graphd might block at exit

[kikimo]

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Describe the bug (required)

Take a look at this issue #3434 , after reviewing GraphDaemon's code, we suspect that it may suffer from the same problem since it call gServer->stop() in signal handler(gServer is a ThriftServer instance, if the handler is triggered and scheduled to execute in one of the ThriftServer's worker thread, it may block forever):

nebula/src/daemons/GraphDaemon.cpp

Lines 199 to 209 in cd2f4a6

	void signalHandler(int sig) {
	switch (sig) {
	case SIGINT:
	case SIGTERM:
	FLOG_INFO("Signal %d(%s) received, stopping this server", sig, ::strsignal(sig));
	gServer->stop();
	break;
	default:
	FLOG_ERROR("Signal %d(%s) received but ignored", sig, ::strsignal(sig));
	}
	}

Your Environments (required)

• OS: uname -a
• Compiler: g++ --version or clang++ --version
• CPU: lscpu
• Commit id (e.g. a3ffc7d8)

How To Reproduce(required)

Steps to reproduce the behavior:

1. Step 1
2. Step 2
3. Step 3

Expected behavior

Additional context