[Tarantool-patches] [PATCH 1.10 5/9] module api: luaT_temp_luastate & luaT_release_temp_luastate
Timur Safin
tsafin at tarantool.org
Fri Sep 25 00:00:37 MSK 2020
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
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