block/null: Implement bdrv_refresh_filename()

The null block driver ignores any filename used for creating its BDSs,
which allows creating such BDSs even without any filename at all. In
that case, we currently construct a JSON filename when queried instead
of a plain "null-co://" or "null-aio://". This patch implements
bdrv_refresh_filename() to remedy this behavior.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20160610185750.30956-4-mreitz@redhat.com
[mreitz@redhat.com: Added commit message]
Signed-off-by: Max Reitz <mreitz@redhat.com>

block/null.c

@@ -12,6 +12,8 @@
 
 #include "qemu/osdep.h"
 #include "qapi/error.h"
+#include "qapi/qmp/qdict.h"
+#include "qapi/qmp/qstring.h"
 #include "block/block_int.h"
 
 #define NULL_OPT_LATENCY "latency-ns"
@@ -223,6 +225,20 @@ static int64_t coroutine_fn null_co_get_block_status(BlockDriverState *bs,
     }
 }
 
+static void null_refresh_filename(BlockDriverState *bs, QDict *opts)
+{
+    QINCREF(opts);
+    qdict_del(opts, "filename");
+
+    if (!qdict_size(opts)) {
+        snprintf(bs->exact_filename, sizeof(bs->exact_filename), "%s://",
+                 bs->drv->format_name);
+    }
+
+    qdict_put(opts, "driver", qstring_from_str(bs->drv->format_name));
+    bs->full_open_options = opts;
+}
+
 static BlockDriver bdrv_null_co = {
     .format_name            = "null-co",
     .protocol_name          = "null-co",
@@ -238,6 +254,8 @@ static BlockDriver bdrv_null_co = {
     .bdrv_reopen_prepare    = null_reopen_prepare,
 
     .bdrv_co_get_block_status   = null_co_get_block_status,
+
+    .bdrv_refresh_filename  = null_refresh_filename,
 };
 
 static BlockDriver bdrv_null_aio = {
@@ -255,6 +273,8 @@ static BlockDriver bdrv_null_aio = {
     .bdrv_reopen_prepare    = null_reopen_prepare,
 
     .bdrv_co_get_block_status   = null_co_get_block_status,
+
+    .bdrv_refresh_filename  = null_refresh_filename,
 };
 
 static void bdrv_null_init(void)