[v13,09/14] qemu-img: Empty image after commit

diff --git a/qemu-img-cmds.hx b/qemu-img-cmds.hx
index 55aec6b..e2ceadf 100644
--- a/qemu-img-cmds.hx
+++ b/qemu-img-cmds.hx
@@ -22,9 +22,9 @@  STEXI
 ETEXI
 
 DEF("commit", img_commit,
-    "commit [-q] [-f fmt] [-t cache] filename")
+    "commit [-q] [-f fmt] [-t cache] [-d] filename")
 STEXI
-@item commit [-q] [-f @var{fmt}] [-t @var{cache}] @var{filename}
+@item commit [-q] [-f @var{fmt}] [-t @var{cache}] [-d] @var{filename}
 ETEXI
 
 DEF("compare", img_compare,
diff --git a/qemu-img.c b/qemu-img.c
index f1f2857..5093bfa 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -754,14 +754,14 @@  static int img_commit(int argc, char **argv)
     const char *filename, *fmt, *cache;
     BlockBackend *blk;
     BlockDriverState *bs, *base_bs;
-    bool quiet = false;
+    bool quiet = false, drop = false;
     Error *local_err = NULL;
     CommonBlockJobCBInfo cbi;
 
     fmt = NULL;
     cache = BDRV_DEFAULT_CACHE;
     for(;;) {
-        c = getopt(argc, argv, "f:ht:q");
+        c = getopt(argc, argv, "f:ht:dq");
         if (c == -1) {
             break;
         }
@@ -776,6 +776,9 @@  static int img_commit(int argc, char **argv)
         case 't':
             cache = optarg;
             break;
+        case 'd':
+            drop = true;
+            break;
         case 'q':
             quiet = true;
             break;
@@ -786,7 +789,7 @@  static int img_commit(int argc, char **argv)
     }
     filename = argv[optind++];
 
-    flags = BDRV_O_RDWR;
+    flags = BDRV_O_RDWR | BDRV_O_UNMAP;
     ret = bdrv_parse_cache_flags(cache, &flags);
     if (ret < 0) {
         error_report("Invalid cache option: %s", cache);
@@ -819,7 +822,32 @@  static int img_commit(int argc, char **argv)
         goto done;
     }
 
+    /* The block job will swap base_bs and bs (which is not what we really want
+     * here, but okay) and unref base_bs (after the swap, i.e., the old top
+     * image). In order to still be able to empty that top image afterwards,
+     * increment the reference counter here preemptively. */
+    if (!drop) {
+        bdrv_ref(base_bs);
+    }
+
     run_block_job(bs->job, &local_err);
+    if (local_err) {
+        goto unref_backing;
+    }
+
+    if (!drop && base_bs->drv->bdrv_make_empty) {
+        ret = base_bs->drv->bdrv_make_empty(base_bs);
+        if (ret) {
+            error_setg_errno(&local_err, -ret, "Could not empty %s",
+                             filename);
+            goto unref_backing;
+        }
+    }
+
+unref_backing:
+    if (!drop) {
+        bdrv_unref(base_bs);
+    }
 
 done:
     blk_unref(blk);
diff --git a/qemu-img.texi b/qemu-img.texi
index e3ef4a0..420bd91 100644
--- a/qemu-img.texi
+++ b/qemu-img.texi
@@ -167,7 +167,7 @@  this case. @var{backing_file} will never be modified unless you use the
 The size can also be specified using the @var{size} option with @code{-o},
 it doesn't need to be specified separately in this case.
 
-@item commit [-f @var{fmt}] [-t @var{cache}] @var{filename}
+@item commit [-f @var{fmt}] [-t @var{cache}] [-d] @var{filename}
 
 Commit the changes recorded in @var{filename} in its base image or backing file.
 If the backing file is smaller than the snapshot, then the backing file will be
@@ -176,6 +176,10 @@  the backing file, the backing file will not be truncated.  If you want the
 backing file to match the size of the smaller snapshot, you can safely truncate
 it yourself once the commit operation successfully completes.
 
+The image @var{filename} is emptied after the operation has succeeded. If you do
+not need @var{filename} afterwards and intend to drop it, you may skip emptying
+@var{filename} by specifying the @code{-d} flag.
+
 @item compare [-f @var{fmt}] [-F @var{fmt}] [-T @var{src_cache}] [-p] [-s] [-q] @var{filename1} @var{filename2}
 
 Check if two images have the same content. You can compare images with