qcow2: Handle full/falloc preallocation in qcow2_co_create()

Once qcow2_co_create() can be called directly on an already existing
node, we must provide the 'full' and 'falloc' preallocation modes
outside of creating the image on the protocol layer. Fortunately, we
have preallocated truncate now which can provide this functionality.

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

block/qcow2.c

@@ -2908,6 +2908,25 @@ qcow2_co_create(BlockdevCreateOptions *create_options, Error **errp)
     }
     blk_set_allow_write_beyond_eof(blk, true);
 
+    /* Clear the protocol layer and preallocate it if necessary */
+    ret = blk_truncate(blk, 0, PREALLOC_MODE_OFF, errp);
+    if (ret < 0) {
+        goto out;
+    }
+
+    if (qcow2_opts->preallocation == PREALLOC_MODE_FULL ||
+        qcow2_opts->preallocation == PREALLOC_MODE_FALLOC)
+    {
+        int64_t prealloc_size =
+            qcow2_calc_prealloc_size(qcow2_opts->size, cluster_size,
+                                     refcount_order);
+
+        ret = blk_truncate(blk, prealloc_size, qcow2_opts->preallocation, errp);
+        if (ret < 0) {
+            goto out;
+        }
+    }
+
     /* Write the header */
     QEMU_BUILD_BUG_ON((1 << MIN_CLUSTER_BITS) < sizeof(*header));
     header = g_malloc0(cluster_size);
@@ -3145,15 +3164,6 @@ static int coroutine_fn qcow2_co_create_opts(const char *filename, QemuOpts *opt
 
 
     /* Create and open the file (protocol layer) */
-    if (prealloc == PREALLOC_MODE_FULL || prealloc == PREALLOC_MODE_FALLOC) {
-        int refcount_order = ctz32(refcount_bits);
-        int64_t prealloc_size =
-            qcow2_calc_prealloc_size(size, cluster_size, refcount_order);
-        qemu_opt_set_number(opts, BLOCK_OPT_SIZE, prealloc_size, &error_abort);
-        qemu_opt_set(opts, BLOCK_OPT_PREALLOC, PreallocMode_str(prealloc),
-                     &error_abort);
-    }
-
     ret = bdrv_create_file(filename, opts, errp);
     if (ret < 0) {
         goto finish;