qcow2: fix image corruption on commit with persistent bitmap

If an image contains persistent bitmaps, we cannot use the
fast path of bdrv_make_empty() to clear the image during
qemu-img commit, because that will lose the clusters related
to the bitmaps.

Also leave a comment in qcow2_read_extensions to remind future
feature additions to think about fast-path removal, since we
just barely fixed the same bug for LUKS encryption.

It's a pain that qemu-img has not yet been taught to manipulate,
or even at a very minimum display, information about persistent
bitmaps; instead, we have to use QMP commands.  It's also a
pain that only qeury-block and x-debug-block-dirty-bitmap-sha256
will allow bitmap introspection; but the former requires the
node to be hooked to a block device, and the latter is experimental.

Signed-off-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 48 insertions, 7 deletions

diff --git a/tests/qemu-iotests/176 b/tests/qemu-iotests/176
index 950b28720e..0f31a20294 100755
--- a/tests/qemu-iotests/176
+++ b/tests/qemu-iotests/176
@@ -3,10 +3,11 @@
 # Commit changes into backing chains and empty the top image if the
 # backing image is not explicitly specified.
 #
-# Variant of 097, which includes snapshots to test different codepath
-# in qcow2
+# Variant of 097, which includes snapshots and persistent bitmaps, to
+# tickle the slow codepath in qcow2. See also 198, for another feature
+# that tickles the slow codepath.
 #
-# Copyright (C) 2014 Red Hat, Inc.
+# Copyright (C) 2014, 2017 Red Hat, Inc.
 #
 # This program is free software; you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by
@@ -43,11 +44,18 @@ trap "_cleanup; exit \$status" 0 1 2 3 15
 . ./common.filter
 . ./common.pattern
 
-# Any format supporting backing files and bdrv_make_empty
+# This test is specific to qcow2
 _supported_fmt qcow2
 _supported_proto file
 _supported_os Linux
 
+function run_qemu()
+{
+    $QEMU -nographic -qmp stdio -serial none "$@" 2>&1 \
+	| _filter_testdir | _filter_qmp | _filter_qemu
+}
+
+for reason in snapshot bitmap; do
 
 # Four passes:
 #  0: Two-layer backing chain, commit to upper backing file (implicitly)
@@ -66,14 +74,29 @@ _supported_os Linux
 for i in 0 1 2 3; do
 
 echo
-echo "=== Test pass $i ==="
+echo "=== Test pass $reason.$i ==="
 echo
 
 len=$((2100 * 1024 * 1024 + 512)) # larger than 2G, and not cluster aligned
 TEST_IMG="$TEST_IMG.base" _make_test_img $len
 TEST_IMG="$TEST_IMG.itmd" _make_test_img -b "$TEST_IMG.base" $len
 _make_test_img -b "$TEST_IMG.itmd" $len
-$QEMU_IMG snapshot -c snap "$TEST_IMG"
+# Update the top image to use a feature that is incompatible with fast path
+case $reason in
+    snapshot) $QEMU_IMG snapshot -c snap "$TEST_IMG" ;;
+    bitmap)
+	run_qemu <<EOF
+{ "execute": "qmp_capabilities" }
+{ "execute": "blockdev-add",
+  "arguments": { "driver": "qcow2", "node-name": "drive0",
+     "file": { "driver": "file", "filename": "$TEST_IMG" } } }
+{ "execute": "block-dirty-bitmap-add",
+  "arguments": { "node": "drive0", "name": "bitmap0",
+     "persistent": true, "autoload": true } }
+{ "execute": "quit" }
+EOF
+	;;
+esac
 
 $QEMU_IO -c "write -P 1 0x7ffd0000 192k" "$TEST_IMG.base" | _filter_qemu_io
 $QEMU_IO -c "write -P 2 0x7ffe0000 128k" "$TEST_IMG.itmd" | _filter_qemu_io
@@ -122,8 +145,26 @@ $QEMU_IMG map "$TEST_IMG.base" | _filter_qemu_img_map
 $QEMU_IMG map "$TEST_IMG.itmd" | _filter_qemu_img_map
 $QEMU_IMG map "$TEST_IMG" | _filter_qemu_img_map
 
-done
+# Check that the reason for slow path is still present, as appropriate
+case $reason in
+    snapshot)
+	$QEMU_IMG snapshot -l "$TEST_IMG" |
+	    sed 's/^\(.\{20\}\).*/\1/; s/ *$//' ;;
+    bitmap)
+	run_qemu <<EOF
+{ "execute": "qmp_capabilities" }
+{ "execute": "blockdev-add",
+  "arguments": { "driver": "qcow2", "node-name": "drive0",
+     "file": { "driver": "file", "filename": "$TEST_IMG" } } }
+{ "execute": "x-debug-block-dirty-bitmap-sha256",
+  "arguments": { "node": "drive0", "name": "bitmap0" } }
+{ "execute": "quit" }
+EOF
+	;;
+esac
 
+done
+done
 
 # success, all done
 echo "*** done"