[PATCH 26/30] virtiofsd: add --fd=FDNUM fd passing option

["Dr. David Alan Gilbert (git)" <dgilbert@redhat.com>]

From: Stefan Hajnoczi <stefanha@redhat.com>

Although --socket-path=PATH is useful for manual invocations, management
tools typically create the UNIX domain socket themselves and pass it to
the vhost-user device backend.  This way QEMU can be launched
immediately with a valid socket.  No waiting for the vhost-user device
backend is required when fd passing is used.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
---
 contrib/virtiofsd/fuse_i.h        |  1 +
 contrib/virtiofsd/fuse_lowlevel.c | 13 ++++++++---
 contrib/virtiofsd/fuse_virtio.c   | 39 +++++++++++++++++++++++--------
 3 files changed, 40 insertions(+), 13 deletions(-)

diff --git a/contrib/virtiofsd/fuse_i.h b/contrib/virtiofsd/fuse_i.h
index 5b6ef09df5..4b718f1aec 100644
--- a/contrib/virtiofsd/fuse_i.h
+++ b/contrib/virtiofsd/fuse_i.h
@@ -68,6 +68,7 @@ struct fuse_session {
 	size_t bufsize;
 	int error;
 	char *vu_socket_path;
+	int   vu_listen_fd;
 	int   vu_socketfd;
 	struct fv_VuDev *virtio_dev;
 };
diff --git a/contrib/virtiofsd/fuse_lowlevel.c b/contrib/virtiofsd/fuse_lowlevel.c
index b1f44ca17e..da2f151927 100644
--- a/contrib/virtiofsd/fuse_lowlevel.c
+++ b/contrib/virtiofsd/fuse_lowlevel.c
@@ -2077,6 +2077,7 @@ static const struct fuse_opt fuse_ll_opts[] = {
 	LL_OPTION("allow_root", deny_others, 1),
 	LL_OPTION("--socket-path=%s", vu_socket_path, 0),
 	LL_OPTION("vhost_user_socket=%s", vu_socket_path, 0),
+	LL_OPTION("--fd=%d", vu_listen_fd, 0),
 	FUSE_OPT_END
 };
 
@@ -2095,6 +2096,7 @@ void fuse_lowlevel_help(void)
 "    -o allow_root              allow access by root\n"
 "    --socket-path=PATH         path for the vhost-user socket\n"
 "    -o vhost_user_socket=PATH  path for the vhost-user socket\n"
+"    --fd=FDNUM                 fd number of vhost-user socket\n"
 "    -o auto_unmount            auto unmount on process termination\n");
 }
 
@@ -2134,6 +2136,7 @@ struct fuse_session *fuse_session_new(struct fuse_args *args,
 		goto out1;
 	}
 	se->fd = -1;
+	se->vu_listen_fd = -1;
 	se->conn.max_write = UINT_MAX;
 	se->conn.max_readahead = UINT_MAX;
 
@@ -2163,8 +2166,12 @@ struct fuse_session *fuse_session_new(struct fuse_args *args,
 		goto out4;
 	}
 
-	if (!se->vu_socket_path) {
-		fprintf(stderr, "fuse: missing -o vhost_user_socket option\n");
+	if (!se->vu_socket_path && se->vu_listen_fd < 0) {
+		fuse_log(FUSE_LOG_ERR, "fuse: missing --socket-path or --fd option\n");
+		goto out4;
+	}
+	if (se->vu_socket_path && se->vu_listen_fd >= 0) {
+		fuse_log(FUSE_LOG_ERR, "fuse: --socket-path and --fd cannot be given together\n");
 		goto out4;
 	}
 
@@ -2205,7 +2212,7 @@ void fuse_session_unmount(struct fuse_session *se)
 
 int fuse_lowlevel_is_virtio(struct fuse_session *se)
 {
-	return se->vu_socket_path != NULL;
+	return !!se->virtio_dev;
 }
 
 #ifdef linux
diff --git a/contrib/virtiofsd/fuse_virtio.c b/contrib/virtiofsd/fuse_virtio.c
index f52d815596..cc393c3cac 100644
--- a/contrib/virtiofsd/fuse_virtio.c
+++ b/contrib/virtiofsd/fuse_virtio.c
@@ -635,21 +635,20 @@ int virtio_loop(struct fuse_session *se)
     return 0;
 }
 
-int virtio_session_mount(struct fuse_session *se)
+static int fv_create_listen_socket(struct fuse_session *se)
 {
     struct sockaddr_un un;
 
+    /* Nothing to do if fd is already initialized */
+    if (se->vu_listen_fd >= 0) {
+        return 0;
+    }
+
     if (strlen(se->vu_socket_path) >= sizeof(un.sun_path)) {
         fuse_log(FUSE_LOG_ERR, "Socket path too long\n");
         return -1;
     }
 
-    /*
-     * Poison the fuse FD so we spot if we accidentally use it;
-     * DO NOT check for this value, check for fuse_lowlevel_is_virtio()
-     */
-    se->fd = 0xdaff0d11;
-
     /*
      * Create the Unix socket to communicate with qemu
      * based on QEMU's vhost-user-bridge
@@ -675,15 +674,35 @@ int virtio_session_mount(struct fuse_session *se)
         return -1;
     }
 
+    se->vu_listen_fd = listen_sock;
+    return 0;
+}
+
+int virtio_session_mount(struct fuse_session *se)
+{
+    int ret;
+
+    ret = fv_create_listen_socket(se);
+    if (ret < 0) {
+        return ret;
+    }
+
+    /*
+     * Poison the fuse FD so we spot if we accidentally use it;
+     * DO NOT check for this value, check fuse_lowlevel_is_virtio()
+     */
+    se->fd = 0xdaff0d11;
+
     fuse_log(FUSE_LOG_INFO, "%s: Waiting for vhost-user socket connection...\n",
              __func__);
-    int data_sock = accept(listen_sock, NULL, NULL);
+    int data_sock = accept(se->vu_listen_fd, NULL, NULL);
     if (data_sock == -1) {
         fuse_log(FUSE_LOG_ERR, "vhost socket accept: %m\n");
-        close(listen_sock);
+        close(se->vu_listen_fd);
         return -1;
     }
-    close(listen_sock);
+    close(se->vu_listen_fd);
+    se->vu_listen_fd = -1;
     fuse_log(FUSE_LOG_INFO, "%s: Received vhost-user socket connection\n",
              __func__);
     se->vu_socketfd = data_sock;
-- 
2.23.0