block: Let format drivers pass @exact

When truncating a format node, the @exact parameter is generally handled
simply by virtue of the format storing the new size in the image
metadata.  Such formats do not need to pass on the parameter to their
file nodes.

There are exceptions, though:
- raw and crypto cannot store the image size, and thus must pass on
  @exact.

- When using qcow2 with an external data file, it just makes sense to
  keep its size in sync with the qcow2 virtual disk (because the
  external data file is the virtual disk).  Therefore, we should pass
  @exact when truncating it.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190918095144.955-7-mreitz@redhat.com
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

1 files changed, 14 insertions, 1 deletions

diff --git a/block/qcow2.c b/block/qcow2.c
index de284ad..9f32dae 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -4027,6 +4027,13 @@ static int coroutine_fn qcow2_co_truncate(BlockDriverState *bs, int64_t offset,
         if ((last_cluster + 1) * s->cluster_size < old_file_size) {
             Error *local_err = NULL;
 
+            /*
+             * Do not pass @exact here: It will not help the user if
+             * we get an error here just because they wanted to shrink
+             * their qcow2 image (on a block device) with qemu-img.
+             * (And on the qcow2 layer, the @exact requirement is
+             * always fulfilled, so there is no need to pass it on.)
+             */
             bdrv_co_truncate(bs->file, (last_cluster + 1) * s->cluster_size,
                              false, PREALLOC_MODE_OFF, &local_err);
             if (local_err) {
@@ -4045,7 +4052,12 @@ static int coroutine_fn qcow2_co_truncate(BlockDriverState *bs, int64_t offset,
     switch (prealloc) {
     case PREALLOC_MODE_OFF:
         if (has_data_file(bs)) {
-            ret = bdrv_co_truncate(s->data_file, offset, false, prealloc, errp);
+            /*
+             * If the caller wants an exact resize, the external data
+             * file should be resized to the exact target size, too,
+             * so we pass @exact here.
+             */
+            ret = bdrv_co_truncate(s->data_file, offset, exact, prealloc, errp);
             if (ret < 0) {
                 goto fail;
             }
@@ -4130,6 +4142,7 @@ static int coroutine_fn qcow2_co_truncate(BlockDriverState *bs, int64_t offset,
         /* Allocate the data area */
         new_file_size = allocation_start +
                         nb_new_data_clusters * s->cluster_size;
+        /* Image file grows, so @exact does not matter */
         ret = bdrv_co_truncate(bs->file, new_file_size, false, prealloc, errp);
         if (ret < 0) {
             error_prepend(errp, "Failed to resize underlying file: ");