Re: [PATCH 10/13] qemu-io: add -g to write past the end of the image

[Eric Blake <eblake@redhat.com>]

On Wed, Sep 30, 2026 at 06:49:56PM +0200, Denis V. Lunev wrote:
> From: Denis V. Lunev <den@openvz.org>
> 
> The preallocate filter only does anything for a write that lands beyond
> the end of its file, and that is what a format driver above it produces.
> qemu-io could not produce it at all: the backend rejects such a request
> with EIO, and the write path asserts without the resize permission, so
> the filter could only be reached by stacking a real format on top and
> letting it choose the offsets.
> 
> Take the resize permission and allow writes beyond the end when -g is
> given, which leaves the tool free to address the filter directly.
> 
> Signed-off-by: Denis V. Lunev <den@openvz.org>
> CC: Kevin Wolf <kwolf@redhat.com>
> CC: Hanna Reitz <hreitz@redhat.com>
> CC: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
> CC: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
> ---
>  qemu-io.c | 32 +++++++++++++++++++++++++++++---
>  1 file changed, 29 insertions(+), 3 deletions(-)
> 
> diff --git a/qemu-io.c b/qemu-io.c
> index 2b684ccf55..87254b32ff 100644
> --- a/qemu-io.c
> +++ b/qemu-io.c
> @@ -45,6 +45,7 @@ static bool quit_qemu_io;
>  static int ncmdline;

> @@ -149,7 +166,7 @@ static const cmdinfo_t open_cmd = {
>      .argmin     = 1,
>      .argmax     = -1,
>      .flags      = CMD_NOFILE_OK,
> -    .args       = "[-rsCnkU] [-t cache] [-d discard] [-o options] [path]",
> +    .args       = "[-grsCnkU] [-t cache] [-d discard] [-o options] [path]",

No real pattern to how we sort the letters in this help synopsis, so
your choice of sticking -g first is no worse.

> @@ -175,7 +192,7 @@ static int open_f(BlockBackend *blk, int argc, char **argv, Error **errp)
>      QDict *opts;
>      bool force_share = false;
>  
> -    while ((c = getopt(argc, argv, "snCro:ki:t:d:U")) != -1) {
> +    while ((c = getopt(argc, argv, "gsnCro:ki:t:d:U")) != -1) {
>          switch (c) {
>          case 's':
>              flags |= BDRV_O_SNAPSHOT;
> @@ -228,6 +245,9 @@ static int open_f(BlockBackend *blk, int argc, char **argv, Error **errp)
>          case 'U':
>              force_share = true;
>              break;
> +        case 'g':
> +            write_beyond_eof = true;
> +            break;
>          default:

With a maintainer's hat on, I do find it easier to patch a getopt()
call where the short-option string was in alphabetical order (mixed
case okay) and then the case statements are in the same order. Even if
the option string is NOT in alphabetical order, keeping the case
statements in the same order makes some sense (ie. since you put 'g'
before 's' in the optstring, it should be before 's' in the case
statements).  However, while the maintainance of sticking 'g' at the
end rather than in any particular order is harder, the logic is not
wrong; and shuffling cases is minor enough that whether or not you do
that on a respin, you can add:

Reviewed-by: Eric Blake <eblake@redhat.com>

> @@ -522,6 +544,7 @@ int main(int argc, char **argv)
>          { "object", required_argument, NULL, OPTION_OBJECT },
>          { "image-opts", no_argument, NULL, OPTION_IMAGE_OPTS },
>          { "force-share", no_argument, 0, 'U'},
> +        { "grow", no_argument, NULL, 'g' },

Pre-existing, but since you're touching this, it might be nice to use
NULL in place of 0 on the force-share line.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.
Virtualization:  qemu.org | libguestfs.org