block: Add blkdebug hook for copy-on-read

Make it possible to inject errors on writes performed during a
read operation due to copy-on-read semantics.
Signed-off-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NJeff Cody <jcody@redhat.com>
Reviewed-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NJohn Snow <jsnow@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/io.c

@@ -983,6 +983,7 @@ static int coroutine_fn bdrv_co_do_copy_on_readv(BdrvChild *child,
         goto err;
     }
 
+    bdrv_debug_event(bs, BLKDBG_COR_WRITE);
     if (drv->bdrv_co_pwrite_zeroes &&
         buffer_is_zero(bounce_buffer, iov.iov_len)) {
         /* FIXME: Should we (perhaps conditionally) be setting

qapi/block-core.json

@@ -2538,6 +2538,8 @@
 #
 # @l1_shrink_free_l2_clusters: discard the l2 tables. (since 2.11)
 #
+# @cor_write: a write due to copy-on-read (since 2.11)
+#
 # Since: 2.9
 ##
 { 'enum': 'BlkdebugEvent', 'prefix': 'BLKDBG',
@@ -2555,7 +2557,8 @@
             'flush_to_disk', 'pwritev_rmw_head', 'pwritev_rmw_after_head',
             'pwritev_rmw_tail', 'pwritev_rmw_after_tail', 'pwritev',
             'pwritev_zero', 'pwritev_done', 'empty_image_prepare',
-            'l1_shrink_write_table', 'l1_shrink_free_l2_clusters' ] }
+            'l1_shrink_write_table', 'l1_shrink_free_l2_clusters',
+            'cor_write'] }
 
 ##
 # @BlkdebugInjectErrorOptions: