qcow2: Fix full preallocation with external data file

preallocate_co() already gave the data file the full size without
forwarding the requested preallocation mode to the protocol. When
bdrv_co_truncate() was called later with the preallocation mode, the
file didn't actually grow any more, so the data file stayed unallocated
even if full preallocation was requested.

Pass the right preallocation mode to preallocate_co() and remove the
second bdrv_co_truncate() to fix this. As a side effect, the ugly
one-byte write in preallocate_co() is replaced with a truncate call,
now leaving the last block unallocated on the protocol level as it
should be.

Cc: qemu-stable@nongnu.org
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

block/qcow2.c

@@ -2721,11 +2721,13 @@ static int qcow2_set_up_encryption(BlockDriverState *bs,
  * Returns: 0 on success, -errno on failure.
  */
 static int coroutine_fn preallocate_co(BlockDriverState *bs, uint64_t offset,
-                                       uint64_t new_length, Error **errp)
+                                       uint64_t new_length, PreallocMode mode,
+                                       Error **errp)
 {
     BDRVQcow2State *s = bs->opaque;
     uint64_t bytes;
     uint64_t host_offset = 0;
+    int64_t file_length;
     unsigned int cur_bytes;
     int ret;
     QCowL2Meta *meta;
@@ -2772,12 +2774,19 @@ static int coroutine_fn preallocate_co(BlockDriverState *bs, uint64_t offset,
      * all of the allocated clusters (otherwise we get failing reads after
      * EOF). Extend the image to the last allocated sector.
      */
-    if (host_offset != 0) {
-        uint8_t data = 0;
-        ret = bdrv_pwrite(s->data_file, (host_offset + cur_bytes) - 1,
-                          &data, 1);
+    file_length = bdrv_getlength(s->data_file->bs);
+    if (file_length < 0) {
+        error_setg_errno(errp, -file_length, "Could not get file size");
+        return file_length;
+    }
+
+    if (host_offset + cur_bytes > file_length) {
+        if (mode == PREALLOC_MODE_METADATA) {
+            mode = PREALLOC_MODE_OFF;
+        }
+        ret = bdrv_co_truncate(s->data_file, host_offset + cur_bytes, mode,
+                               errp);
         if (ret < 0) {
-            error_setg_errno(errp, -ret, "Writing to EOF failed");
             return ret;
         }
     }
@@ -3748,10 +3757,16 @@ 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, prealloc, errp);
+            if (ret < 0) {
+                goto fail;
+            }
+        }
         break;
 
     case PREALLOC_MODE_METADATA:
-        ret = preallocate_co(bs, old_length, offset, errp);
+        ret = preallocate_co(bs, old_length, offset, prealloc, errp);
         if (ret < 0) {
             goto fail;
         }
@@ -3768,7 +3783,7 @@ static int coroutine_fn qcow2_co_truncate(BlockDriverState *bs, int64_t offset,
         /* With a data file, preallocation means just allocating the metadata
          * and forwarding the truncate request to the data file */
         if (has_data_file(bs)) {
-            ret = preallocate_co(bs, old_length, offset, errp);
+            ret = preallocate_co(bs, old_length, offset, prealloc, errp);
             if (ret < 0) {
                 goto fail;
             }
@@ -3883,16 +3898,6 @@ static int coroutine_fn qcow2_co_truncate(BlockDriverState *bs, int64_t offset,
 
     bs->total_sectors = offset / BDRV_SECTOR_SIZE;
 
-    if (has_data_file(bs)) {
-        if (prealloc == PREALLOC_MODE_METADATA) {
-            prealloc = PREALLOC_MODE_OFF;
-        }
-        ret = bdrv_co_truncate(s->data_file, offset, prealloc, errp);
-        if (ret < 0) {
-            goto fail;
-        }
-    }
-
     /* write updated header.size */
     offset = cpu_to_be64(offset);
     ret = bdrv_pwrite_sync(bs->file, offsetof(QCowHeader, size),