[Tarantool-patches] [PATCH v2 3/3] lua: expose temporary Lua state for iproto calls

[Alexander Turenko]

There is code that may save some time and resources for creating a new
Lua state when it is present in the fiber storage of a current fiber.
There are not so much of them: running a Lua trigger and construction of
a next tuple in a merge source.

Before the patch, fiber->storage.lua.stack is filled only for the main
fiber and fibers created from Lua using fiber.create() or fiber.new(),
but not for background fibers (which serve binary protocol requests).

This patch fills fiber->storage.lua.stack for background fibers that
serve a Lua call or eval: we already have this state and nothing prevent
us from exposing it via the fiber storage.

Follows up #4954
---
 src/box/lua/call.c   | 27 +++++++++++++++++++++++++++
 src/lib/core/fiber.h | 14 ++++++++++----
 2 files changed, 37 insertions(+), 4 deletions(-)

diff --git a/src/box/lua/call.c b/src/box/lua/call.c
index 6588ec2fa..ccdef6662 100644
--- a/src/box/lua/call.c
+++ b/src/box/lua/call.c
@@ -537,12 +537,39 @@ box_process_lua(lua_CFunction handler, struct execute_lua_ctx *ctx,
 	port_lua_create(ret, L);
 	((struct port_lua *) ret)->ref = coro_ref;
 
+	/*
+	 * A code that need a temporary fiber-local Lua state may
+	 * save some time and resources for creating a new state
+	 * and use this one.
+	 */
+	bool has_lua_stack = fiber()->storage.lua.stack != NULL;
+	if (!has_lua_stack)
+		fiber()->storage.lua.stack = L;
+
 	lua_pushcfunction(L, handler);
 	lua_pushlightuserdata(L, ctx);
 	if (luaT_call(L, 1, LUA_MULTRET) != 0) {
+		if (!has_lua_stack)
+			fiber()->storage.lua.stack = NULL;
 		port_lua_destroy(ret);
 		return -1;
 	}
+
+	/*
+	 * Since this field is optional we're not obligated to
+	 * keep it until the Lua state will be unreferenced in
+	 * port_lua_destroy().
+	 *
+	 * There is no much sense to keep it beyond the Lua call,
+	 * so let's zap now.
+	 *
+	 * But: keep the stack if it was present before the call,
+	 * because it would be counter-intuitive if the existing
+	 * state pointer would be zapped after this function call.
+	 */
+	if (!has_lua_stack)
+		fiber()->storage.lua.stack = NULL;
+
 	return 0;
 }
 
diff --git a/src/lib/core/fiber.h b/src/lib/core/fiber.h
index cd9346a55..2ff0b4009 100644
--- a/src/lib/core/fiber.h
+++ b/src/lib/core/fiber.h
@@ -495,12 +495,18 @@ struct fiber {
 		struct session *session;
 		struct credentials *credentials;
 		struct txn *txn;
-		/**
-		 * Lua stack and the optional
-		 * fiber.storage Lua reference.
-		 */
+		/** Fields related to Lua code execution. */
 		struct {
+			/**
+			 * Optional Lua state (may be NULL).
+			 * Useful as a temporary Lua state to save
+			 * time and resources on creating it.
+			 * Should not be used in other fibers.
+			 */
 			struct lua_State *stack;
+			/**
+			 * Optional fiber.storage Lua reference.
+			 */
 			int ref;
 		} lua;
 		/**
-- 
2.25.0