box: export current session's identifier to C API

In order to send IPROTO packets using `box_iproto_send` (#7897) we need to
have some session identifier source in the C API: for now, export an analog
of `box.session.id`.

Closes #7895

@TarantoolBot document
Title: Document export of current session's identifier to C API
For the API description and usage examples, see:
* [design document](https://www.notion.so/tarantool/box-iproto-override-44935a6ac7e04fb5a2c81ca713ed1bce#0900ff9cb6b148378ce0b185d3f628b9);
* tarantool/tarantool#7895.

changelogs/unreleased/gh-7895-export-session-id-to-c-api.md

+## feature/box
+
+* Exported current session's identifier to C API via `box_session_id` (gh-7895).

extra/exports

@@ -104,6 +104,7 @@ box_sequence_current
 box_sequence_next
 box_sequence_reset
 box_sequence_set
+box_session_id
 box_session_push
 box_space_id_by_name
 box_truncate

src/box/box.cc

@@ -3201,6 +3201,12 @@ box_session_push(const char *data, const char *data_end)
 	return rc;
 }
 
+API_EXPORT uint64_t
+box_session_id(void)
+{
+	return current_session()->id;
+}
+
 static inline void
 box_register_replica(uint32_t id, const struct tt_uuid *uuid)
 {

src/box/box.h

@@ -599,6 +599,12 @@ box_sequence_reset(uint32_t seq_id);
 API_EXPORT int
 box_session_push(const char *data, const char *data_end);
 
+/**
+ * \return current session's unique monotonic identifier (\sa box.session.id)
+ */
+API_EXPORT uint64_t
+box_session_id(void);
+
 /** \endcond public */
 
 /**

src/box/lua/session.c

@@ -73,17 +73,12 @@ lbox_session_create(struct lua_State *L)
 }
 
 /**
- * Return a unique monotonic session
- * identifier. The identifier can be used
- * to check whether or not a session is alive.
- * 0 means there is no session (e.g.
- * a procedure is running in a detached
- * fiber).
+ * Lua wrapper for `box_session_id`.
  */
 static int
 lbox_session_id(struct lua_State *L)
 {
-	lua_pushnumber(L, current_session()->id);
+	luaL_pushuint64(L, box_session_id());
 	return 1;
 }
 

test/app-tap/module_api.c

@@ -2994,6 +2994,19 @@ test_box_schema_version(struct lua_State *L)
 
 /* }}} Helpers for schema version Lua/C API test cases */
 
+/* {{{ Helpers for current session identifier Lua/C API test cases */
+
+static int
+test_box_session_id(struct lua_State *L)
+{
+	fail_unless(lua_gettop(L) == 1);
+	fail_unless(lua_isnumber(L, 1));
+	lua_pushboolean(L, luaL_touint64(L, 1) == box_session_id());
+	return 1;
+}
+
+/* }}} Helpers for current session identifier Lua/C API test cases */
+
 LUA_API int
 luaopen_module_api(lua_State *L)
 {
@@ -3045,6 +3058,7 @@ luaopen_module_api(lua_State *L)
 		{"isdecimal", test_isdecimal},
 		{"isdecimal_ptr", test_isdecimal_ptr},
 		{"box_schema_version_matches", test_box_schema_version},
+		{"box_session_id_matches", test_box_session_id},
 		{NULL, NULL}
 	};
 	luaL_register(L, "module_api", lib);

test/app-tap/module_api.test.lua

@@ -488,8 +488,16 @@ local function test_box_schema_version_matches(test, module)
     s:drop()
 end
 
+local function test_box_session_id_matches(test, module)
+    test:plan(1)
+
+    test:ok(module.box_session_id_matches(box.session.id()),
+            'verify that Lua/C current session identifier APIs return the ' ..
+            'same value')
+end
+
 require('tap').test("module_api", function(test)
-    test:plan(44)
+    test:plan(45)
     local status, module = pcall(require, 'module_api')
     test:is(status, true, "module")
     test:ok(status, "module is loaded")
@@ -522,6 +530,7 @@ require('tap').test("module_api", function(test)
     test:test("pushdecimal", test_pushdecimal, module)
     test:test("isdecimal", test_isdecimal, module)
     test:test("box_schema_version_matches", test_box_schema_version_matches, module)
+    test:test("box_session_id_matches", test_box_session_id_matches, module)
 
     space:drop()
 end)