[PATCH 3/4] qemu-img: add --shallow option for qemu-img compare --stat

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

Allow compare only top images of backing chains. That's useful for
comparing two increments from the same chain of incremental backups.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 docs/tools/qemu-img.rst |  8 +++++++-
 qemu-img.c              | 14 ++++++++++++--
 qemu-img-cmds.hx        |  4 ++--
 3 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/docs/tools/qemu-img.rst b/docs/tools/qemu-img.rst
index 4b382ca2b0..c8ae96be6a 100644
--- a/docs/tools/qemu-img.rst
+++ b/docs/tools/qemu-img.rst
@@ -176,6 +176,12 @@ Parameters to compare subcommand:
     - If both files don't specify cluster-size, use default of 64K
     - If only one file specify cluster-size, just use it.
 
+.. option:: --shallow
+
+  Only allowed with ``--stat``. This option prevents opening and comparing
+  any backing files. This is useful to compare incremental images from
+  the chain of incremental backups.
+
 Parameters to convert subcommand:
 
 .. program:: qemu-img-convert
@@ -395,7 +401,7 @@ Command description:
 
   The rate limit for the commit process is specified by ``-r``.
 
-.. option:: compare [--object OBJECTDEF] [--image-opts] [-f FMT] [-F FMT] [-T SRC_CACHE] [-p] [-q] [-s] [-U] [--stat [--block-size BLOCK_SIZE]] FILENAME1 FILENAME2
+.. option:: compare [--object OBJECTDEF] [--image-opts] [-f FMT] [-F FMT] [-T SRC_CACHE] [-p] [-q] [-s] [-U] [--stat [--block-size BLOCK_SIZE] [--shallow]] FILENAME1 FILENAME2
 
   Check if two images have the same content. You can compare images with
   different format or settings.
diff --git a/qemu-img.c b/qemu-img.c
index 61e7f470bb..e8ae412c38 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -85,6 +85,7 @@ enum {
     OPTION_SKIP_BROKEN = 277,
     OPTION_STAT = 277,
     OPTION_BLOCK_SIZE = 278,
+    OPTION_SHALLOW = 279,
 };
 
 typedef enum OutputFormat {
@@ -1482,7 +1483,7 @@ static int img_compare(int argc, char **argv)
     int64_t block_end;
     int ret = 0; /* return value - 0 Ident, 1 Different, >1 Error */
     bool progress = false, quiet = false, strict = false;
-    int flags;
+    int flags = 0;
     bool writethrough;
     int64_t total_size;
     int64_t offset = 0;
@@ -1504,6 +1505,7 @@ static int img_compare(int argc, char **argv)
             {"force-share", no_argument, 0, 'U'},
             {"stat", no_argument, 0, OPTION_STAT},
             {"block-size", required_argument, 0, OPTION_BLOCK_SIZE},
+            {"shallow", no_argument, 0, OPTION_SHALLOW},
             {0, 0, 0, 0}
         };
         c = getopt_long(argc, argv, ":hf:F:T:pqsU",
@@ -1569,6 +1571,9 @@ static int img_compare(int argc, char **argv)
                 exit(EXIT_SUCCESS);
             }
             break;
+        case OPTION_SHALLOW:
+            flags |= BDRV_O_NO_BACKING;
+            break;
         }
     }
 
@@ -1590,10 +1595,15 @@ static int img_compare(int argc, char **argv)
         goto out;
     }
 
+    if (!do_stat && (flags & BDRV_O_NO_BACKING)) {
+        error_report("--shallow can be used only together with --stat");
+        ret = 1;
+        goto out;
+    }
+
     /* Initialize before goto out */
     qemu_progress_init(progress, 2.0);
 
-    flags = 0;
     ret = bdrv_parse_cache_mode(cache, &flags, &writethrough);
     if (ret < 0) {
         error_report("Invalid source cache option: %s", cache);
diff --git a/qemu-img-cmds.hx b/qemu-img-cmds.hx
index 96a193eea8..a295bc6860 100644
--- a/qemu-img-cmds.hx
+++ b/qemu-img-cmds.hx
@@ -40,9 +40,9 @@ SRST
 ERST
 
 DEF("compare", img_compare,
-    "compare [--object objectdef] [--image-opts] [-f fmt] [-F fmt] [-T src_cache] [-p] [-q] [-s] [-U] [--stat [--block-size BLOCK_SIZE]] filename1 filename2")
+    "compare [--object objectdef] [--image-opts] [-f fmt] [-F fmt] [-T src_cache] [-p] [-q] [-s] [-U] [--stat [--block-size BLOCK_SIZE] [--shallow]] filename1 filename2")
 SRST
-.. option:: compare [--object OBJECTDEF] [--image-opts] [-f FMT] [-F FMT] [-T SRC_CACHE] [-p] [-q] [-s] [-U] [--stat [--block-size BLOCK_SIZE]] FILENAME1 FILENAME2
+.. option:: compare [--object OBJECTDEF] [--image-opts] [-f FMT] [-F FMT] [-T SRC_CACHE] [-p] [-q] [-s] [-U] [--stat [--block-size BLOCK_SIZE] [--shallow]] FILENAME1 FILENAME2
 ERST
 
 DEF("convert", img_convert,
-- 
2.29.2