[Tarantool-patches] [PATCH 1.10 5/9] module api: luaT_temp_luastate & luaT_release_temp_luastate

[Timur Safin <tsafin@tarantool.org>]

Moved `luaT_temp_luastate` & `luaT_release_temp_luastate` from
merger to ustil.c and made them public.

They were necessary for external merger support

Part of #5273
---
 src/lua/utils.c | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
 src/lua/utils.h | 35 +++++++++++++++++++++++++++++++++++
 2 files changed, 84 insertions(+)

diff --git a/src/lua/utils.c b/src/lua/utils.c
index e555607cf..58c54f8fb 100644
--- a/src/lua/utils.c
+++ b/src/lua/utils.c
@@ -1090,6 +1090,55 @@ luaT_newthread(struct lua_State *L)
 	return L1;
 }
 
+struct lua_State *
+luaT_temp_luastate(int *coro_ref, int *top)
+{
+	if (fiber()->storage.lua.stack != NULL) {
+		/*
+		 * Reuse existing stack. In the releasing function
+		 * we should drop a stack top to its initial
+		 * value to don't exhaust available slots by
+		 * many requests in row.
+		 */
+		struct lua_State *L = fiber()->storage.lua.stack;
+		*coro_ref = LUA_NOREF;
+		*top = lua_gettop(L);
+		return L;
+	}
+
+	/* Popped by luaL_ref(). */
+	struct lua_State *L = luaT_newthread(tarantool_L);
+	if (L == NULL)
+		return NULL;
+	/*
+	 * We should remove the reference to the newly created Lua
+	 * thread from tarantool_L, because of two reasons:
+	 *
+	 * First, if we'll push something to tarantool_L and
+	 * yield, then another fiber will not know that a stack
+	 * top is changed and may operate on a wrong slot.
+	 *
+	 * Second, many requests that push a value to tarantool_L
+	 * and yield may exhaust available slots on the stack. It
+	 * is limited by LUAI_MAXSTACK build time constant (~65K).
+	 *
+	 * We cannot just pop the value, but should keep the
+	 * reference in the registry while it is in use.
+	 * Otherwise it may be garbage collected.
+	 */
+	*coro_ref = luaL_ref(tarantool_L, LUA_REGISTRYINDEX);
+	*top = -1;
+	return L;
+}
+
+void
+luaT_release_temp_luastate(struct lua_State *L, int coro_ref, int top)
+{
+	if (top >= 0)
+		lua_settop(L, top);
+	luaL_unref(tarantool_L, LUA_REGISTRYINDEX, coro_ref);
+}
+
 int
 tarantool_lua_utils_init(struct lua_State *L)
 {
diff --git a/src/lua/utils.h b/src/lua/utils.h
index a818e5eee..4eb5ce566 100644
--- a/src/lua/utils.h
+++ b/src/lua/utils.h
@@ -530,6 +530,41 @@ luaL_iscallable(lua_State *L, int idx);
 struct lua_State *
 luaT_newthread(struct lua_State *L);
 
+/**
+ * Get a temporary Lua state.
+ *
+ * Use case: a function does not accept a Lua state as an argument
+ * to allow using from C code, but uses a Lua value, which is
+ * referenced in LUA_REGISTRYINDEX. A temporary Lua stack is needed
+ * to get and process the value.
+ *
+ * The resulting Lua state has a separate Lua stack, but the same
+ * globals and registry as `tarantool_L` (and all Lua states in
+ * tarantool at the moment of writing this).
+ *
+ * This Lua state should be used only from one fiber: otherwise
+ * one fiber may change the stack and another one will access a
+ * wrong stack slot when it will be scheduled for execution after
+ * yield.
+ *
+ * Return a Lua state on success and set @a coro_ref and @a top.
+ * These values should be passed to
+ * `luaT_release_temp_luastate()`, when the state is not needed
+ * anymore.
+ *
+ * Return NULL and set a diag at failure.
+ */
+struct lua_State *
+luaT_temp_luastate(int *coro_ref, int *top);
+
+/**
+ * Release a temporary Lua state.
+ *
+ * It complements `luaT_temp_luastate()`.
+ */
+void
+luaT_release_temp_luastate(struct lua_State *L, int coro_ref, int top);
+
 /** \endcond public */
 
 void
-- 
2.20.1