Commit 9e6bdef2 authored by Marc-André Lureau's avatar Marc-André Lureau Committed by Paolo Bonzini
Browse files

util: add qemu_write_pidfile() 



There are variants of qemu_create_pidfile() in qemu-pr-helper and
qemu-ga. Let's have a common implementation in libqemuutil.

The code is initially based from pr-helper write_pidfile(), with
various improvements and suggestions from Daniel Berrangé:

  QEMU will leave the pidfile existing on disk when it exits which
  initially made me think it avoids the deletion race. The app
  managing QEMU, however, may well delete the pidfile after it has
  seen QEMU exit, and even if the app locks the pidfile before
  deleting it, there is still a race.

  eg consider the following sequence

        QEMU 1        libvirtd        QEMU 2

  1.    lock(pidfile)

  2.    exit()

  3.                 open(pidfile)

  4.                 lock(pidfile)

  5.                                  open(pidfile)

  6.                 unlink(pidfile)

  7.                 close(pidfile)

  8.                                  lock(pidfile)

  IOW, at step 8 the new QEMU has successfully acquired the lock, but
  the pidfile no longer exists on disk because it was deleted after
  the original QEMU exited.

  While we could just say no external app should ever delete the
  pidfile, I don't think that is satisfactory as people don't read
  docs, and admins don't like stale pidfiles being left around on
  disk.

  To make this robust, I think we might want to copy libvirt's
  approach to pidfile acquisition which runs in a loop and checks that
  the file on disk /after/ acquiring the lock matches the file that
  was locked. Then we could in fact safely let QEMU delete its own
  pidfiles on clean exit..

Signed-off-by: default avatarMarc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20180831145314.14736-2-marcandre.lureau@redhat.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 71bb4ce1

include/qemu/osdep.h

+2 −1
Original line number Diff line number Diff line
@@ -448,7 +448,8 @@ bool qemu_has_ofd_lock(void); 
#define FMT_pid "%d"

#endif



int qemu_create_pidfile(const char *filename);

bool qemu_write_pidfile(const char *pidfile, Error **errp);



int qemu_get_thread_id(void);



#ifndef CONFIG_IOVEC

os-posix.c

+0 −24
Original line number Diff line number Diff line
@@ -344,30 +344,6 @@ void os_set_line_buffering(void) 
    setvbuf(stdout, NULL, _IOLBF, 0);

}



int qemu_create_pidfile(const char *filename)

{

    char buffer[128];

    int len;

    int fd;



    fd = qemu_open(filename, O_RDWR | O_CREAT, 0600);

    if (fd == -1) {

        return -1;

    }

    if (lockf(fd, F_TLOCK, 0) == -1) {

        close(fd);

        return -1;

    }

    len = snprintf(buffer, sizeof(buffer), FMT_pid "\n", getpid());

    if (write(fd, buffer, len) != len) {

        close(fd);

        return -1;

    }



    /* keep pidfile open & locked forever */

    return 0;

}



bool is_daemonized(void)

{

    return daemonize;

os-win32.c

+0 −25
Original line number Diff line number Diff line
@@ -97,28 +97,3 @@ int os_parse_cmd_args(int index, const char *optarg) 
{

    return -1;

}
 


int qemu_create_pidfile(const char *filename)

{

    char buffer[128];

    int len;

    HANDLE file;

    OVERLAPPED overlap;

    BOOL ret;

    memset(&overlap, 0, sizeof(overlap));



    file = CreateFile(filename, GENERIC_WRITE, FILE_SHARE_READ, NULL,

                      OPEN_ALWAYS, FILE_ATTRIBUTE_NORMAL, NULL);



    if (file == INVALID_HANDLE_VALUE) {

        return -1;

    }

    len = snprintf(buffer, sizeof(buffer), "%d\n", getpid());

    ret = WriteFile(file, (LPCVOID)buffer, (DWORD)len,

                    NULL, &overlap);

    CloseHandle(file);

    if (ret == 0) {

        return -1;

    }

    return 0;

}

qga/main.c

+10 −44
Original line number Diff line number Diff line
@@ -340,46 +340,6 @@ static FILE *ga_open_logfile(const char *logfile) 
    return f;

}



#ifndef _WIN32

static bool ga_open_pidfile(const char *pidfile)

{

    int pidfd;

    char pidstr[32];



    pidfd = qemu_open(pidfile, O_CREAT|O_WRONLY, S_IRUSR|S_IWUSR);

    if (pidfd == -1 || lockf(pidfd, F_TLOCK, 0)) {

        g_critical("Cannot lock pid file, %s", strerror(errno));

        if (pidfd != -1) {

            close(pidfd);

        }

        return false;

    }



    if (ftruncate(pidfd, 0)) {

        g_critical("Failed to truncate pid file");

        goto fail;

    }

    snprintf(pidstr, sizeof(pidstr), "%d\n", getpid());

    if (write(pidfd, pidstr, strlen(pidstr)) != strlen(pidstr)) {

        g_critical("Failed to write pid file");

        goto fail;

    }



    /* keep pidfile open & locked forever */

    return true;



fail:

    unlink(pidfile);

    close(pidfd);

    return false;

}

#else /* _WIN32 */

static bool ga_open_pidfile(const char *pidfile)

{

    return true;

}

#endif



static gint ga_strcmp(gconstpointer str1, gconstpointer str2)

{

    return strcmp(str1, str2);
@@ -479,8 +439,11 @@ void ga_unset_frozen(GAState *s) 
    ga_enable_logging(s);

    g_warning("logging re-enabled due to filesystem unfreeze");

    if (s->deferred_options.pid_filepath) {

        if (!ga_open_pidfile(s->deferred_options.pid_filepath)) {

            g_warning("failed to create/open pid file");

        Error *err = NULL;



        if (!qemu_write_pidfile(s->deferred_options.pid_filepath, &err)) {

            g_warning("%s", error_get_pretty(err));

            error_free(err);

        }

        s->deferred_options.pid_filepath = NULL;

    }
@@ -515,8 +478,11 @@ static void become_daemon(const char *pidfile) 
    }



    if (pidfile) {

        if (!ga_open_pidfile(pidfile)) {

            g_critical("failed to create pidfile");

        Error *err = NULL;



        if (!qemu_write_pidfile(pidfile, &err)) {

            g_critical("%s", error_get_pretty(err));

            error_free(err);

            exit(EXIT_FAILURE);

        }

    }

scsi/qemu-pr-helper.c

+5 −35
Original line number Diff line number Diff line
@@ -117,39 +117,6 @@ QEMU_COPYRIGHT "\n" 
    , name);

}



static void write_pidfile(void)

{

    int pidfd;

    char pidstr[32];



    pidfd = qemu_open(pidfile, O_CREAT|O_WRONLY, S_IRUSR|S_IWUSR);

    if (pidfd == -1) {

        error_report("Cannot open pid file, %s", strerror(errno));

        exit(EXIT_FAILURE);

    }



    if (lockf(pidfd, F_TLOCK, 0)) {

        error_report("Cannot lock pid file, %s", strerror(errno));

        goto fail;

    }

    if (ftruncate(pidfd, 0)) {

        error_report("Failed to truncate pid file");

        goto fail;

    }



    snprintf(pidstr, sizeof(pidstr), "%d\n", getpid());

    if (write(pidfd, pidstr, strlen(pidstr)) != strlen(pidstr)) {

        error_report("Failed to write pid file");

        goto fail;

    }

    return;



fail:

    unlink(pidfile);

    close(pidfd);

    exit(EXIT_FAILURE);

}



/* SG_IO support */



typedef struct PRHelperSGIOData {
@@ -1080,8 +1047,11 @@ int main(int argc, char **argv) 
        }

    }



    if (daemonize || pidfile_specified)

        write_pidfile();

    if ((daemonize || pidfile_specified) &&

        !qemu_write_pidfile(pidfile, &local_err)) {

        error_report_err(local_err);

        exit(EXIT_FAILURE);

    }



#ifdef CONFIG_LIBCAP

    if (drop_privileges() < 0) {