block: Allow NULL file for bdrv_get_block_status()

Not all callers care about which BDS owns the mapping for a given
range of the file.  This patch merely simplifies the callers by
consolidating the logic in the common call point, while guaranteeing
a non-NULL file to all the driver callbacks, for no semantic change.
The only caller that does not care about pnum is bdrv_is_allocated,
as invoked by vvfat; we can likewise add assertions that the rest
of the stack does not have to worry about a NULL pnum.

Furthermore, this will also set the stage for a future cleanup: when
a caller does not care about which BDS owns an offset, it would be
nice to allow the driver to optimize things to not have to return
BDRV_BLOCK_OFFSET_VALID in the first place.  In the case of fragmented
allocation (for example, it's fairly easy to create a qcow2 image
where consecutive guest addresses are not at consecutive host
addresses), the current contract requires bdrv_get_block_status()
to clamp *pnum to the limit where host addresses are no longer
consecutive, but allowing a NULL file means that *pnum could be
set to the full length of known-allocated data.

Signed-off-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 27 insertions, 22 deletions

diff --git a/block/io.c b/block/io.c
index 0854e0fdac..93231b0e26 100644
--- a/block/io.c
+++ b/block/io.c
@@ -718,7 +718,6 @@ int bdrv_make_zero(BdrvChild *child, BdrvRequestFlags flags)
 {
     int64_t target_sectors, ret, nb_sectors, sector_num = 0;
     BlockDriverState *bs = child->bs;
-    BlockDriverState *file;
     int n;
 
     target_sectors = bdrv_nb_sectors(bs);
@@ -731,7 +730,7 @@ int bdrv_make_zero(BdrvChild *child, BdrvRequestFlags flags)
         if (nb_sectors <= 0) {
             return 0;
         }
-        ret = bdrv_get_block_status(bs, sector_num, nb_sectors, &n, &file);
+        ret = bdrv_get_block_status(bs, sector_num, nb_sectors, &n, NULL);
         if (ret < 0) {
             error_report("error getting block status at sector %" PRId64 ": %s",
                          sector_num, strerror(-ret));
@@ -1820,8 +1819,9 @@ int64_t coroutine_fn bdrv_co_get_block_status_from_backing(BlockDriverState *bs,
  * beyond the end of the disk image it will be clamped; if 'pnum' is set to
  * the end of the image, then the returned value will include BDRV_BLOCK_EOF.
  *
- * If returned value is positive and BDRV_BLOCK_OFFSET_VALID bit is set, 'file'
- * points to the BDS which the sector range is allocated in.
+ * If returned value is positive, BDRV_BLOCK_OFFSET_VALID bit is set, and
+ * 'file' is non-NULL, then '*file' points to the BDS which the sector range
+ * is allocated in.
  */
 static int64_t coroutine_fn bdrv_co_get_block_status(BlockDriverState *bs,
                                                      int64_t sector_num,
@@ -1831,20 +1831,23 @@ static int64_t coroutine_fn bdrv_co_get_block_status(BlockDriverState *bs,
     int64_t total_sectors;
     int64_t n;
     int64_t ret, ret2;
+    BlockDriverState *local_file = NULL;
 
-    *file = NULL;
+    assert(pnum);
+    *pnum = 0;
     total_sectors = bdrv_nb_sectors(bs);
     if (total_sectors < 0) {
-        return total_sectors;
+        ret = total_sectors;
+        goto early_out;
     }
 
     if (sector_num >= total_sectors) {
-        *pnum = 0;
-        return BDRV_BLOCK_EOF;
+        ret = BDRV_BLOCK_EOF;
+        goto early_out;
     }
     if (!nb_sectors) {
-        *pnum = 0;
-        return 0;
+        ret = 0;
+        goto early_out;
     }
 
     n = total_sectors - sector_num;
@@ -1860,23 +1863,23 @@ static int64_t coroutine_fn bdrv_co_get_block_status(BlockDriverState *bs,
         }
         if (bs->drv->protocol_name) {
             ret |= BDRV_BLOCK_OFFSET_VALID | (sector_num * BDRV_SECTOR_SIZE);
-            *file = bs;
+            local_file = bs;
         }
-        return ret;
+        goto early_out;
     }
 
     bdrv_inc_in_flight(bs);
     ret = bs->drv->bdrv_co_get_block_status(bs, sector_num, nb_sectors, pnum,
-                                            file);
+                                            &local_file);
     if (ret < 0) {
         *pnum = 0;
         goto out;
     }
 
     if (ret & BDRV_BLOCK_RAW) {
-        assert(ret & BDRV_BLOCK_OFFSET_VALID && *file);
-        ret = bdrv_co_get_block_status(*file, ret >> BDRV_SECTOR_BITS,
-                                       *pnum, pnum, file);
+        assert(ret & BDRV_BLOCK_OFFSET_VALID && local_file);
+        ret = bdrv_co_get_block_status(local_file, ret >> BDRV_SECTOR_BITS,
+                                       *pnum, pnum, &local_file);
         goto out;
     }
 
@@ -1894,14 +1897,13 @@ static int64_t coroutine_fn bdrv_co_get_block_status(BlockDriverState *bs,
         }
     }
 
-    if (*file && *file != bs &&
+    if (local_file && local_file != bs &&
         (ret & BDRV_BLOCK_DATA) && !(ret & BDRV_BLOCK_ZERO) &&
         (ret & BDRV_BLOCK_OFFSET_VALID)) {
-        BlockDriverState *file2;
         int file_pnum;
 
-        ret2 = bdrv_co_get_block_status(*file, ret >> BDRV_SECTOR_BITS,
-                                        *pnum, &file_pnum, &file2);
+        ret2 = bdrv_co_get_block_status(local_file, ret >> BDRV_SECTOR_BITS,
+                                        *pnum, &file_pnum, NULL);
         if (ret2 >= 0) {
             /* Ignore errors.  This is just providing extra information, it
              * is useful but not necessary.
@@ -1927,6 +1929,10 @@ out:
     if (ret >= 0 && sector_num + *pnum == total_sectors) {
         ret |= BDRV_BLOCK_EOF;
     }
+early_out:
+    if (file) {
+        *file = local_file;
+    }
     return ret;
 }
 
@@ -2026,7 +2032,6 @@ int64_t bdrv_get_block_status(BlockDriverState *bs,
 int coroutine_fn bdrv_is_allocated(BlockDriverState *bs, int64_t offset,
                                    int64_t bytes, int64_t *pnum)
 {
-    BlockDriverState *file;
     int64_t sector_num = offset >> BDRV_SECTOR_BITS;
     int nb_sectors = bytes >> BDRV_SECTOR_BITS;
     int64_t ret;
@@ -2035,7 +2040,7 @@ int coroutine_fn bdrv_is_allocated(BlockDriverState *bs, int64_t offset,
     assert(QEMU_IS_ALIGNED(offset, BDRV_SECTOR_SIZE));
     assert(QEMU_IS_ALIGNED(bytes, BDRV_SECTOR_SIZE) && bytes < INT_MAX);
     ret = bdrv_get_block_status(bs, sector_num, nb_sectors, &psectors,
-                                &file);
+                                NULL);
     if (ret < 0) {
         return ret;
     }