qcow2: simple case support for downgrading of qcow2 images with zstd

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>
Message-Id: <20211223160144.1097696-13-vsementsov@virtuozzo.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>

block/qcow2.c

@@ -5279,6 +5279,38 @@ static int qcow2_load_vmstate(BlockDriverState *bs, QEMUIOVector *qiov,
     return bs->drv->bdrv_co_preadv_part(bs, offset, 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.
@@ -5336,9 +5368,10 @@ 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;
     }
 
@@ -5356,6 +5389,27 @@ 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) {