87,27 +87,31 @@ luaL_testcancel(struct lua_State *L)
 static const char *fiberlib_name = "fiber";
 
 /**
- * @pre: stack top contains a table
- * @post: sets table field specified by name of the table on top
- * of the stack to a weak kv table and pops that weak table.
+ * Trigger invoked when the fiber has stopped execution of its
+ * current request. Only purpose - delete storage.lua.ref and
+ * storage.lua.storage_ref keeping a reference of Lua
+ * fiber and fiber.storage objects. Unlike Lua stack,
+ * Lua fiber storage may be created not only for fibers born from
+ * Lua land. For example, an IProto request may execute a Lua
+ * function, which can create the storage. Trigger guarantees,
+ * that even for non-Lua fibers the Lua storage is destroyed.
  */
-static void
-lbox_create_weak_table(struct lua_State *L, const char *name)
+static int
+lbox_fiber_on_stop(struct trigger *trigger, void *event)
 {
-	lua_newtable(L);
-	/* and a metatable */
-	lua_newtable(L);
-	/* weak keys and values */
-	lua_pushstring(L, "kv");
-	/* pops 'kv' */
-	lua_setfield(L, -2, "__mode");
-	/* pops the metatable */
-	lua_setmetatable(L, -2);
-	/* assigns and pops table */
-	lua_setfield(L, -2, name);
-	/* gets memoize back. */
-	lua_getfield(L, -1, name);
-	assert(! lua_isnil(L, -1));
+	struct fiber *f = (struct fiber *) event;
+	int storage_ref = f->storage.lua.storage_ref;
+	if (storage_ref > 0) {
+		luaL_unref(tarantool_L, LUA_REGISTRYINDEX, storage_ref);
+		f->storage.lua.storage_ref = LUA_NOREF;
+	}
+	int ref = f->storage.lua.ref;
+	assert(ref > 0);
+	luaL_unref(tarantool_L, LUA_REGISTRYINDEX, ref);
+	f->storage.lua.ref = LUA_NOREF;
+	trigger_clear(trigger);
+	free(trigger);
+	return 0;
 }
 
 /**
@@ -116,42 +120,26 @@ lbox_create_weak_table(struct lua_State *L, const char *name)
 static void
 lbox_pushfiber(struct lua_State *L, struct fiber *f)
 {
-	/*
-	 * Use 'memoize'  pattern and keep a single userdata for
-	 * the given fiber. This is important to not run __gc
-	 * twice for a copy of an attached fiber -- __gc should
-	 * not remove attached fiber's coro prematurely.
-	 */
-	luaL_getmetatable(L, fiberlib_name);
-	lua_getfield(L, -1, "memoize");
-	if (lua_isnil(L, -1)) {
-		/* first access - instantiate memoize */
-		/* pop the nil */
-		lua_pop(L, 1);
-		/* create memoize table */
-		lbox_create_weak_table(L, "memoize");
-	}
-	/* Find out whether the fiber is  already in the memoize table. */
-	uint64_t fid = f->fid;
-	luaL_pushuint64(L, fid);
-	lua_gettable(L, -2);
-	if (lua_isnil(L, -1)) {
-		/* no userdata for fiber created so far */
-		/* pop the nil */
-		lua_pop(L, 1);
-		/* push the key back */
-		luaL_pushuint64(L, fid);
+	int ref = f->storage.lua.ref;
+	if (ref <= 0) {
+		struct trigger *t = (struct trigger *)malloc(sizeof(*t));
+		if (t == NULL) {
+			diag_set(OutOfMemory, sizeof(*t), "malloc", "t");
+			luaT_error(L);
+		}
+		trigger_create(t, lbox_fiber_on_stop, NULL, (trigger_f0) free);
+		trigger_add(&f->on_stop, t);
+
+		uint64_t fid = f->fid;
 		/* create a new userdata */
 		uint64_t *ptr = lua_newuserdata(L, sizeof(*ptr));
 		*ptr = fid;
 		luaL_getmetatable(L, fiberlib_name);
 		lua_setmetatable(L, -2);
-		/* memoize it */
-		lua_settable(L, -3);
-		luaL_pushuint64(L, fid);
-		/* get it back */
-		lua_gettable(L, -2);
+		ref = luaL_ref(L, LUA_REGISTRYINDEX);
+		f->storage.lua.ref = ref;
 	}
+	lua_rawgeti(L, LUA_REGISTRYINDEX, ref);
 }
 
 static struct fiber *
@@ -669,28 +657,6 @@ lbox_fiber_name(struct lua_State *L)
 	}
 }
 
-/**
- * Trigger invoked when the fiber has stopped execution of its
- * current request. Only purpose - delete storage.lua.storage_ref keeping
- * a reference of Lua fiber.storage object. Unlike Lua stack,
- * Lua fiber storage may be created not only for fibers born from
- * Lua land. For example, an IProto request may execute a Lua
- * function, which can create the storage. Trigger guarantees,
- * that even for non-Lua fibers the Lua storage is destroyed.
- */
-static int
-lbox_fiber_on_stop(struct trigger *trigger, void *event)
-{
-	struct fiber *f = (struct fiber *) event;
-	int storage_ref = f->storage.lua.storage_ref;
-	assert(storage_ref > 0);
-	luaL_unref(tarantool_L, LUA_REGISTRYINDEX, storage_ref);
-	f->storage.lua.storage_ref = LUA_NOREF;
-	trigger_clear(trigger);
-	free(trigger);
-	return 0;
-}
-
 static int
 lbox_fiber_storage(struct lua_State *L)
 {
@@ -703,8 +669,6 @@ lbox_fiber_storage(struct lua_State *L)
 			diag_set(OutOfMemory, sizeof(*t), "malloc", "t");
 			return luaT_error(L);
 		}
-		trigger_create(t, lbox_fiber_on_stop, NULL, (trigger_f0) free);
-		trigger_add(&f->on_stop, t);
 		lua_newtable(L); /* create local storage on demand */
 		storage_ref = luaL_ref(L, LUA_REGISTRYINDEX);
 		f->storage.lua.storage_ref = storage_ref;
-- 
2.32.0