block: Allow omitting the file name when using driver-specific options

After this patch, using -drive with an empty file name continues to open
the file if driver-specific options are used. If no driver-specific
options are specified, the semantics stay as it was: It defines a drive
without an inserted medium.

In order to achieve this, bdrv_open() must be made safe to work with a
NULL filename parameter. The assumption that is made is that only block
drivers which implement bdrv_parse_filename() support using driver
specific options and could therefore work without a filename. These
drivers must make sure to cope with NULL in their implementation of
.bdrv_open() (this is only NBD for now). For all other drivers, the
block layer code will make sure to error out before calling into their
code - they can't possibly work without a filename.

Now an NBD connection can be opened like this:

  qemu-system-x86_64 -drive file.driver=nbd,file.port=1234,file.host=::1

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

1 files changed, 7 insertions, 3 deletions

diff --git a/blockdev.c b/blockdev.c
index 6f2b7592ce..8cdc9ce16a 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -658,7 +658,11 @@ DriveInfo *drive_init(QemuOpts *all_opts, BlockInterfaceType block_default_type)
         abort();
     }
     if (!file || !*file) {
-        return dinfo;
+        if (qdict_size(bs_opts)) {
+            file = NULL;
+        } else {
+            return dinfo;
+        }
     }
     if (snapshot) {
         /* always use cache=unsafe with snapshot */
@@ -697,10 +701,10 @@ DriveInfo *drive_init(QemuOpts *all_opts, BlockInterfaceType block_default_type)
     if (ret < 0) {
         if (ret == -EMEDIUMTYPE) {
             error_report("could not open disk image %s: not in %s format",
-                         file, drv->format_name);
+                         file ?: dinfo->id, drv->format_name);
         } else {
             error_report("could not open disk image %s: %s",
-                         file, strerror(-ret));
+                         file ?: dinfo->id, strerror(-ret));
         }
         goto err;
     }