'qemuDomainPrepareDiskSourceData' propagates 'detect_zeroes' only for
the disk source image, but the mirror destination has the ambition to
replace the disk source when the job is finished, so we need to
propagate the 'detect_zeroes' setting also in that case.

Unfortunately it would become very hairy to either set 'disk->mirror'
sooner or propagate that we want this done into
'qemuDomainPrepareDiskSourceData', so the most straightforward solution
is to do the propagation inside 'qemuDomainBlockCopyCommon'.

Closes: https://gitlab.com/libvirt/libvirt/-/issues/277
Signed-off-by: Peter Krempa <pkre...@redhat.com>
---
 src/qemu/qemu_driver.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/src/qemu/qemu_driver.c b/src/qemu/qemu_driver.c
index e417d358cd..4fc998cde5 100644
--- a/src/qemu/qemu_driver.c
+++ b/src/qemu/qemu_driver.c
@@ -14977,6 +14977,14 @@ qemuDomainBlockCopyCommon(virDomainObj *vm,
             if (qemuDomainPrepareStorageSourceBlockdev(disk, n, priv, cfg) < 0)
                 goto endjob;
         }
+
+        /* 'qemuDomainPrepareStorageSourceBlockdev' calls
+         * 'qemuDomainPrepareDiskSourceData' which propagates 'detect_zeroes'
+         * into the topmost virStorage source of the disk chain.
+         * Since 'mirror' has the ambition to replace it we need to propagate
+         * it into the mirror too. We do it directly as otherwise we'd need
+         * to modify all callers of 'qemuDomainPrepareStorageSourceBlockdev' */
+        mirror->detect_zeroes = disk->detect_zeroes;
     }

     /* If reusing an external image that includes a backing file but the user
-- 
2.35.1

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