block/nfs: Implement bdrv_dirname()

While the basic idea is obvious and could be handled by the default
bdrv_dirname() implementation, we cannot generate a directory name if
the gid or uid are set, so we have to explicitly return NULL in those
cases.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Message-id: 20190201192935.18394-19-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>

block/nfs.c

@@ -855,6 +855,20 @@ static void nfs_refresh_filename(BlockDriverState *bs, QDict *options)
     bs->full_open_options = opts;
 }
 
+static char *nfs_dirname(BlockDriverState *bs, Error **errp)
+{
+    NFSClient *client = bs->opaque;
+
+    if (client->uid || client->gid) {
+        bdrv_refresh_filename(bs);
+        error_setg(errp, "Cannot generate a base directory for NFS node '%s'",
+                   bs->filename);
+        return NULL;
+    }
+
+    return g_strdup_printf("nfs://%s%s/", client->server->host, client->path);
+}
+
 #ifdef LIBNFS_FEATURE_PAGECACHE
 static void coroutine_fn nfs_co_invalidate_cache(BlockDriverState *bs,
                                                  Error **errp)
@@ -889,6 +903,7 @@ static BlockDriver bdrv_nfs = {
     .bdrv_detach_aio_context        = nfs_detach_aio_context,
     .bdrv_attach_aio_context        = nfs_attach_aio_context,
     .bdrv_refresh_filename          = nfs_refresh_filename,
+    .bdrv_dirname                   = nfs_dirname,
 
 #ifdef LIBNFS_FEATURE_PAGECACHE
     .bdrv_co_invalidate_cache       = nfs_co_invalidate_cache,