[Qemu-block] [PATCH v3 1/4] qemu-img: add --shrink flag for resize

[Pavel Butsykin]

The flag as additional precaution of data loss. Perhaps in the future the
operation shrink without this flag will be blocked for all formats, but while
we need to maintain compatibility with raw.

Signed-off-by: Pavel Butsykin <address@hidden>
---
 qemu-img-cmds.hx       |  4 ++--
 qemu-img.c             | 23 +++++++++++++++++++++++
 qemu-img.texi          |  7 ++++++-
 tests/qemu-iotests/102 |  4 ++--
 4 files changed, 33 insertions(+), 5 deletions(-)

diff --git a/qemu-img-cmds.hx b/qemu-img-cmds.hx
index a39fcdba71..3b2eab9d20 100644
--- a/qemu-img-cmds.hx
+++ b/qemu-img-cmds.hx
@@ -76,9 +76,9 @@ STEXI
 ETEXI
 
 DEF("resize", img_resize,
-    "resize [--object objectdef] [--image-opts] [-q] filename [+ | -]size")
+    "resize [--object objectdef] [--image-opts] [-q] [--shrink] filename [+ | 
-]size")
 STEXI
address@hidden resize [--object @var{objectdef}] [--image-opts] [-q] 
@var{filename} [+ | address@hidden
address@hidden resize [--object @var{objectdef}] [--image-opts] [-q] [--shrink] 
@var{filename} [+ | address@hidden
 ETEXI
 
 DEF("amend", img_amend,
diff --git a/qemu-img.c b/qemu-img.c
index 91ad6bebbf..9773e835e4 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -61,6 +61,7 @@ enum {
     OPTION_FLUSH_INTERVAL = 261,
     OPTION_NO_DRAIN = 262,
     OPTION_TARGET_IMAGE_OPTS = 263,
+    OPTION_SHRINK = 264,
 };
 
 typedef enum OutputFormat {
@@ -3458,6 +3459,7 @@ static int img_resize(int argc, char **argv)
         },
     };
     bool image_opts = false;
+    bool shrink = false;
 
     /* Remove size from argv manually so that negative numbers are not treated
      * as options by getopt. */
@@ -3475,6 +3477,7 @@ static int img_resize(int argc, char **argv)
             {"help", no_argument, 0, 'h'},
             {"object", required_argument, 0, OPTION_OBJECT},
             {"image-opts", no_argument, 0, OPTION_IMAGE_OPTS},
+            {"shrink", no_argument, 0, OPTION_SHRINK},
             {0, 0, 0, 0}
         };
         c = getopt_long(argc, argv, ":f:hq",
@@ -3509,6 +3512,9 @@ static int img_resize(int argc, char **argv)
         case OPTION_IMAGE_OPTS:
             image_opts = true;
             break;
+        case OPTION_SHRINK:
+            shrink = true;
+            break;
         }
     }
     if (optind != argc - 1) {
@@ -3568,6 +3574,23 @@ static int img_resize(int argc, char **argv)
         goto out;
     }
 
+    if (total_size < blk_getlength(blk) && !shrink) {
+        error_report("Warning: Shrinking an image will delete all data beyond"
+                     "the shrunken image's end. Before performing such an"
+                     "operation, make sure there is no important data there.");
+
+        if (g_strcmp0(bdrv_get_format_name(blk_bs(blk)), "raw") != 0) {
+            error_report(
+              "Use the --shrink option to perform a shrink operation.");
+            ret = -1;
+            goto out;
+        } else {
+            error_report("Using the --shrink option will suppress this 
message."
+                         "Note that future versions of qemu-img may refuse to "
+                         "shrink images without this option!");
+        }
+    }
+
     ret = blk_truncate(blk, total_size, &err);
     if (!ret) {
         qprintf(quiet, "Image resized.\n");
diff --git a/qemu-img.texi b/qemu-img.texi
index 5b925ecf41..6324abef48 100644
--- a/qemu-img.texi
+++ b/qemu-img.texi
@@ -499,7 +499,7 @@ qemu-img rebase -b base.img diff.qcow2
 At this point, @code{modified.img} can be discarded, since
 @code{base.img + diff.qcow2} contains the same information.
 
address@hidden resize @var{filename} [+ | address@hidden
address@hidden resize [--shrink] @var{filename} [+ | address@hidden
 
 Change the disk image as if it had been created with @var{size}.
 
@@ -507,6 +507,11 @@ Before using this command to shrink a disk image, you MUST 
use file system and
 partitioning tools inside the VM to reduce allocated file systems and partition
 sizes accordingly.  Failure to do so will result in data loss!
 
address@hidden informs qemu-img that the user is certain about wanting
+to shrink an image and is aware that any data beyond the truncated
+image's end will be lost. Trying to shrink an image without this option
+results in a warning; future versions may make it an error.
+
 After using this command to grow a disk image, you must use file system and
 partitioning tools inside the VM to actually begin using the new space on the
 device.
diff --git a/tests/qemu-iotests/102 b/tests/qemu-iotests/102
index 87db1bb1bf..d7ad8d9840 100755
--- a/tests/qemu-iotests/102
+++ b/tests/qemu-iotests/102
@@ -54,7 +54,7 @@ _make_test_img $IMG_SIZE
 $QEMU_IO -c 'write 0 64k' "$TEST_IMG" | _filter_qemu_io
 # Remove data cluster from image (first cluster: image header, second: 
reftable,
 # third: refblock, fourth: L1 table, fifth: L2 table)
-$QEMU_IMG resize -f raw "$TEST_IMG" $((5 * 64 * 1024))
+$QEMU_IMG resize -f raw --shrink "$TEST_IMG" $((5 * 64 * 1024))
 
 $QEMU_IO -c map "$TEST_IMG"
 $QEMU_IMG map "$TEST_IMG"
@@ -69,7 +69,7 @@ $QEMU_IO -c 'write 0 64k' "$TEST_IMG" | _filter_qemu_io
 
 qemu_comm_method=monitor _launch_qemu -drive if=none,file="$TEST_IMG",id=drv0
 
-$QEMU_IMG resize -f raw "$TEST_IMG" $((5 * 64 * 1024))
+$QEMU_IMG resize -f raw --shrink "$TEST_IMG" $((5 * 64 * 1024))
 
 _send_qemu_cmd $QEMU_HANDLE 'qemu-io drv0 map' 'allocated' \
     | sed -e 's/^(qemu).*qemu-io drv0 map...$/(qemu) qemu-io drv0 map/'
-- 
2.13.0