[Qemu-block] [PATCH v5 3/3] qemu-img: rebase: Reuse in-chain BlockDriverState

[Sam Eiderman]

If a chain was detected, don't open a new BlockBackend from the target
backing file which will create a new BlockDriverState. Instead, create
an empty BlockBackend and attach the already open BlockDriverState.

Permissions for blk_new() were copied from blk_new_open() when
flags = 0.

Reviewed-by: Karl Heubaum <address@hidden>
Reviewed-by: Eyal Moscovici <address@hidden>
Signed-off-by: Sagi Amit <address@hidden>
Co-developed-by: Sagi Amit <address@hidden>
Signed-off-by: Sam Eiderman <address@hidden>
---
 qemu-img.c | 33 +++++++++++++++++++++++----------
 1 file changed, 23 insertions(+), 10 deletions(-)

diff --git a/qemu-img.c b/qemu-img.c
index 2d96a491e2..b0535919b1 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -3358,16 +3358,29 @@ static int img_rebase(int argc, char **argv)
              * in its chain.
              */
             prefix_chain_bs = bdrv_find_backing_image(bs, out_real_path);
-
-            blk_new_backing = blk_new_open(out_real_path, NULL,
-                                           options, src_flags, &local_err);
-            g_free(out_real_path);
-            if (!blk_new_backing) {
-                error_reportf_err(local_err,
-                                  "Could not open new backing file '%s': ",
-                                  out_baseimg);
-                ret = -1;
-                goto out;
+            if (prefix_chain_bs) {
+                g_free(out_real_path);
+                blk_new_backing = blk_new(BLK_PERM_CONSISTENT_READ,
+                                          BLK_PERM_ALL);
+                ret = blk_insert_bs(blk_new_backing, prefix_chain_bs,
+                                    &local_err);
+                if (ret < 0) {
+                    error_reportf_err(local_err,
+                                      "Could not reuse backing file '%s': ",
+                                      out_baseimg);
+                    goto out;
+                }
+            } else {
+                blk_new_backing = blk_new_open(out_real_path, NULL,
+                                               options, src_flags, &local_err);
+                g_free(out_real_path);
+                if (!blk_new_backing) {
+                    error_reportf_err(local_err,
+                                      "Could not open new backing file '%s': ",
+                                      out_baseimg);
+                    ret = -1;
+                    goto out;
+                }
             }
         }
     }
-- 
2.13.3