lock.c

/**
 * \file lock.c cross-platform concurrency locking for fetchmail
 *
 * For license terms, see the file COPYING in this directory.
 */
#include "config.h"
#include "fetchmail.h"

#include <stdio.h>
#include <string.h> /* strcat() */
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <fcntl.h>
#include <signal.h>

#include "i18n.h"
#include "lock.h"

static char *lockfile;		/** name of lockfile */
static int lock_acquired;	/** flag if have we acquired a lock */

void fm_lock_setup(struct runctl *ctl)
/* set up the global lockfile name */
{
    /* set up to do lock protocol */
    const char *const FETCHMAIL_PIDFILE="fetchmail.pid";

    /* command-line option override */
    if (ctl->pidfile) {
	lockfile = xstrdup(ctl->pidfile);
	return;
    }

    /* defaults */
    if (getuid() == ROOT_UID) {
	lockfile = (char *)xmalloc(strlen(PID_DIR)
		+ strlen(FETCHMAIL_PIDFILE) + 2); /* 2: "/" and NUL */
	strcpy(lockfile, PID_DIR);
	strcat(lockfile, "/");
	strcat(lockfile, FETCHMAIL_PIDFILE);
    } else {
	lockfile = (char *)xmalloc(strlen(fmhome)
		+ strlen(FETCHMAIL_PIDFILE) + 3); /* 3: "/", "." and NUL */
	strcpy(lockfile, fmhome);
	strcat(lockfile, "/");
	if (at_home)
	   strcat(lockfile, ".");
	strcat(lockfile, FETCHMAIL_PIDFILE);
    }
}

static void unlockit(void)
/* must-do actions for exit (but we can't count on being able to do malloc) */
{
    if (lockfile && lock_acquired)
	unlink(lockfile) && truncate(lockfile, (off_t)0);
}

void fm_lock_dispose(void)
/* arrange for a lock to be removed on process exit */
{
    atexit(unlockit);
}

int fm_lock_state(void)
{
    long	pid;
    int		st;
    FILE	*lockfp;
    int		bkgd = FALSE;

    if ((lockfp = fopen(lockfile, "r")) != NULL)
    {
	int args = fscanf(lockfp, "%ld %d", &pid, &st);
	bkgd = (args == 2);

	if (ferror(lockfp)) {
	    report(stderr, GT_("fetchmail: error reading lockfile \"%s\": %s\n"),
		    lockfile, strerror(errno));
	    fclose(lockfp); /* not checking should be safe, file mode was "r" */
	    exit(PS_EXCLUDE);
	}
	fclose(lockfp); /* not checking should be safe, file mode was "r" */

	if (args == EOF || args == 0 || kill(pid, 0) == -1) {
	    /* ^ could not read PID  || process does not exist */
	    /* => lockfile is stale, unlink it */
	    pid = 0;
	    report(stderr,GT_("fetchmail: removing stale lockfile \"%s\"\n"), lockfile);
	    if (unlink(lockfile)) {
	       if (errno != ENOENT) {
		   report(stderr, GT_("fetchmail: cannot unlink lockfile \"%s\" (%s), trying to write to it\n"),
			lockfile, strerror(errno));
		   if (outlevel >= O_VERBOSE) {
		       report(stderr, GT_("fetchmail: cannot unlink lockfile \"%s\" (%s), trying to write to it\n"),
			       lockfile, strerror(errno));
		   }
		   /* we complain but we don't exit; it might be
		    * writable for us, but in a directory we cannot
		    * write to. This means we can write the new PID to
		    * the file. Truncate to be safe in case the PID is
		    * recycled by another process later.
		    * \bug we should use fcntl() style locks or
		    * something else instead in a future release. */
		   if (truncate(lockfile, (off_t)0)) {
		       /* but if we cannot truncate the file either,
			* assume that we cannot write to it later,
			* complain and quit. */
		       report(stderr, GT_("fetchmail: cannot write to lockfile \"%s\" either (%s), exiting\n"),
			       strerror(errno), lockfile);
		       exit(PS_EXCLUDE);
		   }
	       }
	    }
	}
    } else {
	pid = 0;
	if (errno != ENOENT) {
	    report(stderr, GT_("fetchmail: error opening lockfile \"%s\": %s\n"),
		    lockfile, strerror(errno));
	    exit(PS_EXCLUDE);
	}
    }

    return(bkgd ? -pid : pid);
}

void fm_lock_assert(void)
/* assert that we already possess a lock */
{
    lock_acquired = TRUE;
}

void fm_lock_or_die(void)
/* get a lock on a given host or exit */
{
    int fd;
    char	tmpbuf[50];

    if (!lock_acquired) {
	int e = 0;

	fd = open(lockfile, O_WRONLY|O_CREAT|O_EXCL, 0666);
	if (fd == -1 && EEXIST == errno) {
		fd = open(lockfile, O_WRONLY|O_TRUNC, 0666);
	}
	if (-1 != fd) {
	    ssize_t wr;

	    snprintf(tmpbuf, sizeof(tmpbuf), "%ld\n", (long)getpid());
	    wr = write(fd, tmpbuf, strlen(tmpbuf));
	    if (wr == -1 || (size_t)wr != strlen(tmpbuf))
	        e = 1;
	    if (run.poll_interval)
	    {
		snprintf(tmpbuf, sizeof(tmpbuf), "%d\n", run.poll_interval);
		wr = write(fd, tmpbuf, strlen(tmpbuf));
		if (wr == -1 || (size_t)wr != strlen(tmpbuf))
		    e = 1;
	    }
	    if (fsync(fd)) e = 1;
	    if (close(fd)) e = 1;
	} else {
	    e = 1;
	}
	if (e == 0) {
	    lock_acquired = TRUE;
	} else {
	    exit(PS_EXCLUDE);
	}
    }
}

void fm_lock_release(void)
/* release a lock on a given host */
{
    if (unlink(lockfile)) {
	    if (truncate(lockfile, (off_t)0)) {
		    report(stderr, GT_("fetchmail: cannot remove or truncate pidfile \"%s\": %s\n"), lockfile, strerror(errno));
	    }
    }
}
/* lock.c ends here */