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From: Gibeom Gwon <gb.gwon@stackframe.dev>
Subject: Handles the masked signal when the thread exits
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Hello,

I am experiencing strange(unintended?) behavior when using pthread with 
signals. If I set the signal mask with pthread_sigmask() in the thread 
function and the process has pending signal when thread is exiting, 
signal handler executed in thread.

It looks like glibc restores original signal mask at the end of the 
thread. So I suspect this eventually triggers masked signal handler. But 
I'm not sure it is intended or not.

Code samples:

sleep.c
-------
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <sys/wait.h>
#include <pthread.h>

pthread_t thread;
pid_t pid;

void stop_threads() {
		pthread_cancel(thread);
		pthread_join(thread, NULL);
}

void sigint_handler(int signum) {
	printf("sigint: %ld\n",pthread_self());
	stop_threads();
	exit(0);
}

void sigchld_handler(int signum) {
	printf("sigchld: %ld\n",pthread_self());
	pid_t pid;
	int status;
	while((pid = waitpid(-1,&status,WNOHANG)) > 0) {}
}

void spawn_sleep() {
	pid = fork();
	if(pid == 0)
		execl("./sleep","sleep",NULL);
}

void* worker(void *arg) {
	sigset_t mask;
	sigfillset(&mask);
	pthread_sigmask(SIG_SETMASK,&mask,NULL);

	pthread_setcancelstate(PTHREAD_CANCEL_DISABLE,NULL);
	int i = 10;
	while(i) {
		printf("worker...\n");
		sleep(1);
		i--;
	}
	pthread_setcancelstate(PTHREAD_CANCEL_ENABLE,NULL);
     	printf("thread close\n");
	pthread_exit(0);
}

void spawn_thread() {
     pthread_create(&thread,NULL,worker,NULL);
     printf("child thread: %ld\n",thread);
}

int main() {
	printf("main thread: %ld\n",pthread_self());
	spawn_sleep();	

	struct sigaction sa1;
	sa1.sa_handler = sigint_handler;
	sigemptyset(&sa1.sa_mask);
	sigaddset(&sa1.sa_mask,SIGCHLD);
	sa1.sa_flags = 0;
	sigaction(SIGINT, &sa1, 0);

	struct sigaction sa2;
	sa2.sa_handler = sigchld_handler;
	sigemptyset(&sa2.sa_mask);
	sa2.sa_flags = 0;
	sigaction(SIGCHLD, &sa2, 0);

	spawn_thread();

	while(1)
		sleep(1);

	return 0;
}


result
======
$ ./signal
main thread: 139882152073024
child thread: 139882152068672
worker...
worker...
worker...
^Csigint: 139882152073024
worker...
worker...
worker...
worker...
worker...
worker...
worker...
thread close
sigchld: 139882152068672

Comprehensive signal mask set in worker() but SIGCHLD handler executed 
after thread function exited.

Regards,
Gibeom Gwon