lua: fix crash on running in background mode

Tarantool with background mode enabled in environment variable could
lead a crash:

NO_WRAP
```
$ TT_PID_FILE=tarantool.pid TT_LOG=tarantool.log TT_BACKGROUND=true TT_LISTEN=3301 tarantool -e 'box.cfg{}'
$ tail -3 tarantool.log
2021-11-02 16:05:43.672 [2341202] main init.c:696 E> LuajitError: cannot read stdin: Resource temporarily unavailable
2021-11-02 16:05:43.672 [2341202] main F> fatal error, exiting the event loop
2021-11-02 16:05:43.672 [2341202] main F> fatal error, exiting the event loop

Patch fixes a crash.

Other testcases were tested too:

$ TT_PID_FILE=tarantool.pid TT_LOG=tarantool.log TT_BACKGROUND=true TT_LISTEN=3301 ./build/src/tarantool < box.cfg.lua
$ echo "box.cfg{}" | TT_PID_FILE=tarantool.pid TT_LOG=tarantool.log TT_BACKGROUND=true TT_LISTEN=3301 ./build/src/tarantool -
$ ./build/src/tarantool -l fiber -e 'fiber.sleep(1) box.cfg{background=true, log="tarantool.log", pid_file="tarantool.pid"}'

(T="$(realpath ./build/src/tarantool)";
 I=$(realpath third_party/luajit/test/luajit-test-init.lua);
 cd third_party/luajit/test/lua-Harness-tests;
 export LUA_PATH="$(realpath .)/?.lua;;";
 "${T}" -e "dofile[[${I}]]" -l profile_tarantool 241-standalone.t)
```
NO_WRAP

I dig a bit to a Git history and found that case with TT_BACKGROUND=true
never worked:

- commit 1b33012 ("box: set box.cfg options via environment variables")
- commit 9db64ae ("lua: add '-' command line option")

Closes tarantool#6128

NO_DOC=bugfix

changelogs/unreleased/gh-6128-fix-crash-with-background-env-option.md

@@ -0,0 +1,4 @@
+## bugfix/core
+
+* Fixed a crash that could happen when Tarantool is started with environment
+  variable `TT_BACKGROUND=true` (gh-6128).

src/cfg.c

@@ -36,12 +36,42 @@
 
 enum { MAX_OPT_NAME_LEN = 256, MAX_OPT_VAL_LEN = 256 };
 
+/*
+ * @brief Toggle panics in cfg_get().
+ */
+static int cfg_panic;
+
+/*
+ * @brief Enables panics in cfg_get().
+ */
+void
+cfg_enable_panic(void)
+{
+	cfg_panic = 1;
+}
+
+/*
+ * @brief Disables panics in cfg_get().
+ */
+void
+cfg_disable_panic(void)
+{
+	cfg_panic = 0;
+}
+
 static void
 cfg_get(const char *param)
 {
+	/* int panic = 1; */
+	/* va_list args; */
+	/* va_start(args, param); */
+	/* panic = va_arg(args, int); */
+	/* printf("cfg_get background %d\n", panic); */
+	/* va_end(args); */
+
 	const char *buf =
 		tt_snprintf(MAX_OPT_NAME_LEN, "return box.cfg.%s", param);
-	if (luaL_dostring(tarantool_L, buf) != 0)
+	if ((luaL_dostring(tarantool_L, buf) != 0) && cfg_panic)
 		panic("cfg_get('%s')", param);
 }
 
@@ -76,6 +106,14 @@ cfg_isnumber(const char *param)
 	return ret;
 }
 
+/*
+ * @brief Returns a value of a boolean parameter whose name is passed.
+ *
+ * @param param - name of a boolean parameter
+ * @return -1 - undefined
+ *          1 - false or nil
+ *          0 - true
+ */
 int
 cfg_getb(const char *param)
 {

src/cfg.h

@@ -39,6 +39,18 @@ struct uri_set;
 extern "C" {
 #endif /* defined(__cplusplus) */
 
+/*
+ * @brief Enables panics in cfg_get().
+ */
+void
+cfg_enable_panic(void);
+
+/*
+ * @brief Disables panics in cfg_get().
+ */
+void
+cfg_disable_panic(void);
+
 /**
  * Fill @a uri_set according to cfg @a param.
  * Return 0 if success, otherwise return -1

src/lua/init.c

@@ -45,6 +45,7 @@
 
 #include <fiber.h>
 #include "version.h"
+#include "cfg.h"
 #include "coio.h"
 #include "core/backtrace.h"
 #include "core/tt_static.h"
@@ -979,14 +980,43 @@ run_script_f(va_list ap)
 
 	int is_a_tty = isatty(STDIN_FILENO);
 
+	/*
+	 * - interactive mode:
+	 *   - option '-i' is passed.
+	 *     Example: tarantool -i.
+	 *   - stdin is terminal AND
+	 *     there are no script ('-e' is considered as a script).
+	 *     Example: tarantool.
+	 * - background mode:
+	 *   - explicitly enabled with box.cfg{ background=true }.
+	 *     Example: tarantool -e 'box.cfg{background=true}'.
+	 *   - explicitly enabled with TT_BACKGROUND via environment.
+	 *   - Example: TT_BACKGROUND=1 tarantool.
+	 * - is_a_tty mode:
+	 *   - whether stdin refers to a terminal.
+	 *     Example: tarantool < script.lua
+	 *     Example: tarantool - < script.lua
+	 *     Example: cat script.lua | tarantool
+	 */
+
+	cfg_disable_panic();
+	int background = cfg_geti("background");
+	cfg_enable_panic();
+
+	/* tarantool script.lua */
 	if (path && strcmp(path, "-") != 0 && access(path, F_OK) == 0) {
 		/* Execute script. */
 		if (luaL_loadfile(L, path) != 0)
 			goto luajit_error;
 		if (lua_main(L, argc, argv) != 0)
 			goto error;
-	} else if ((!interactive && !is_a_tty) ||
-			(path && strcmp(path, "-") == 0)) {
+	/*
+	 * tarantool < script.lua
+	 * tarantool - < script.lua
+	 * cat script.lua | tarantool -
+	 */
+	} else if (((!interactive && !is_a_tty) ||
+			(path && strcmp(path, "-") == 0)) && (!background)) {
 		/* Execute stdin */
 		if (luaL_loadfile(L, NULL) != 0)
 			goto luajit_error;

test/box-tap/gh-5602-environment-cfg-test-cases.lua

@@ -70,5 +70,12 @@ if arg[1] == '5' then
     end
     test:is(err_msg, exp_err, 'bad strip_core value')
 end
+if arg[1] == '6' then
+    test:plan(4)
+    test:ok(status, 'box.cfg is successful')
+    test:is(box.cfg['listen'], listen_sock, 'listen')
+    test:is(box.cfg['pid_file'], 'tarantool.pid', 'pid file')
+    test:is(box.cfg['log'], 'tarantool.log', 'log file')
+end
 
 os.exit(test:check() and 0 or 1)

test/box-tap/gh-5602-environment-vars-cfg.result

@@ -29,5 +29,16 @@ TAP version 13
 ok - box.cfg is not successful
 ok - bad strip_core value
 TAP version 13
-1..1
-ok - exit status list
+1..4
+ok - box.cfg is successful
+ok - listen
+ok - pid file
+ok - log file
+TAP version 13
+1..6
+ok - tarantool's exit code is ok (0)
+ok - tarantool's exit code is ok (0)
+ok - tarantool's exit code is ok (0)
+ok - tarantool's exit code is ok (0)
+ok - tarantool's exit code is ok (0)
+ok - tarantool's exit code is ok (0)

test/box-tap/gh-5602-environment-vars-cfg.test.lua

@@ -29,30 +29,42 @@ local repl1_sock = fio.pathjoin(fio.cwd(), 'repl1.sock')
 local repl2_sock = fio.pathjoin(fio.cwd(), 'repl2.sock')
 
 local cases = {
+    -- testcase 1
     ('%s %s %s %s %s'):format(
         string.format('TT_LISTEN=%s', listen_sock),
         'TT_READAHEAD=10000',
         'TT_STRIP_CORE=false',
         'TT_LOG_FORMAT=json',
         'TT_LOG_NONBLOCK=false'),
+
+    -- testcase 2
     ('%s %s %s %s'):format(
         string.format('TT_LISTEN=%s', listen_sock),
         string.format('TT_REPLICATION=%s,%s', repl1_sock,repl2_sock),
         'TT_REPLICATION_CONNECT_TIMEOUT=0.01',
         'TT_REPLICATION_SYNCHRO_QUORUM=\'4 + 1\''),
+
+    -- testcase 3
     'TT_BACKGROUND=true TT_VINYL_TIMEOUT=60.1',
+
+    -- testcase 4
     'TT_SQL_CACHE_SIZE=a',
+
+    -- testcase 5
     'TT_STRIP_CORE=a',
+
+    -- testcase 6
+    ('%s %s %s %s'):format(
+        string.format('TT_LISTEN=%s', listen_sock),
+        'TT_LOG=tarantool.log',
+        'TT_PID_FILE=tarantool.pid',
+        'TT_BACKGROUND=true'),
 }
 
-test:plan(1)
-local exit_status_list = {}
-local exit_status_list_exp = {}
+test:plan(#cases)
 for i, case in ipairs(cases) do
-    exit_status_list[i] = os.execute(shell_command(case, i))
-    exit_status_list_exp[i] = 0
+    local rc = os.execute(shell_command(case, i))
+    test:is(rc, 0, "tarantool's exit code is ok (0)")
 end
 
-test:is_deeply(exit_status_list, exit_status_list_exp, 'exit status list')
-
 os.exit(test:check() and 0 or 1)