qcow2: Fix preallocation on images with unaligned sizes

When resizing an image with qcow2_co_truncate() using the falloc or
full preallocation modes the code assumes that both the old and new
sizes are cluster-aligned.

There are two problems with this:

  1) The calculation of how many clusters are involved does not always
     get the right result.

     Example: creating a 60KB image and resizing it (with
     preallocation=full) to 80KB won't allocate the second cluster.

  2) No copy-on-write is performed, so in the previous example if
     there is a backing file then the first 60KB of the first cluster
     won't be filled with data from the backing file.

This patch fixes both issues.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-Id: <20200617140036.20311-1-berto@igalia.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

1 files changed, 24 insertions, 0 deletions

diff --git a/tests/qemu-iotests/125 b/tests/qemu-iotests/125
index d510984..7cb1c19 100755
--- a/tests/qemu-iotests/125
+++ b/tests/qemu-iotests/125
@@ -164,6 +164,30 @@ for GROWTH_SIZE in 16 48 80; do
 done
 done
 
+# Test image resizing using preallocation and unaligned offsets
+$QEMU_IMG create -f raw "$TEST_IMG.base" 128k | _filter_img_create
+$QEMU_IO -c 'write -q -P 1 0 128k' -f raw "$TEST_IMG.base"
+for orig_size in 31k 33k; do
+    echo "--- Resizing image from $orig_size to 96k ---"
+    _make_test_img -F raw -b "$TEST_IMG.base" -o cluster_size=64k "$orig_size"
+    $QEMU_IMG resize -f "$IMGFMT" --preallocation=full "$TEST_IMG" 96k
+    # The first part of the image should contain data from the backing file
+    $QEMU_IO -c "read -q -P 1 0 ${orig_size}" "$TEST_IMG"
+    # The resized part of the image should contain zeroes
+    $QEMU_IO -c "read -q -P 0 ${orig_size} 63k" "$TEST_IMG"
+    # If the image does not have an external data file we can also verify its
+    # actual size. The resized image should have 7 clusters:
+    # header, L1 table, L2 table, refcount table, refcount block, 2 data clusters
+    if ! _get_data_file "$TEST_IMG" > /dev/null; then
+        expected_file_length=$((65536 * 7))
+        file_length=$(stat -c '%s' "$TEST_IMG_FILE")
+        if [ "$file_length" != "$expected_file_length" ]; then
+            echo "ERROR: file length $file_length (expected $expected_file_length)"
+        fi
+    fi
+    echo
+done
+
 # success, all done
 echo '*** done'
 rm -f $seq.full