[Qemu-devel] [PULL 16/21] qemu-img: rebase: Reuse parent BlockDriverState

[Max Reitz <mreitz@redhat.com>]

From: Sam Eiderman <shmuel.eiderman@oracle.com>

In safe mode we open the entire chain, including the parent backing
file of the rebased file.
Do not open a new BlockBackend for the parent backing file, which
saves opening the rest of the chain twice, which for long chains
saves many "pricy" bdrv_open() calls.

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

Reviewed-by: Karl Heubaum <karl.heubaum@oracle.com>
Reviewed-by: Eyal Moscovici <eyal.moscovici@oracle.com>
Signed-off-by: Sagi Amit <sagi.amit@oracle.com>
Co-developed-by: Sagi Amit <sagi.amit@oracle.com>
Signed-off-by: Sam Eiderman <shmuel.eiderman@oracle.com>
Message-id: 20190523163337.4497-2-shmuel.eiderman@oracle.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 qemu-img.c | 29 +++++++++--------------------
 1 file changed, 9 insertions(+), 20 deletions(-)

diff --git a/qemu-img.c b/qemu-img.c
index 28fba1e7a7..9bd0bb1e47 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -3309,29 +3309,18 @@ static int img_rebase(int argc, char **argv)
 
     /* For safe rebasing we need to compare old and new backing file */
     if (!unsafe) {
-        char backing_name[PATH_MAX];
         QDict *options = NULL;
+        BlockDriverState *base_bs = backing_bs(bs);
 
-        if (bs->backing) {
-            if (bs->backing_format[0] != '\0') {
-                options = qdict_new();
-                qdict_put_str(options, "driver", bs->backing_format);
-            }
-
-            if (force_share) {
-                if (!options) {
-                    options = qdict_new();
-                }
-                qdict_put_bool(options, BDRV_OPT_FORCE_SHARE, true);
-            }
-            bdrv_get_backing_filename(bs, backing_name, sizeof(backing_name));
-            blk_old_backing = blk_new_open(backing_name, NULL,
-                                           options, src_flags, &local_err);
-            if (!blk_old_backing) {
+        if (base_bs) {
+            blk_old_backing = blk_new(BLK_PERM_CONSISTENT_READ,
+                                      BLK_PERM_ALL);
+            ret = blk_insert_bs(blk_old_backing, base_bs,
+                                &local_err);
+            if (ret < 0) {
                 error_reportf_err(local_err,
-                                  "Could not open old backing file '%s': ",
-                                  backing_name);
-                ret = -1;
+                                  "Could not reuse old backing file '%s': ",
+                                  base_bs->filename);
                 goto out;
             }
         } else {
-- 
2.21.0