annotate contrib/chg/procutil.c @ 50336:cf4d2f31660d stable

chg: populate CHGHG if not set

Normally, chg determines which `hg` executable to use by first consulting the
`$CHGHG` and `$HG` environment variables, and if neither are present defaults
to the `hg` found in the user's `$PATH`. If built with the `HGPATHREL` compiler
flag, chg will instead assume that there exists an `hg` executable in the same
directory as the `chg` binary and attempt to use that.

This can cause problems in situations where there are multiple actively-used
Mercurial installations on the same system. When a `chg` client connects to a
running command server, the server process performs some basic validation to
determine whether a new command server needs to be spawned. These checks include
things like checking certain "sensitive" environment variables and config
sections, as well as checking whether the mtime of the extensions, hg's
`__version__.py` module, and the Python interpreter have changed.

Crucially, the command server doesn't explicitly check whether the executable it
is running from matches the executable that the `chg` client would have
otherwise invoked had there been no existing command server process. Without
`HGPATHREL`, this still gets implicitly checked during the validation step,
because the only way to specify an alternate hg executable (apart from `$PATH`)
is via the `$CHGHG` and `$HG` environment variables, both of which are checked.

With `HGPATHREL`, however, the command server has no way of knowing which hg
executable the client would have run. This means that a client located at
`/version_B/bin/chg` will happily connect to a command server running
`/version_A/bin/hg` instead of `/version_B/bin/hg` as expected. A simple
solution is to have the client set `$CHGHG` itself, which then allows the
command server's environment validation to work as intended.

I have tested this manually using two locally built hg installations and it
seems to work with no ill effects. That said, I'm not sure how to write an
automated test for this since the `chg` available to the tests isn't even built
with the `HGPATHREL` compiler flag to begin with.

/*
 * 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;
	}
	/* forward user-defined signals */
	if (sigaction(SIGUSR1, &sa, NULL) < 0) {
		goto error;
	}
	if (sigaction(SIGUSR2, &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, const char *envp[])
{
	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 =
		    execle("/bin/sh", "/bin/sh", "-c", pagercmd, NULL, envp);
		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;
	}
}