block: Add bdrv_supports_compressed_writes()

Filters cannot compress data themselves but they have to implement
.bdrv_co_pwritev_compressed() still (or they cannot forward compressed
writes).  Therefore, checking whether
bs->drv->bdrv_co_pwritev_compressed is non-NULL is not sufficient to
know whether the node can actually handle compressed writes.  This
function looks down the filter chain to see whether there is a
non-filter that can actually convert the compressed writes into
compressed data (and thus normal writes).

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>

block.c

@@ -5065,6 +5065,29 @@ bool bdrv_is_sg(BlockDriverState *bs)
     return bs->sg;
 }
 
+/**
+ * Return whether the given node supports compressed writes.
+ */
+bool bdrv_supports_compressed_writes(BlockDriverState *bs)
+{
+    BlockDriverState *filtered;
+
+    if (!bs->drv || !block_driver_can_compress(bs->drv)) {
+        return false;
+    }
+
+    filtered = bdrv_filter_bs(bs);
+    if (filtered) {
+        /*
+         * Filters can only forward compressed writes, so we have to
+         * check the child.
+         */
+        return bdrv_supports_compressed_writes(filtered);
+    }
+
+    return true;
+}
+
 const char *bdrv_get_format_name(BlockDriverState *bs)
 {
     return bs->drv ? bs->drv->format_name : NULL;

include/block/block.h

@@ -532,6 +532,7 @@ BlockDriverState *bdrv_next(BdrvNextIterator *it);
 void bdrv_next_cleanup(BdrvNextIterator *it);
 
 BlockDriverState *bdrv_next_monitor_owned(BlockDriverState *bs);
+bool bdrv_supports_compressed_writes(BlockDriverState *bs);
 void bdrv_iterate_format(void (*it)(void *opaque, const char *name),
                          void *opaque, bool read_only);
 const char *bdrv_get_node_name(const BlockDriverState *bs);