view contrib/chg/procutil.c @ 32697:19b9fc40cc51

revlog: skeleton support for version 2 revlogs

There are a number of improvements we want to make to revlogs
that will require a new version - version 2. It is unclear what the
full set of improvements will be or when we'll be done with them.
What I do know is that the process will likely take longer than a
single release, will require input from various stakeholders to
evaluate changes, and will have many contentious debates and
bikeshedding.

It is unrealistic to develop revlog version 2 up front: there
are just too many uncertainties that we won't know until things
are implemented and experiments are run. Some changes will also
be invasive and prone to bit rot, so sitting on dozens of patches
is not practical.

This commit introduces skeleton support for version 2 revlogs in
a way that is flexible and not bound by backwards compatibility
concerns.

An experimental repo requirement for denoting revlog v2 has been
added. The requirement string has a sub-version component to it.
This will allow us to declare multiple requirements in the course
of developing revlog v2. Whenever we change the in-development
revlog v2 format, we can tweak the string, creating a new
requirement and locking out old clients. This will allow us to
make as many backwards incompatible changes and experiments to
revlog v2 as we want. In other words, we can land code and make
meaningful progress towards revlog v2 while still maintaining
extreme format flexibility up until the point we freeze the
format and remove the experimental labels.

To enable the new repo requirement, you must supply an experimental
and undocumented config option. But not just any boolean flag
will do: you need to explicitly use a value that no sane person
should ever type. This is an additional guard against enabling
revlog v2 on an installation it shouldn't be enabled on. The
specific scenario I'm trying to prevent is say a user with a
4.4 client with a frozen format enabling the option but then
downgrading to 4.3 and accidentally creating repos with an
outdated and unsupported repo format. Requiring a "challenge"
string should prevent this.

Because the format is not yet finalized and I don't want to take
any chances, revlog v2's version is currently 0xDEAD. I figure
squatting on a value we're likely never to use as an actual revlog
version to mean "internal testing only" is acceptable. And
"dead" is easily recognized as something meaningful.

There is a bunch of cleanup that is needed before work on revlog
v2 begins in earnest. I plan on doing that work once this patch
is accepted and we're comfortable with the idea of starting down
this path.

/*
 * 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;
	}
}