[Qemu-block] [PATCH v4] backup: allow target without .bdrv_get_info

[Vladimir Sementsov-Ogievskiy]

Currently backup to nbd target is broken, as nbd doesn't have
.bdrv_get_info realization.

Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
---

v4: use error_report()
    add article

v3: fix compilation (I feel like an idiot)
    adjust wording (Fam)
    
v2: add WARNING

===

Since commit

commit 4c9bca7e39a6e07ad02c1dcde3478363344ec60b
Author: John Snow <address@hidden>
Date:   Thu Feb 25 15:58:30 2016 -0500

    block/backup: avoid copying less than full target clusters

backup to nbd target is broken, we have "Couldn't determine the cluster size of
the target image".

Proposed NBD protocol extension - NBD_OPT_INFO should finally solve this 
problem.
But until it is not realized, we need allow backup to nbd target due to backward
compatibility.

Furthermore, is it entirely ok to disallow backup if bds lacks .bdrv_get_info?
Which behavior should be default: to fail backup or to use default cluster size?

 block/backup.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/block/backup.c b/block/backup.c
index ea38733849..ea160e9e82 100644
--- a/block/backup.c
+++ b/block/backup.c
@@ -24,6 +24,7 @@
 #include "qemu/cutils.h"
 #include "sysemu/block-backend.h"
 #include "qemu/bitmap.h"
+#include "qemu/error-report.h"
 
 #define BACKUP_CLUSTER_SIZE_DEFAULT (1 << 16)
 #define SLICE_TIME 100000000ULL /* ns */
@@ -638,7 +639,16 @@ BlockJob *backup_job_create(const char *job_id, 
BlockDriverState *bs,
      * backup cluster size is smaller than the target cluster size. Even for
      * targets with a backing file, try to avoid COW if possible. */
     ret = bdrv_get_info(target, &bdi);
-    if (ret < 0 && !target->backing) {
+    if (ret == -ENOTSUP) {
+        /* Cluster size is not defined */
+        error_report("WARNING: The target block device doesn't provide "
+                     "information about the block size and it doesn't have a "
+                     "backing file. The default block size of %u bytes is "
+                     "used. If the actual block size of the target exceeds "
+                     "this default, the backup may be unusable",
+                     BACKUP_CLUSTER_SIZE_DEFAULT);
+        job->cluster_size = BACKUP_CLUSTER_SIZE_DEFAULT;
+    } else if (ret < 0 && !target->backing) {
         error_setg_errno(errp, -ret,
             "Couldn't determine the cluster size of the target image, "
             "which has no backing file");
-- 
2.11.1