chg: document why we send SIGHUP and SIGINT to process group

This makes the code more consistent - other signals are documented.

contrib/chg/procutil.c

@@ -1,227 +1,231
 /*
  * Utilities about process handling - signal and subprocess (ex. pager)
  *
  * Copyright (c) 2011 Yuya Nishihara <[e-mail]>
  *
  * This software may be used and distributed according to the terms of the
  * GNU General Public License version 2 or any later version.
  */
 
 #include <assert.h>
 #include <errno.h>
 #include <signal.h>
 #include <stdio.h>
 #include <string.h>
 #include <sys/wait.h>
 #include <unistd.h>
 
 #include "procutil.h"
 #include "util.h"
 
 static pid_t pagerpid = 0;
 static pid_t peerpgid = 0;
 static pid_t peerpid = 0;
 
 static void forwardsignal(int sig)
 {
 	assert(peerpid > 0);
 	if (kill(peerpid, sig) < 0)
 		abortmsgerrno("cannot kill %d", peerpid);
 	debugmsg("forward signal %d", sig);
 }
 
 static void forwardsignaltogroup(int sig)
 {
 	/* prefer kill(-pgid, sig), fallback to pid if pgid is invalid */
 	pid_t killpid = peerpgid > 1 ? -peerpgid : peerpid;
 	if (kill(killpid, sig) < 0)
 		abortmsgerrno("cannot kill %d", killpid);
 	debugmsg("forward signal %d to %d", sig, killpid);
 }
 
 static void handlestopsignal(int sig)
 {
 	sigset_t unblockset, oldset;
 	struct sigaction sa, oldsa;
 	if (sigemptyset(&unblockset) < 0)
 		goto error;
 	if (sigaddset(&unblockset, sig) < 0)
 		goto error;
 	memset(&sa, 0, sizeof(sa));
 	sa.sa_handler = SIG_DFL;
 	sa.sa_flags = SA_RESTART;
 	if (sigemptyset(&sa.sa_mask) < 0)
 		goto error;
 
 	forwardsignal(sig);
 	if (raise(sig) < 0)  /* resend to self */
 		goto error;
 	if (sigaction(sig, &sa, &oldsa) < 0)
 		goto error;
 	if (sigprocmask(SIG_UNBLOCK, &unblockset, &oldset) < 0)
 		goto error;
 	/* resent signal will be handled before sigprocmask() returns */
 	if (sigprocmask(SIG_SETMASK, &oldset, NULL) < 0)
 		goto error;
 	if (sigaction(sig, &oldsa, NULL) < 0)
 		goto error;
 	return;
 
 error:
 	abortmsgerrno("failed to handle stop signal");
 }
 
 static void handlechildsignal(int sig UNUSED_)
 {
 	if (peerpid == 0 || pagerpid == 0)
 		return;
 	/* if pager exits, notify the server with SIGPIPE immediately.
 	 * otherwise the server won't get SIGPIPE if it does not write
 	 * anything. (issue5278) */
 	if (waitpid(pagerpid, NULL, WNOHANG) == pagerpid)
 		kill(peerpid, SIGPIPE);
 }
 
 void setupsignalhandler(pid_t pid, pid_t pgid)
 {
 	if (pid <= 0)
 		return;
 	peerpid = pid;
 	peerpgid = (pgid <= 1 ? 0 : pgid);
 
 	struct sigaction sa;
 	memset(&sa, 0, sizeof(sa));
+
+	/* deadly signals meant to be sent to a process group:
+	 * - SIGHUP: usually generated by the kernel, when termination of a
+	 *   process causes that process group to become orphaned
+	 * - SIGINT: usually generated by the terminal */
 	sa.sa_handler = forwardsignaltogroup;
 	sa.sa_flags = SA_RESTART;
 	if (sigemptyset(&sa.sa_mask) < 0)
 		goto error;
-
 	if (sigaction(SIGHUP, &sa, NULL) < 0)
 		goto error;
 	if (sigaction(SIGINT, &sa, NULL) < 0)
 		goto error;
 
 	/* terminate frontend by double SIGTERM in case of server freeze */
 	sa.sa_handler = forwardsignal;
 	sa.sa_flags |= SA_RESETHAND;
 	if (sigaction(SIGTERM, &sa, NULL) < 0)
 		goto error;
 
 	/* notify the worker about window resize events */
 	sa.sa_flags = SA_RESTART;
 	if (sigaction(SIGWINCH, &sa, NULL) < 0)
 		goto error;
 	/* propagate job control requests to worker */
 	sa.sa_handler = forwardsignal;
 	sa.sa_flags = SA_RESTART;
 	if (sigaction(SIGCONT, &sa, NULL) < 0)
 		goto error;
 	sa.sa_handler = handlestopsignal;
 	sa.sa_flags = SA_RESTART;
 	if (sigaction(SIGTSTP, &sa, NULL) < 0)
 		goto error;
 	/* get notified when pager exits */
 	sa.sa_handler = handlechildsignal;
 	sa.sa_flags = SA_RESTART;
 	if (sigaction(SIGCHLD, &sa, NULL) < 0)
 		goto error;
 
 	return;
 
 error:
 	abortmsgerrno("failed to set up signal handlers");
 }
 
 void restoresignalhandler(void)
 {
 	struct sigaction sa;
 	memset(&sa, 0, sizeof(sa));
 	sa.sa_handler = SIG_DFL;
 	sa.sa_flags = SA_RESTART;
 	if (sigemptyset(&sa.sa_mask) < 0)
 		goto error;
 
 	if (sigaction(SIGHUP, &sa, NULL) < 0)
 		goto error;
 	if (sigaction(SIGTERM, &sa, NULL) < 0)
 		goto error;
 	if (sigaction(SIGWINCH, &sa, NULL) < 0)
 		goto error;
 	if (sigaction(SIGCONT, &sa, NULL) < 0)
 		goto error;
 	if (sigaction(SIGTSTP, &sa, NULL) < 0)
 		goto error;
 	if (sigaction(SIGCHLD, &sa, NULL) < 0)
 		goto error;
 
 	/* ignore Ctrl+C while shutting down to make pager exits cleanly */
 	sa.sa_handler = SIG_IGN;
 	if (sigaction(SIGINT, &sa, NULL) < 0)
 		goto error;
 
 	peerpid = 0;
 	return;
 
 error:
 	abortmsgerrno("failed to restore signal handlers");
 }
 
 /* This implementation is based on hgext/pager.py (post 369741ef7253)
  * Return 0 if pager is not started, or pid of the pager */
 pid_t setuppager(const char *pagercmd)
 {
 	assert(pagerpid == 0);
 	if (!pagercmd)
 		return 0;
 
 	int pipefds[2];
 	if (pipe(pipefds) < 0)
 		return 0;
 	pid_t pid = fork();
 	if (pid < 0)
 		goto error;
 	if (pid > 0) {
 		close(pipefds[0]);
 		if (dup2(pipefds[1], fileno(stdout)) < 0)
 			goto error;
 		if (isatty(fileno(stderr))) {
 			if (dup2(pipefds[1], fileno(stderr)) < 0)
 				goto error;
 		}
 		close(pipefds[1]);
 		pagerpid = pid;
 		return pid;
 	} else {
 		dup2(pipefds[0], fileno(stdin));
 		close(pipefds[0]);
 		close(pipefds[1]);
 
 		int r = execlp("/bin/sh", "/bin/sh", "-c", pagercmd, NULL);
 		if (r < 0) {
 			abortmsgerrno("cannot start pager '%s'", pagercmd);
 		}
 		return 0;
 	}
 
 error:
 	close(pipefds[0]);
 	close(pipefds[1]);
 	abortmsgerrno("failed to prepare pager");
 	return 0;
 }
 
 void waitpager(void)
 {
 	if (pagerpid == 0)
 		return;
 
 	/* close output streams to notify the pager its input ends */
 	fclose(stdout);
 	fclose(stderr);
 	while (1) {
 		pid_t ret = waitpid(pagerpid, NULL, 0);
 		if (ret == -1 && errno == EINTR)
 			continue;
 		break;
 	}
 }