[Tarantool-patches] [PATCH 1.10 05/16] WIP: module api/lua: add luaL_iscdata() function

[Alexander Turenko <alexander.turenko@tarantool.org>]

It is useful to provide a module specific error when cdata expected, but
a value of another type is passed.

Alternative would be using of lua_type() to check against LUA_TCDATA,
but this constant is not exposed for modules. See more in the
luaL_iscdata() API comment.

XXX: Add a test to test/app-tap/module_api.{c,test.lua}.

Part of #5273

(cherry picked from commit 98551e8b90ebd2839dee57fee13f446aee515e42)
---
 extra/exports   |  1 +
 src/lua/utils.c |  6 ++++++
 src/lua/utils.h | 20 ++++++++++++++++++++
 3 files changed, 27 insertions(+)

diff --git a/extra/exports b/extra/exports
index 52bb29d68..a7d8ef81b 100644
--- a/extra/exports
+++ b/extra/exports
@@ -117,6 +117,7 @@ coio_close
 coio_call
 coio_getaddrinfo
 luaL_pushcdata
+luaL_iscdata
 luaL_checkcdata
 luaL_setcdatagc
 luaL_ctypeid
diff --git a/src/lua/utils.c b/src/lua/utils.c
index d88efc560..e555607cf 100644
--- a/src/lua/utils.c
+++ b/src/lua/utils.c
@@ -102,6 +102,12 @@ luaL_pushcdata(struct lua_State *L, uint32_t ctypeid)
 	return cdataptr(cd);
 }
 
+int
+luaL_iscdata(struct lua_State *L, int idx)
+{
+	return lua_type(L, idx) == LUA_TCDATA;
+}
+
 void *
 luaL_checkcdata(struct lua_State *L, int idx, uint32_t *ctypeid)
 {
diff --git a/src/lua/utils.h b/src/lua/utils.h
index 5164eb5bf..fe6728986 100644
--- a/src/lua/utils.h
+++ b/src/lua/utils.h
@@ -69,6 +69,26 @@ extern struct ibuf *tarantool_lua_ibuf;
 
 /** \cond public */
 
+/**
+ * @brief Checks whether a value on the Lua stack is a cdata.
+ *
+ * Unlike <luaL_checkcdata>() this function does not raise an
+ * error. It is useful to raise a domain specific error.
+ *
+ * Lua API and module API don't expose LUA_TCDATA constant.
+ * We have no guarantee that this constant will remain the same in
+ * future LuaJIT versions. So this function should be used in
+ * modules instead of `lua_type(L, idx) == LUA_TCDATA`.
+ *
+ * @param L Lua State
+ * @param idx stack index
+ *
+ * @return 1 if the value at the given acceptable index is a cdata
+ * and 0 otherwise.
+ */
+LUA_API int
+luaL_iscdata(struct lua_State *L, int idx);
+
 /**
  * @brief Push cdata of given \a ctypeid onto the stack.
  * CTypeID must be used from FFI at least once. Allocated memory returned
-- 
2.25.0