block: Fail gracefully when using a format driver on protocol level

Specifying the wrong driver could fail an assertion:

$ qemu-system-x86_64 -drive file.driver=qcow2,file=x
qemu-system-x86_64: block.c:721: bdrv_open_common: Assertion `file !=
((void *)0)' failed.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block.c

@@ -718,6 +718,13 @@ static int bdrv_open_common(BlockDriverState *bs, BlockDriverState *file,
         assert(drv->bdrv_parse_filename || filename != NULL);
         ret = drv->bdrv_file_open(bs, filename, options, open_flags);
     } else {
+        if (file == NULL) {
+            qerror_report(ERROR_CLASS_GENERIC_ERROR, "Can't use '%s' as a "
+                          "block driver for the protocol level",
+                          drv->format_name);
+            ret = -EINVAL;
+            goto free_and_fail;
+        }
         assert(file != NULL);
         bs->file = file;
         ret = drv->bdrv_open(bs, options, open_flags);

tests/qemu-iotests/051

@@ -142,6 +142,13 @@ run_qemu -drive media=cdrom,cache=writethrough
 run_qemu -drive media=cdrom,cache=unsafe
 run_qemu -drive media=cdrom,cache=invalid_value
 
+echo
+echo === Specifying the protocol layer ===
+echo
+
+run_qemu -drive file=$TEST_IMG,file.driver=file
+run_qemu -drive file=$TEST_IMG,file.driver=qcow2
+
 # success, all done
 echo "*** done"
 rm -f $seq.full

tests/qemu-iotests/051.out

@@ -159,4 +159,14 @@ qququiquit
 Testing: -drive media=cdrom,cache=invalid_value
 QEMU_PROG: -drive media=cdrom,cache=invalid_value: invalid cache option
 
+
+=== Specifying the protocol layer ===
+
+Testing: -drive file=TEST_DIR/t.qcow2,file.driver=file
+qququiquit
+
+Testing: -drive file=TEST_DIR/t.qcow2,file.driver=qcow2
+QEMU_PROG: -drive file=TEST_DIR/t.qcow2,file.driver=qcow2: Can't use 'qcow2' as a block driver for the protocol level
+QEMU_PROG: -drive file=TEST_DIR/t.qcow2,file.driver=qcow2: could not open disk image TEST_DIR/t.qcow2: Invalid argument
+
 *** done