qcow2: Prohibit backing file changes in 'qemu-img amend'

This was deprecated back in bc5ee6da7 (qcow2: Deprecate use of
qemu-img amend to change backing file), and no one in the meantime has
given any reasons why it should be supported.  Time to make change
attempts a hard error (but for convenience, specifying the _same_
backing chain is not forbidden).  Update a couple of iotests to match.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210503213600.569128-2-eblake@redhat.com>
Reviewed-by: Connor Kuehl <ckuehl@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/qcow2.c

@@ -5620,15 +5620,10 @@ static int qcow2_amend_options(BlockDriverState *bs, QemuOpts *opts,
     if (backing_file || backing_format) {
         if (g_strcmp0(backing_file, s->image_backing_file) ||
             g_strcmp0(backing_format, s->image_backing_format)) {
-            warn_report("Deprecated use of amend to alter the backing file; "
+            error_setg(errp, "Cannot amend the backing file");
-                        "use qemu-img rebase instead");
+            error_append_hint(errp,
-        }
+                              "You can use 'qemu-img rebase' instead.\n");
-        ret = qcow2_change_backing_file(bs,
+            return -EINVAL;
-                    backing_file ?: s->image_backing_file,
-                    backing_format ?: s->image_backing_format);
-        if (ret < 0) {
-            error_setg_errno(errp, -ret, "Failed to change the backing file");
-            return ret;
         }
     }
 

docs/system/deprecated.rst

@@ -282,18 +282,6 @@ this CPU is also deprecated.
 Related binaries
 ----------------
 
-qemu-img amend to adjust backing file (since 5.1)
-'''''''''''''''''''''''''''''''''''''''''''''''''
-
-The use of ``qemu-img amend`` to modify the name or format of a qcow2
-backing image is deprecated; this functionality was never fully
-documented or tested, and interferes with other amend operations that
-need access to the original backing image (such as deciding whether a
-v3 zero cluster may be left unallocated when converting to a v2
-image).  Rather, any changes to the backing chain should be performed
-with ``qemu-img rebase -u`` either before or after the remaining
-changes being performed by amend, as appropriate.
-
 qemu-img backing file without format (since 5.1)
 ''''''''''''''''''''''''''''''''''''''''''''''''
 

docs/system/removed-features.rst

@@ -491,6 +491,18 @@ topologies described with -smp include all possible cpus, i.e.
 The ``enforce-config-section`` property was replaced by the
 ``-global migration.send-configuration={on|off}`` option.
 
+qemu-img amend to adjust backing file (removed in 6.1)
+''''''''''''''''''''''''''''''''''''''''''''''''''''''
+
+The use of ``qemu-img amend`` to modify the name or format of a qcow2
+backing image was never fully documented or tested, and interferes
+with other amend operations that need access to the original backing
+image (such as deciding whether a v3 zero cluster may be left
+unallocated when converting to a v2 image).  Any changes to the
+backing chain should be performed with ``qemu-img rebase -u`` either
+before or after the remaining changes being performed by amend, as
+appropriate.
+
 Block devices
 -------------
 

tests/qemu-iotests/061

@@ -167,6 +167,9 @@ _make_test_img -o "compat=1.1" 64M
 TEST_IMG="$TEST_IMG.base" _make_test_img -o "compat=1.1" 64M
 $QEMU_IO -c "write -P 0x2a 0 128k" "$TEST_IMG.base" | _filter_qemu_io
 $QEMU_IO -c "read -P 0 0 128k" "$TEST_IMG" | _filter_qemu_io
+$QEMU_IMG amend -o "backing_file=$TEST_IMG.base,backing_fmt=qcow2" \
+	  "$TEST_IMG" && echo "unexpected pass"
+$QEMU_IMG rebase -u -b "$TEST_IMG.base" -F qcow2 "$TEST_IMG"
 $QEMU_IMG amend -o "backing_file=$TEST_IMG.base,backing_fmt=qcow2" "$TEST_IMG"
 $QEMU_IO -c "read -P 0x2a 0 128k" "$TEST_IMG" | _filter_qemu_io
 _check_test_img

tests/qemu-iotests/061.out

@@ -370,7 +370,8 @@ wrote 131072/131072 bytes at offset 0
 128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 read 131072/131072 bytes at offset 0
 128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-qemu-img: warning: Deprecated use of amend to alter the backing file; use qemu-img rebase instead
+qemu-img: Cannot amend the backing file
+You can use 'qemu-img rebase' instead.
 read 131072/131072 bytes at offset 0
 128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 No errors were found on the image.

tests/qemu-iotests/082.out

@@ -808,12 +808,14 @@ Amend options for 'qcow2':
   size=<size>            - Virtual disk size
 
 Testing: amend -f qcow2 -o backing_file=TEST_DIR/t.qcow2,,help TEST_DIR/t.qcow2
-qemu-img: warning: Deprecated use of amend to alter the backing file; use qemu-img rebase instead
+qemu-img: Cannot amend the backing file
+You can use 'qemu-img rebase' instead.
 
 Testing: rebase -u -b  -f qcow2 TEST_DIR/t.qcow2
 
 Testing: amend -f qcow2 -o backing_file=TEST_DIR/t.qcow2,,? TEST_DIR/t.qcow2
-qemu-img: warning: Deprecated use of amend to alter the backing file; use qemu-img rebase instead
+qemu-img: Cannot amend the backing file
+You can use 'qemu-img rebase' instead.
 
 Testing: rebase -u -b  -f qcow2 TEST_DIR/t.qcow2