qemu-img: Add -F shorthand to convert

Although we have long supported 'qemu-img convert -o
backing_file=foo,backing_fmt=bar', the fact that we have a shortcut -B
for backing_file but none for backing_fmt has made it more likely that
users accidentally run into:

qemu-img: warning: Deprecated use of backing file without explicit backing format

when using -B instead of -o.  For similarity with other qemu-img
commands, such as create and compare, add '-F $fmt' as the shorthand
for '-o backing_fmt=$fmt'.  Update iotest 122 for coverage of both
spellings.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210913131735.1948339-1-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>

docs/tools/qemu-img.rst

@@ -415,7 +415,7 @@ Command description:
   4
     Error on reading data
 
-.. option:: convert [--object OBJECTDEF] [--image-opts] [--target-image-opts] [--target-is-zero] [--bitmaps [--skip-broken-bitmaps]] [-U] [-C] [-c] [-p] [-q] [-n] [-f FMT] [-t CACHE] [-T SRC_CACHE] [-O OUTPUT_FMT] [-B BACKING_FILE] [-o OPTIONS] [-l SNAPSHOT_PARAM] [-S SPARSE_SIZE] [-r RATE_LIMIT] [-m NUM_COROUTINES] [-W] FILENAME [FILENAME2 [...]] OUTPUT_FILENAME
+.. option:: convert [--object OBJECTDEF] [--image-opts] [--target-image-opts] [--target-is-zero] [--bitmaps [--skip-broken-bitmaps]] [-U] [-C] [-c] [-p] [-q] [-n] [-f FMT] [-t CACHE] [-T SRC_CACHE] [-O OUTPUT_FMT] [-B BACKING_FILE [-F backing_fmt]] [-o OPTIONS] [-l SNAPSHOT_PARAM] [-S SPARSE_SIZE] [-r RATE_LIMIT] [-m NUM_COROUTINES] [-W] FILENAME [FILENAME2 [...]] OUTPUT_FILENAME
 
   Convert the disk image *FILENAME* or a snapshot *SNAPSHOT_PARAM*
   to disk image *OUTPUT_FILENAME* using format *OUTPUT_FMT*. It can
@@ -439,7 +439,7 @@ Command description:
   You can use the *BACKING_FILE* option to force the output image to be
   created as a copy on write image of the specified base image; the
   *BACKING_FILE* should have the same content as the input's base image,
-  however the path, image format, etc may differ.
+  however the path, image format (as given by *BACKING_FMT*), etc may differ.
 
   If a relative path name is given, the backing file is looked up relative to
   the directory containing *OUTPUT_FILENAME*.

qemu-img-cmds.hx

@@ -46,7 +46,7 @@ SRST
 ERST
 
 DEF("convert", img_convert,
-    "convert [--object objectdef] [--image-opts] [--target-image-opts] [--target-is-zero] [--bitmaps] [-U] [-C] [-c] [-p] [-q] [-n] [-f fmt] [-t cache] [-T src_cache] [-O output_fmt] [-B backing_file] [-o options] [-l snapshot_param] [-S sparse_size] [-r rate_limit] [-m num_coroutines] [-W] [--salvage] filename [filename2 [...]] output_filename")
+    "convert [--object objectdef] [--image-opts] [--target-image-opts] [--target-is-zero] [--bitmaps] [-U] [-C] [-c] [-p] [-q] [-n] [-f fmt] [-t cache] [-T src_cache] [-O output_fmt] [-B backing_file [-F backing_fmt]] [-o options] [-l snapshot_param] [-S sparse_size] [-r rate_limit] [-m num_coroutines] [-W] [--salvage] filename [filename2 [...]] output_filename")
 SRST
 .. option:: convert [--object OBJECTDEF] [--image-opts] [--target-image-opts] [--target-is-zero] [--bitmaps] [-U] [-C] [-c] [-p] [-q] [-n] [-f FMT] [-t CACHE] [-T SRC_CACHE] [-O OUTPUT_FMT] [-B BACKING_FILE] [-o OPTIONS] [-l SNAPSHOT_PARAM] [-S SPARSE_SIZE] [-r RATE_LIMIT] [-m NUM_COROUTINES] [-W] [--salvage] FILENAME [FILENAME2 [...]] OUTPUT_FILENAME
 ERST

qemu-img.c

@@ -2183,7 +2183,8 @@ static int img_convert(int argc, char **argv)
     int c, bs_i, flags, src_flags = BDRV_O_NO_SHARE;
     const char *fmt = NULL, *out_fmt = NULL, *cache = "unsafe",
                *src_cache = BDRV_DEFAULT_CACHE, *out_baseimg = NULL,
-               *out_filename, *out_baseimg_param, *snapshot_name = NULL;
+               *out_filename, *out_baseimg_param, *snapshot_name = NULL,
+               *backing_fmt = NULL;
     BlockDriver *drv = NULL, *proto_drv = NULL;
     BlockDriverInfo bdi;
     BlockDriverState *out_bs;
@@ -2223,7 +2224,7 @@ static int img_convert(int argc, char **argv)
             {"skip-broken-bitmaps", no_argument, 0, OPTION_SKIP_BROKEN},
             {0, 0, 0, 0}
         };
-        c = getopt_long(argc, argv, ":hf:O:B:Cco:l:S:pt:T:qnm:WUr:",
+        c = getopt_long(argc, argv, ":hf:O:B:CcF:o:l:S:pt:T:qnm:WUr:",
                         long_options, NULL);
         if (c == -1) {
             break;
@@ -2253,6 +2254,9 @@ static int img_convert(int argc, char **argv)
         case 'c':
             s.compressed = true;
             break;
+        case 'F':
+            backing_fmt = optarg;
+            break;
         case 'o':
             if (accumulate_options(&options, optarg) < 0) {
                 goto fail_getopt;
@@ -2521,7 +2525,7 @@ static int img_convert(int argc, char **argv)
 
         qemu_opt_set_number(opts, BLOCK_OPT_SIZE,
                             s.total_sectors * BDRV_SECTOR_SIZE, &error_abort);
-        ret = add_old_style_options(out_fmt, opts, out_baseimg, NULL);
+        ret = add_old_style_options(out_fmt, opts, out_baseimg, backing_fmt);
         if (ret < 0) {
             goto out;
         }

tests/qemu-iotests/122

@@ -67,7 +67,7 @@ echo
 _make_test_img -b "$TEST_IMG".base -F $IMGFMT
 
 $QEMU_IO -c "write -P 0 0 3M" "$TEST_IMG" 2>&1 | _filter_qemu_io | _filter_testdir
-$QEMU_IMG convert -O $IMGFMT -B "$TEST_IMG".base -o backing_fmt=$IMGFMT \
+$QEMU_IMG convert -O $IMGFMT -B "$TEST_IMG".base -F $IMGFMT \
     "$TEST_IMG" "$TEST_IMG".orig
 $QEMU_IO -c "read -P 0 0 3M" "$TEST_IMG".orig 2>&1 | _filter_qemu_io | _filter_testdir
 $QEMU_IMG convert -O $IMGFMT -c -B "$TEST_IMG".base -o backing_fmt=$IMGFMT \