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Date: Wed, 14 May 2014 09:43:38 -0400
From: Jeff Cody <jcody@redhat.com>
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Subject: Re: [Qemu-devel] [PULL 03/17] qemu-img: sort block formats in help
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To: Mike Day <ncmike@ncultra.org>
Cc: Cornelia Huck <cornelia.huck@de.ibm.com>, Stefan Hajnoczi <stefanha@gmail.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, Stefan Hajnoczi <stefanha@redhat.com>, Peter Maydell <peter.maydell@linaro.org>

On Wed, May 14, 2014 at 09:27:31AM -0400, Mike Day wrote:
> On Wed, May 14, 2014 at 8:35 AM, Stefan Hajnoczi <stefanha@gmail.com> wrote:
> > Jeff Cody recently wanted to eliminate duplicate entries in the list.  I
> > thought part of your intention was to address the duplicates with your
> > patch.
> >
> > We can back out the sequence API if it's not supported on older glib but
> > it would be nice to eliminate duplicates later, too.
> 
> I agree. I can submit an additional patch that uses an older API.
> What, exactly is the cause of duplicate entries in the list? I've only
> seen it one time but its disconcerting when it happens.
>

Some block drivers register multiple drivers for a given format name,
for instance, gluster:

https://github.com/qemu/qemu/blob/master/block/gluster.c#L818

    static void bdrv_gluster_init(void)
    {
        bdrv_register(&bdrv_gluster_rdma);
        bdrv_register(&bdrv_gluster_unix);
        bdrv_register(&bdrv_gluster_tcp);
        bdrv_register(&bdrv_gluster);
    }

Each of those drivers has a format_name of "gluster", in this example.

In qemu-img, it lists the supported formats by simply calling
bdrv_iterate_format(), which calls a callback function for each
format_name in the driver list.

Prior to a recent commit, this function did not make distinction on
duplicates.  As of commit e855e4fb7, duplicates are not longer printed
in the help message:

e855e4fb7: Ignore duplicate or NULL format_name in bdrv_iterate_format):

https://github.com/qemu/qemu/commit/e855e4fb7b97f7f605e1f44427b98022e39e6f8f#diff-ea36ba0f79150cc299732696a069caba