lua/log: do not allow to set json for boot logger

For some reason we've missed that say_set_log_format
doesn't support json format not only for syslog but
for boottime logging as well.

Part-of #689

Reviewed-by: Oleg Babin <olegrok@tarantool.org>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>

src/lua/log.lua

@@ -160,8 +160,11 @@ local function log_format(name)
     end
 
     if fmt_str2num[name] == ffi.C.SF_JSON then
-        if ffi.C.log_type() == ffi.C.SAY_LOGGER_SYSLOG then
-            error("log_format: 'json' can't be used with syslog logger")
+        if ffi.C.log_type() == ffi.C.SAY_LOGGER_SYSLOG or
+            ffi.C.log_type() == ffi.C.SAY_LOGGER_BOOT then
+            local m = "log_format: %s can't be used with " ..
+                    "syslog or boot-time logger"
+            error(m:format(fmt_num2str[ffi.C.SF_JSON]))
         end
         ffi.C.say_set_log_format(ffi.C.SF_JSON)
     else

test/app-tap/logger.test.lua

@@ -1,12 +1,13 @@
 #!/usr/bin/env tarantool
 
 local test = require('tap').test('log')
-test:plan(24)
+test:plan(25)
 
 -- gh-3946: Assertion failure when using log_format() before box.cfg()
 local log = require('log')
-log.log_format('json')
 log.log_format('plain')
+_, err = pcall(log.log_format, 'json')
+test:ok(err:find("log_format: json can\'t be used") ~= nil)
 
 --
 -- Check that Tarantool creates ADMIN session for #! script