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[libvirt] [PATCH] virNetSocketNewConnectUNIX: Use flocks when spawning a daemon



https://bugzilla.redhat.com/show_bug.cgi?id=1200149

Even though we have a mutex mechanism so that two clients don't spawn
two daemons, it's not strong enough. It can happen that while one
client is spawning the daemon, the other one fails to connect.
Basically two possible errors can happen:

  error: Failed to connect socket to '/home/mprivozn/.cache/libvirt/libvirt-sock': Connection refused

or:

  error: Failed to connect socket to '/home/mprivozn/.cache/libvirt/libvirt-sock': No such file or directory

The problem in both cases is, the daemon is only starting up, while we
are trying to connect (and fail). We should postpone the connecting
phase until the daemon is started (by the other thread that is
spawning it). In order to do that, create a file lock 'libvirt-lock'
in the directory where session daemon would create its socket. So even
when called from multiple processes, spawning a daemon will serialize
on the file lock. So only the first to come will spawn the daemon.

Signed-off-by: Michal Privoznik <mprivozn redhat com>
---

diff to v1:
-hopefully this one is race free

 src/rpc/virnetsocket.c | 148 ++++++++++++-------------------------------------
 1 file changed, 36 insertions(+), 112 deletions(-)

diff --git a/src/rpc/virnetsocket.c b/src/rpc/virnetsocket.c
index 6b019cc..2b891ae 100644
--- a/src/rpc/virnetsocket.c
+++ b/src/rpc/virnetsocket.c
@@ -123,7 +123,7 @@ VIR_ONCE_GLOBAL_INIT(virNetSocket)
 
 
 #ifndef WIN32
-static int virNetSocketForkDaemon(const char *binary, int passfd)
+static int virNetSocketForkDaemon(const char *binary)
 {
     int ret;
     virCommandPtr cmd = virCommandNewArgList(binary,
@@ -136,10 +136,6 @@ static int virNetSocketForkDaemon(const char *binary, int passfd)
     virCommandAddEnvPassBlockSUID(cmd, "XDG_RUNTIME_DIR", NULL);
     virCommandClearCaps(cmd);
     virCommandDaemonize(cmd);
-    if (passfd) {
-        virCommandPassFD(cmd, passfd, VIR_COMMAND_PASS_FD_CLOSE_PARENT);
-        virCommandPassListenFDs(cmd);
-    }
     ret = virCommandRun(cmd, NULL);
     virCommandFree(cmd);
     return ret;
@@ -543,10 +539,10 @@ int virNetSocketNewConnectUNIX(const char *path,
                                const char *binary,
                                virNetSocketPtr *retsock)
 {
-    char *binname = NULL;
-    char *pidpath = NULL;
-    int fd, passfd = -1;
-    int pidfd = -1;
+    char *lockpath = NULL;
+    int lockfd = -1;
+    int fd = -1;
+    int retries = 100;
     virSocketAddr localAddr;
     virSocketAddr remoteAddr;
 
@@ -561,6 +557,22 @@ int virNetSocketNewConnectUNIX(const char *path,
         return -1;
     }
 
+    if (spawnDaemon) {
+        if (virPidFileConstructPath(false, NULL, "libvirt-lock", &lockpath) < 0)
+            goto error;
+
+        if ((lockfd = open(lockpath, O_RDWR | O_CREAT, 0600)) < 0 ||
+            virSetCloseExec(lockfd) < 0) {
+            virReportSystemError(errno, _("Unable to create lock '%s'"), lockpath);
+            goto error;
+        }
+
+        if (virFileLock(lockfd, false, 0, 1, true) < 0) {
+            virReportSystemError(errno, _("Unable to lock '%s'"), lockpath);
+            goto error;
+        }
+    }
+
     if ((fd = socket(PF_UNIX, SOCK_STREAM, 0)) < 0) {
         virReportSystemError(errno, "%s", _("Failed to create socket"));
         goto error;
@@ -574,108 +586,25 @@ int virNetSocketNewConnectUNIX(const char *path,
     if (remoteAddr.data.un.sun_path[0] == '@')
         remoteAddr.data.un.sun_path[0] = '\0';
 
- retry:
-    if (connect(fd, &remoteAddr.data.sa, remoteAddr.len) < 0) {
-        int status = 0;
-        pid_t pid = 0;
-
-        if (!spawnDaemon) {
+    while (retries &&
+           connect(fd, &remoteAddr.data.sa, remoteAddr.len) < 0) {
+        if (!(spawnDaemon && errno == ENOENT)) {
             virReportSystemError(errno, _("Failed to connect socket to '%s'"),
                                  path);
             goto error;
         }
 
-        if (!(binname = last_component(binary)) || binname[0] == '\0') {
-            virReportError(VIR_ERR_INTERNAL_ERROR,
-                           _("Cannot determine basename for binary '%s'"),
-                           binary);
-            goto error;
-        }
-
-        if (virPidFileConstructPath(false, NULL, binname, &pidpath) < 0)
-            goto error;
-
-        pidfd = virPidFileAcquirePath(pidpath, false, getpid());
-        if (pidfd < 0) {
-            /*
-             * This can happen in a very rare case of two clients spawning two
-             * daemons at the same time, and the error in the logs that gets
-             * reset here can be a clue to some future debugging.
-             */
-            virResetLastError();
-            spawnDaemon = false;
-            goto retry;
-        }
-
-        if ((passfd = socket(PF_UNIX, SOCK_STREAM, 0)) < 0) {
-            virReportSystemError(errno, "%s", _("Failed to create socket"));
+        if (virNetSocketForkDaemon(binary) < 0)
             goto error;
-        }
 
-        /*
-         * We already even acquired the pidfile, so no one else should be using
-         * @path right now.  So we're OK to unlink it and paying attention to
-         * the return value makes no real sense here.  Only if it's not an
-         * abstract socket, of course.
-         */
-        if (path[0] != '@')
-            unlink(path);
-
-        /*
-         * We have to fork() here, because umask() is set per-process, chmod()
-         * is racy and fchmod() has undefined behaviour on sockets according to
-         * POSIX, so it doesn't work outside Linux.
-         */
-        if ((pid = virFork()) < 0)
-            goto error;
-
-        if (pid == 0) {
-            umask(0077);
-            if (bind(passfd, &remoteAddr.data.sa, remoteAddr.len) < 0)
-                _exit(EXIT_FAILURE);
-
-            _exit(EXIT_SUCCESS);
-        }
+        retries--;
+        usleep(5000);
+    }
 
-        if (virProcessWait(pid, &status, false) < 0)
-            goto error;
-
-        if (status != EXIT_SUCCESS) {
-            /*
-             * OK, so the child failed to bind() the socket.  This may mean that
-             * another daemon was starting at the same time and succeeded with
-             * its bind() (even though it should not happen because we using a
-             * pidfile for the race).  So we'll try connecting again, but this
-             * time without spawning the daemon.
-             */
-            spawnDaemon = false;
-            virPidFileDeletePath(pidpath);
-            VIR_FORCE_CLOSE(pidfd);
-            VIR_FORCE_CLOSE(passfd);
-            goto retry;
-        }
-
-        if (listen(passfd, 0) < 0) {
-            virReportSystemError(errno, "%s",
-                                 _("Failed to listen on socket that's about "
-                                   "to be passed to the daemon"));
-            goto error;
-        }
-
-        if (connect(fd, &remoteAddr.data.sa, remoteAddr.len) < 0) {
-            virReportSystemError(errno, _("Failed to connect socket to '%s'"),
-                                 path);
-            goto error;
-        }
-
-        /*
-         * Do we need to eliminate the super-rare race here any more?  It would
-         * need incorporating the following VIR_FORCE_CLOSE() into a
-         * virCommandHook inside a virNetSocketForkDaemon().
-         */
-        VIR_FORCE_CLOSE(pidfd);
-        if (virNetSocketForkDaemon(binary, passfd) < 0)
-            goto error;
+    if (lockfd) {
+        unlink(lockpath);
+        VIR_FORCE_CLOSE(lockfd);
+        VIR_FREE(lockpath);
     }
 
     localAddr.len = sizeof(localAddr.data);
@@ -687,19 +616,14 @@ int virNetSocketNewConnectUNIX(const char *path,
     if (!(*retsock = virNetSocketNew(&localAddr, &remoteAddr, true, fd, -1, 0)))
         goto error;
 
-    VIR_FREE(pidpath);
-
     return 0;
 
  error:
-    if (pidfd >= 0)
-        virPidFileDeletePath(pidpath);
-    VIR_FREE(pidpath);
+    if (lockfd)
+        unlink(lockpath);
+    VIR_FREE(lockpath);
     VIR_FORCE_CLOSE(fd);
-    VIR_FORCE_CLOSE(passfd);
-    VIR_FORCE_CLOSE(pidfd);
-    if (spawnDaemon)
-        unlink(path);
+    VIR_FORCE_CLOSE(lockfd);
     return -1;
 }
 #else
-- 
2.0.5


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