[v5,09/12] qemu-img: Specify backing file for commit

diff --git a/qemu-img-cmds.hx b/qemu-img-cmds.hx
index 8bc55cd..e8a5d34 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] [-p] filename")
+    "commit [-q] [-f fmt] [-t cache] [-b base] [-p] filename")
 STEXI
-@item commit [-q] [-f @var{fmt}] [-t @var{cache}] [-p] @var{filename}
+@item commit [-q] [-f @var{fmt}] [-t @var{cache}] [-b @var{base}] [-p] @var{filename}
 ETEXI
 
 DEF("compare", img_compare,
diff --git a/qemu-img.c b/qemu-img.c
index 0d65fed..d048453 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -727,7 +727,7 @@  static void run_block_job(BlockJob *job, Error **errp)
 static int img_commit(int argc, char **argv)
 {
     int c, ret, flags;
-    const char *filename, *fmt, *cache;
+    const char *filename, *fmt, *cache, *base;
     BlockDriverState *bs, *base_bs, *backing_bs;
     bool progress = false, quiet = false;
     Error *local_err = NULL;
@@ -738,8 +738,9 @@  static int img_commit(int argc, char **argv)
 
     fmt = NULL;
     cache = BDRV_DEFAULT_CACHE;
+    base = NULL;
     for(;;) {
-        c = getopt(argc, argv, "f:ht:qp");
+        c = getopt(argc, argv, "f:ht:b:qp");
         if (c == -1) {
             break;
         }
@@ -754,6 +755,9 @@  static int img_commit(int argc, char **argv)
         case 't':
             cache = optarg;
             break;
+        case 'b':
+            base = optarg;
+            break;
         case 'p':
             progress = true;
             break;
@@ -788,12 +792,16 @@  static int img_commit(int argc, char **argv)
     qemu_progress_init(progress, 1.f);
     qemu_progress_print(0.f, 100);
 
-    /* This is different from QMP, which by default uses the deepest file in the
-     * backing chain (i.e., the very base); however, the traditional behavior of
-     * qemu-img commit is using the immediate backing file. */
-    base_bs = bs->backing_hd;
+    if (base) {
+        base_bs = bdrv_find_backing_image(bs, base);
+    } else {
+        /* This is different from QMP, which by default uses the deepest file in
+         * the backing chain (i.e., the very base); however, the traditional
+         * behavior of qemu-img commit is using the immediate backing file. */
+        base_bs = bs->backing_hd;
+    }
     if (!base_bs) {
-        error_set(&local_err, QERR_BASE_NOT_FOUND, "NULL");
+        error_set(&local_err, QERR_BASE_NOT_FOUND, base ?: "NULL");
         goto done;
     }
 
diff --git a/qemu-img.texi b/qemu-img.texi
index 1a9c08f..27b65ae 100644
--- a/qemu-img.texi
+++ b/qemu-img.texi
@@ -140,7 +140,7 @@  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 [-q] [-f @var{fmt}] [-t @var{cache}] [-p] @var{filename}
+@item commit [-q] [-f @var{fmt}] [-t @var{cache}] [-b @var{base}] [-p] @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
@@ -149,6 +149,12 @@  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.
 
+If the backing chain of the given image file @var{filename} has more than one
+layer, the backing file into which the changes will be committed may be
+specified as @var{base} (which has to be part of @var{filename}'s backing
+chain). If @var{base} is not specified, the immediate backing file of the top
+image (which is @var{filename}) will be used.
+
 @item compare [-f @var{fmt}] [-F @var{fmt}] [-p] [-s] [-q] @var{filename1} @var{filename2}
 
 Check if two images have the same content. You can compare images with