block/blkio: retry blkio_connect() if it fails using `fd`

libblkio 1.3.0 added support of "fd" property for virtio-blk-vhost-vdpa
driver. In QEMU, starting from commit cad2ccc395 ("block/blkio: use
qemu_open() to support fd passing for virtio-blk") we are using
`blkio_get_int(..., "fd")` to check if the "fd" property is supported
for all the virtio-blk-* driver.

Unfortunately that property is also available for those driver that do
not support it, such as virtio-blk-vhost-user.

So, `blkio_get_int()` is not enough to check whether the driver supports
the `fd` property or not. This is because the virito-blk common libblkio
driver only checks whether or not `fd` is set during `blkio_connect()`
and fails with -EINVAL for those transports that do not support it
(all except vhost-vdpa for now).

So let's handle the `blkio_connect()` failure, retrying it using `path`
directly.

Fixes: cad2ccc395 ("block/blkio: use qemu_open() to support fd passing for virtio-blk")
Suggested-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-id: 20230727161020.84213-3-sgarzare@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

block/blkio.c

@@ -733,6 +733,35 @@ static int blkio_virtio_blk_connect(BlockDriverState *bs, QDict *options,
     }
 
     ret = blkio_connect(s->blkio);
+    /*
+     * If the libblkio driver doesn't support the `fd` property, blkio_connect()
+     * will fail with -EINVAL. So let's try calling blkio_connect() again by
+     * directly setting `path`.
+     */
+    if (fd_supported && ret == -EINVAL) {
+        qemu_close(fd);
+
+        /*
+         * We need to clear the `fd` property we set previously by setting
+         * it to -1.
+         */
+        ret = blkio_set_int(s->blkio, "fd", -1);
+        if (ret < 0) {
+            error_setg_errno(errp, -ret, "failed to set fd: %s",
+                             blkio_get_error_msg());
+            return ret;
+        }
+
+        ret = blkio_set_str(s->blkio, "path", path);
+        if (ret < 0) {
+            error_setg_errno(errp, -ret, "failed to set path: %s",
+                             blkio_get_error_msg());
+            return ret;
+        }
+
+        ret = blkio_connect(s->blkio);
+    }
+
     if (ret < 0) {
         error_setg_errno(errp, -ret, "blkio_connect failed: %s",
                          blkio_get_error_msg());