lua/fiber: do not raise on printing a dead fiber

An attempt to print a dead fiber raised a fatal error, which is quite
unexpected. This patch updates __tostring metamethod of fiber_object so
that it pushes the "fiber: <fid> (dead)" string instead of the error.
The __serialize metamethod is patched similarly.

Closes tarantool#4265

NO_DOC=bugfix

changelogs/unreleased/gh-4265-printing-a-finished-fiber-stops-the-program.md

@@ -0,0 +1,3 @@
+## bugfix/core
+
+* An error is no longer raised on an attempt to print a dead fiber (gh-4265).

src/lua/fiber.c

@@ -138,14 +138,24 @@ lbox_pushfiber(struct lua_State *L, struct fiber *f)
 	lua_rawgeti(L, LUA_REGISTRYINDEX, fid_ref);
 }
 
+/**
+ * Returns fiber id for the fiber located at the given `index` on the Lua stack.
+ */
+static uint64_t
+lbox_checkfiberid(struct lua_State *L, int index)
+{
+	uint64_t *fid = luaL_checkudata(L, index, fiberlib_name);
+	return *fid;
+}
+
 static struct fiber *
 lbox_checkfiber(struct lua_State *L, int index)
 {
 	uint64_t fid;
 	if (lua_type(L, index) == LUA_TNUMBER) {
 		fid = luaL_touint64(L, index);
 	} else {
-		fid = *(uint64_t *)luaL_checkudata(L, index, fiberlib_name);
+		fid = lbox_checkfiberid(L, index);
 	}
 	struct fiber *f = fiber_find(fid);
 	if (f == NULL)
@@ -160,7 +170,7 @@ lbox_fiber_id(struct lua_State *L)
 	if (lua_gettop(L)  == 0)
 		fid = fiber()->fid;
 	else
-		fid = *(uint64_t *)luaL_checkudata(L, 1, fiberlib_name);
+		fid = lbox_checkfiberid(L, 1);
 	luaL_pushuint64(L, fid);
 	return 1;
 }
@@ -506,8 +516,8 @@ static struct fiber *
 lbox_get_fiber(struct lua_State *L)
 {
 	if (lua_gettop(L) != 0) {
-		uint64_t *fid = luaL_checkudata(L, 1, fiberlib_name);
-		return fiber_find(*fid);
+		uint64_t fid = lbox_checkfiberid(L, 1);
+		return fiber_find(fid);
 	} else {
 		return fiber();
 	}
@@ -726,12 +736,16 @@ lbox_fiber_cancel(struct lua_State *L)
 static int
 lbox_fiber_serialize(struct lua_State *L)
 {
-	struct fiber *f = lbox_checkfiber(L, 1);
+	uint64_t fid = lbox_checkfiberid(L, 1);
+	struct fiber *f = fiber_find(fid);
+
 	lua_createtable(L, 0, 1);
-	luaL_pushuint64(L, f->fid);
+	luaL_pushuint64(L, fid);
 	lua_setfield(L, -2, "id");
-	lua_pushstring(L, fiber_name(f));
-	lua_setfield(L, -2, "name");
+	if (f != NULL) {
+		lua_pushstring(L, fiber_name(f));
+		lua_setfield(L, -2, "name");
+	}
 	lbox_fiber_status(L);
 	lua_setfield(L, -2, "status");
 	return 1;
@@ -740,10 +754,12 @@ lbox_fiber_serialize(struct lua_State *L)
 static int
 lbox_fiber_tostring(struct lua_State *L)
 {
-	char buf[32];
-	struct fiber *f = lbox_checkfiber(L, 1);
-	snprintf(buf, sizeof(buf), "fiber: %llu",
-		 (long long)f->fid);
+	char buf[35];
+	uint64_t fid = lbox_checkfiberid(L, 1);
+	bool is_dead = fiber_find(fid) == NULL;
+
+	snprintf(buf, sizeof(buf), "fiber: %llu%s", (long long)fid,
+		 is_dead ? " (dead)" : "");
 	lua_pushstring(L, buf);
 	return 1;
 }

test/app-luatest/fiber_test.lua

@@ -0,0 +1,36 @@
+local fiber = require('fiber')
+local t = require('luatest')
+local g = t.group('fiber')
+
+-- Test __serialize metamethod of the fiber.
+g.test_serialize = function()
+    local f = fiber.new(function() end)
+    local fid = f:id()
+    f:name('test fiber')
+
+    -- Serialize a ready fiber.
+    t.assert_equals(f:__serialize(),
+                    { id = fid, name = 'test fiber', status = 'suspended' })
+
+    -- gh-4265: Serializing a finished fiber should not raise an error.
+    fiber.yield()
+    t.assert_equals(f:__serialize(), { id = fid, status = 'dead' })
+
+    -- Serialize a running fiber.
+    t.assert_equals(fiber.self():__serialize(),
+                    { id = fiber.self():id(),
+                      name = 'luatest',
+                      status = 'running' })
+end
+
+-- Test __tostring metamethod of the fiber.
+g.test_tostring = function()
+    local f = fiber.new(function() end)
+    local fid = f:id()
+
+    t.assert_equals(tostring(f), "fiber: " .. fid)
+
+    -- gh-4265: Printing a finished fiber should not raise an error.
+    fiber.yield()
+    t.assert_equals(tostring(f), "fiber: " .. fid .. " (dead)")
+end

test/app/fiber.result

@@ -961,9 +961,8 @@ fiber.create(function()end):name()
 --
 -- gh-1926
 --
-fiber.create(function() fiber.wakeup(fiber.self()) end)
+_ = fiber.create(function() fiber.wakeup(fiber.self()) end)
 ---
-- the fiber is dead
 ...
 --
 -- gh-2066 test for fiber wakeup

test/app/fiber.test.lua

@@ -387,7 +387,7 @@ fiber.create(function()end):name()
 --
 -- gh-1926
 --
-fiber.create(function() fiber.wakeup(fiber.self()) end)
+_ = fiber.create(function() fiber.wakeup(fiber.self()) end)
 
 --
 -- gh-2066 test for fiber wakeup