[PATCH 2/3] block/qed: Keep auto_backing_file if possible

[Hanna Reitz <hreitz@redhat.com>]

Just like qcow2, qed invokes its open function in its
.bdrv_co_invalidate_cache() implementation.  Therefore, just like done
for qcow2 in HEAD^, update auto_backing_file only if the backing file
string in the image header differs from the one we have read before.

Signed-off-by: Hanna Reitz <hreitz@redhat.com>
---
 block/qed.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/block/qed.c b/block/qed.c
index 40943e679b..324ca0e95a 100644
--- a/block/qed.c
+++ b/block/qed.c
@@ -445,6 +445,8 @@ static int coroutine_fn bdrv_qed_do_open(BlockDriverState *bs, QDict *options,
     }
 
     if ((s->header.features & QED_F_BACKING_FILE)) {
+        g_autofree char *backing_file_str = NULL;
+
         if ((uint64_t)s->header.backing_filename_offset +
             s->header.backing_filename_size >
             s->header.cluster_size * s->header.header_size) {
@@ -452,16 +454,21 @@ static int coroutine_fn bdrv_qed_do_open(BlockDriverState *bs, QDict *options,
             return -EINVAL;
         }
 
+        backing_file_str = g_malloc(sizeof(bs->backing_file));
         ret = qed_read_string(bs->file, s->header.backing_filename_offset,
                               s->header.backing_filename_size,
-                              bs->auto_backing_file,
-                              sizeof(bs->auto_backing_file));
+                              backing_file_str, sizeof(bs->backing_file));
         if (ret < 0) {
             error_setg(errp, "Failed to read backing filename");
             return ret;
         }
-        pstrcpy(bs->backing_file, sizeof(bs->backing_file),
-                bs->auto_backing_file);
+
+        if (!g_str_equal(backing_file_str, bs->backing_file)) {
+            pstrcpy(bs->backing_file, sizeof(bs->backing_file),
+                    backing_file_str);
+            pstrcpy(bs->auto_backing_file, sizeof(bs->auto_backing_file),
+                    backing_file_str);
+        }
 
         if (s->header.features & QED_F_BACKING_FORMAT_NO_PROBE) {
             pstrcpy(bs->backing_format, sizeof(bs->backing_format), "raw");
-- 
2.36.1