qemu-img rebase: use empty string to rebase without backing file

This patch allows an empty filename to be passed as the new base image name
for qemu-img rebase to mean base the image on no backing file (i.e.
independent of any backing file). According to Eric Blake, qemu-img rebase
already supports this when '-u' is used; this adds support when -u is not
used.
Signed-off-by: NAlex Bligh <alex@alex.org.uk>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

qemu-img.c

@@ -1562,13 +1562,15 @@ static int img_rebase(int argc, char **argv)
             error_report("Could not open old backing file '%s'", backing_name);
             goto out;
         }
-
-        bs_new_backing = bdrv_new("new_backing");
-        ret = bdrv_open(bs_new_backing, out_baseimg, BDRV_O_FLAGS,
+        if (out_baseimg[0]) {
+            bs_new_backing = bdrv_new("new_backing");
+            ret = bdrv_open(bs_new_backing, out_baseimg, BDRV_O_FLAGS,
                         new_backing_drv);
-        if (ret) {
-            error_report("Could not open new backing file '%s'", out_baseimg);
-            goto out;
+            if (ret) {
+                error_report("Could not open new backing file '%s'",
+                             out_baseimg);
+                goto out;
+            }
         }
     }
 
@@ -1584,7 +1586,7 @@ static int img_rebase(int argc, char **argv)
     if (!unsafe) {
         uint64_t num_sectors;
         uint64_t old_backing_num_sectors;
-        uint64_t new_backing_num_sectors;
+        uint64_t new_backing_num_sectors = 0;
         uint64_t sector;
         int n;
         uint8_t * buf_old;
@@ -1596,7 +1598,9 @@ static int img_rebase(int argc, char **argv)
 
         bdrv_get_geometry(bs, &num_sectors);
         bdrv_get_geometry(bs_old_backing, &old_backing_num_sectors);
-        bdrv_get_geometry(bs_new_backing, &new_backing_num_sectors);
+        if (bs_new_backing) {
+            bdrv_get_geometry(bs_new_backing, &new_backing_num_sectors);
+        }
 
         if (num_sectors != 0) {
             local_progress = (float)100 /
@@ -1636,7 +1640,7 @@ static int img_rebase(int argc, char **argv)
                 }
             }
 
-            if (sector >= new_backing_num_sectors) {
+            if (sector >= new_backing_num_sectors || !bs_new_backing) {
                 memset(buf_new, 0, n * BDRV_SECTOR_SIZE);
             } else {
                 if (sector + n > new_backing_num_sectors) {
@@ -1682,7 +1686,12 @@ static int img_rebase(int argc, char **argv)
      * backing file are overwritten in the COW file now, so the visible content
      * doesn't change when we switch the backing file.
      */
-    ret = bdrv_change_backing_file(bs, out_baseimg, out_basefmt);
+    if (out_baseimg && *out_baseimg) {
+        ret = bdrv_change_backing_file(bs, out_baseimg, out_basefmt);
+    } else {
+        ret = bdrv_change_backing_file(bs, NULL, NULL);
+    }
+
     if (ret == -ENOSPC) {
         error_report("Could not change the backing file to '%s': No "
                      "space left in the file header", out_baseimg);

qemu-img.texi

@@ -148,7 +148,9 @@ Changes the backing file of an image. Only the formats @code{qcow2} and
 
 The backing file is changed to @var{backing_file} and (if the image format of
 @var{filename} supports this) the backing file format is changed to
-@var{backing_fmt}.
+@var{backing_fmt}. If @var{backing_file} is specified as ``'' (the empty
+string), then the image is rebased onto no backing file (i.e. it will exist
+independently of any backing file).
 
 There are two different modes in which @code{rebase} can operate:
 @table @option