module api/lua: add luaL_iscdata() function

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.

Part of #5273

src/exports.h

@@ -356,6 +356,7 @@ EXPORT(luaL_findtable)
 EXPORT(luaL_getmetafield)
 EXPORT(luaL_gsub)
 EXPORT(luaL_iscallable)
+EXPORT(luaL_iscdata)
 EXPORT(luaL_loadbuffer)
 EXPORT(luaL_loadbufferx)
 EXPORT(luaL_loadfile)

src/lua/utils.c

@@ -113,6 +113,12 @@ luaL_pushuuid(struct lua_State *L)
 	return luaL_pushcdata(L, CTID_UUID);
 }
 
+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)
 {

src/lua/utils.h

@@ -78,6 +78,26 @@ luaL_pushuuid(struct lua_State *L);
 
 /** \cond public */
 
+/**
+ * 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  Acceptable index on the Lua stack.
+ *
+ * @retval 1   If the value at the given index is a cdata.
+ * @retval 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

test/app-tap/module_api.c

@@ -455,6 +455,27 @@ test_iscallable(lua_State *L)
 	return 1;
 }
 
+static int
+test_iscdata(struct lua_State *L)
+{
+	assert(lua_gettop(L) == 2);
+
+	int exp = lua_toboolean(L, 2);
+
+	/* Basic test. */
+	int res = luaL_iscdata(L, 1);
+	int ok = res == exp;
+	assert(lua_gettop(L) == 2);
+
+	/* Use negative index. */
+	res = luaL_iscdata(L, -2);
+	ok = ok && res == exp;
+	assert(lua_gettop(L) == 2);
+
+	lua_pushboolean(L, ok);
+	return 1;
+}
+
 /* {{{ test_box_region */
 
 /**
@@ -562,6 +583,7 @@ luaopen_module_api(lua_State *L)
 		{"test_state", test_state},
 		{"test_tostring", test_tostring},
 		{"iscallable", test_iscallable},
+		{"iscdata", test_iscdata},
 		{"test_box_region", test_box_region},
 		{NULL, NULL}
 	};

test/app-tap/module_api.test.lua

@@ -116,8 +116,68 @@ local function test_iscallable(test, module)
     end
 end
 
+local function test_iscdata(test, module)
+    local ffi = require('ffi')
+    ffi.cdef([[
+        struct foo { int bar; };
+    ]])
+
+    local cases = {
+        {
+            obj = nil,
+            exp = false,
+            description = 'nil',
+        },
+        {
+            obj = 1,
+            exp = false,
+            description = 'number',
+        },
+        {
+            obj = 'hello',
+            exp = false,
+            description = 'string',
+        },
+        {
+            obj = {},
+            exp = false,
+            description = 'table',
+        },
+        {
+            obj = function() end,
+            exp = false,
+            description = 'function',
+        },
+        {
+            obj = ffi.new('struct foo'),
+            exp = true,
+            description = 'cdata',
+        },
+        {
+            obj = ffi.new('struct foo *'),
+            exp = true,
+            description = 'cdata pointer',
+        },
+        {
+            obj = ffi.new('struct foo &'),
+            exp = true,
+            description = 'cdata reference',
+        },
+        {
+            obj = 1LL,
+            exp = true,
+            description = 'cdata number',
+        },
+    }
+
+    test:plan(#cases)
+    for _, case in ipairs(cases) do
+        test:ok(module.iscdata(case.obj, case.exp), case.description)
+    end
+end
+
 local test = require('tap').test("module_api", function(test)
-    test:plan(25)
+    test:plan(26)
     local status, module = pcall(require, 'module_api')
     test:is(status, true, "module")
     test:ok(status, "module is loaded")
@@ -143,6 +203,7 @@ local test = require('tap').test("module_api", function(test)
 
     test:test("pushcdata", test_pushcdata, module)
     test:test("iscallable", test_iscallable, module)
+    test:test("iscdata", test_iscdata, module)
 
     space:drop()
 end)