[Qemu-block] [PATCH v3 1/2] block: sync bdrv_co_get_block_status_above() with bdrv_is_allocated_above()

[Denis V. Lunev]

They should work very similar, covering same areas if backing store is
shorter than the image. This change is necessary for the followup patch
switching to bdrv_get_block_status_above() in mirror to avoid assert
in check_block.

This change should be made very carefully. Let us assume that we have
top image and 2 backing stores L0->L1->L2.
  L0: --------------
  L1: -------
  L2: -------=======
The data marked as '=' in L2 should not appear as BDRV_BLOCK_ALLOCATED
and we should return it as filled in L0 image with properly calculated
*pnum value.

Signed-off-by: Denis V. Lunev <address@hidden>
CC: Stefan Hajnoczi <address@hidden>
CC: Fam Zheng <address@hidden>
CC: Kevin Wolf <address@hidden>
CC: Max Reitz <address@hidden>
CC: Jeff Cody <address@hidden>
---
 block/io.c | 25 ++++++++++++++++++++-----
 1 file changed, 20 insertions(+), 5 deletions(-)

diff --git a/block/io.c b/block/io.c
index 420944d..067d465 100644
--- a/block/io.c
+++ b/block/io.c
@@ -1741,18 +1741,33 @@ static int64_t coroutine_fn 
bdrv_co_get_block_status_above(BlockDriverState *bs,
         BlockDriverState **file)
 {
     BlockDriverState *p;
-    int64_t ret = 0;
+    int64_t ret = 0, res = nb_sectors;
 
     assert(bs != base);
     for (p = bs; p != base; p = backing_bs(p)) {
-        ret = bdrv_co_get_block_status(p, sector_num, nb_sectors, pnum, file);
-        if (ret < 0 || ret & BDRV_BLOCK_ALLOCATED) {
-            break;
+        int sc;
+        ret = bdrv_co_get_block_status(p, sector_num, nb_sectors, &sc, file);
+        if (ret < 0) {
+            return ret;
+        } else if (ret & BDRV_BLOCK_ALLOCATED) {
+            *pnum = sc;
+            return ret;
+        }
+
+        if (res > sc && (p == bs || sector_num + sc < p->total_sectors)) {
+            res = sc;
         }
+
         /* [sector_num, pnum] unallocated on this layer, which could be only
          * the first part of [sector_num, nb_sectors].  */
-        nb_sectors = MIN(nb_sectors, *pnum);
+        nb_sectors = MIN(nb_sectors, sc);
+
+        if (nb_sectors == 0) {
+            break;
+        }
     }
+
+    *pnum = res;
     return ret;
 }
 
-- 
2.7.4