virtiofsd: Prevent multiply running with same vhost_user_socket

virtiofsd can run multiply even if the vhost_user_socket is same path.

  ]# ./virtiofsd -o vhost_user_socket=/tmp/vhostqemu -o source=/tmp/share &
  [1] 244965
  virtio_session_mount: Waiting for vhost-user socket connection...
  ]# ./virtiofsd -o vhost_user_socket=/tmp/vhostqemu -o source=/tmp/share &
  [2] 244966
  virtio_session_mount: Waiting for vhost-user socket connection...
  ]#

The user will get confused about the situation and maybe the cause of the
unexpected problem. So it's better to prevent the multiple running.

Create a regular file under localstatedir directory to exclude the
vhost_user_socket. To create and lock the file, use qemu_write_pidfile()
because the API has some sanity checks and file lock.

Signed-off-by: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
  Applied fixes from Stefan's review and moved osdep include
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

tools/virtiofsd/fuse_lowlevel.c

@@ -18,6 +18,7 @@
 
 #include <assert.h>
 #include <errno.h>
+#include <glib.h>
 #include <limits.h>
 #include <stdbool.h>
 #include <stddef.h>

tools/virtiofsd/fuse_virtio.c

@@ -13,11 +13,12 @@
 
 #include "qemu/osdep.h"
 #include "qemu/iov.h"
-#include "fuse_virtio.h"
+#include "qapi/error.h"
 #include "fuse_i.h"
 #include "standard-headers/linux/fuse.h"
 #include "fuse_misc.h"
 #include "fuse_opt.h"
+#include "fuse_virtio.h"
 
 #include <assert.h>
 #include <errno.h>
@@ -743,6 +744,42 @@ int virtio_loop(struct fuse_session *se)
     return 0;
 }
 
+static void strreplace(char *s, char old, char new)
+{
+    for (; *s; ++s) {
+        if (*s == old) {
+            *s = new;
+        }
+    }
+}
+
+static bool fv_socket_lock(struct fuse_session *se)
+{
+    g_autofree gchar *sk_name = NULL;
+    g_autofree gchar *pidfile = NULL;
+    g_autofree gchar *dir = NULL;
+    Error *local_err = NULL;
+
+    dir = qemu_get_local_state_pathname("run/virtiofsd");
+
+    if (g_mkdir_with_parents(dir, S_IRWXU) < 0) {
+        fuse_log(FUSE_LOG_ERR, "%s: Failed to create directory %s: %s",
+                 __func__, dir, strerror(errno));
+        return false;
+    }
+
+    sk_name = g_strdup(se->vu_socket_path);
+    strreplace(sk_name, '/', '.');
+    pidfile = g_strdup_printf("%s/%s.pid", dir, sk_name);
+
+    if (!qemu_write_pidfile(pidfile, &local_err)) {
+        error_report_err(local_err);
+        return false;
+    }
+
+    return true;
+}
+
 static int fv_create_listen_socket(struct fuse_session *se)
 {
     struct sockaddr_un un;
@@ -758,6 +795,16 @@ static int fv_create_listen_socket(struct fuse_session *se)
         return -1;
     }
 
+    if (!strlen(se->vu_socket_path)) {
+        fuse_log(FUSE_LOG_ERR, "Socket path is empty\n");
+        return -1;
+    }
+
+    /* Check the vu_socket_path is already used */
+    if (!fv_socket_lock(se)) {
+        return -1;
+    }
+
     /*
      * Create the Unix socket to communicate with qemu
      * based on QEMU's vhost-user-bridge