Re: [Qemu-devel] [PATCH] qemu-img rebase: allow backing file to be specified as '-'

[Alex Bligh]

Eric,

--On 15 October 2012 12:11:02 -0600 Eric Blake <address@hidden> wrote:

> On the other hand, if you don't use -u, then qemu-img complains:
>
> $ qemu-img rebase  -b ''  bar
> qemu-img: Could not open new backing file ''
>
> So I think a better patch would be to allow rebase-by-pull to work the
> same as unsafe rebase, by honoring the empty string as a request to pull
> the entire chain into the destination and leave no backing file.

Yes, that's exactly the case we're missing. How about the attached (as
a single patch)

Also available at:
 https://github.com/abligh/qemu
as commits
 https://github.com/abligh/qemu/commit/4cce9c961fa52a71bd6520a9c499f4dc4b174b97
and
 https://github.com/abligh/qemu/commit/4d5b3b431d8dd276f4c564d8a82c6d25cb111381

--
Alex Bligh

diff --git a/qemu-img.c b/qemu-img.c
index f17f187..7a4e73f 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -1558,13 +1558,14 @@ 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;
+            }
        }
    }

@@ -1580,7 +1581,7 @@ 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;
@@ -1592,7 +1593,8 @@ 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);

        local_progress = (float)100 /
            (num_sectors / MIN(num_sectors, IO_BUF_SIZE / 512));
@@ -1629,7 +1631,7 @@ 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) {
@@ -1675,7 +1677,11 @@ 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 (bs_new_backing)
+        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);
diff --git a/qemu-img.texi b/qemu-img.texi
index 8b05f2c..42ec392 100644
--- a/qemu-img.texi
+++ b/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
address@hidden
address@hidden 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