[PATCH v2 10/17] qcow2: simple case support for downgrading of qcow2 images with zstd

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

If image doesn't have any compressed cluster we can easily switch to
zlib compression, which may allow to downgrade the image.

That's mostly needed to support IMGOPTS='compression_type=zstd' in some
iotests which do qcow2 downgrade.

While being here also fix checkpatch complain against '#' in printf
formatting.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
---
 block/qcow2.c | 58 +++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 56 insertions(+), 2 deletions(-)

diff --git a/block/qcow2.c b/block/qcow2.c
index 9f1b6461c8..a841fa4c72 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -5250,6 +5250,38 @@ static int qcow2_load_vmstate(BlockDriverState *bs, QEMUIOVector *qiov,
                                         qiov->size, qiov, 0, 0);
 }
 
+static int qcow2_has_compressed_clusters(BlockDriverState *bs)
+{
+    int64_t offset = 0;
+    int64_t bytes = bdrv_getlength(bs);
+
+    if (bytes < 0) {
+        return bytes;
+    }
+
+    while (bytes != 0) {
+        int ret;
+        QCow2SubclusterType type;
+        unsigned int cur_bytes = MIN(INT_MAX, bytes);
+        uint64_t host_offset;
+
+        ret = qcow2_get_host_offset(bs, offset, &cur_bytes, &host_offset,
+                                    &type);
+        if (ret < 0) {
+            return ret;
+        }
+
+        if (type == QCOW2_SUBCLUSTER_COMPRESSED) {
+            return 1;
+        }
+
+        offset += cur_bytes;
+        bytes -= cur_bytes;
+    }
+
+    return 0;
+}
+
 /*
  * Downgrades an image's version. To achieve this, any incompatible features
  * have to be removed.
@@ -5307,9 +5339,10 @@ static int qcow2_downgrade(BlockDriverState *bs, int target_version,
      * the first place; if that happens nonetheless, returning -ENOTSUP is the
      * best thing to do anyway */
 
-    if (s->incompatible_features) {
+    if (s->incompatible_features & ~QCOW2_INCOMPAT_COMPRESSION) {
         error_setg(errp, "Cannot downgrade an image with incompatible features "
-                   "%#" PRIx64 " set", s->incompatible_features);
+                   "0x%" PRIx64 " set",
+                   s->incompatible_features & ~QCOW2_INCOMPAT_COMPRESSION);
         return -ENOTSUP;
     }
 
@@ -5327,6 +5360,27 @@ static int qcow2_downgrade(BlockDriverState *bs, int target_version,
         return ret;
     }
 
+    if (s->incompatible_features & QCOW2_INCOMPAT_COMPRESSION) {
+        ret = qcow2_has_compressed_clusters(bs);
+        if (ret < 0) {
+            error_setg(errp, "Failed to check block status");
+            return -EINVAL;
+        }
+        if (ret) {
+            error_setg(errp, "Cannot downgrade an image with zstd compression "
+                       "type and existing compressed clusters");
+            return -ENOTSUP;
+        }
+        /*
+         * No compressed clusters for now, so just chose default zlib
+         * compression.
+         */
+        s->incompatible_features &= ~QCOW2_INCOMPAT_COMPRESSION;
+        s->compression_type = QCOW2_COMPRESSION_TYPE_ZLIB;
+    }
+
+    assert(s->incompatible_features == 0);
+
     s->qcow_version = target_version;
     ret = qcow2_update_header(bs);
     if (ret < 0) {
-- 
2.29.2