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Re: [Qemu-devel] [PATCH] char: Fix removing wrong GSource that be found

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH] char: Fix removing wrong GSource that be found by fd_in_tag
Date: Tue, 18 Apr 2017 08:36:29 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.0

On 04/14/2017 05:10 AM, Marc-André Lureau wrote:
> Hi
> ----- Original Message -----
>> We use fd_in_tag to find a GSource, fd_in_tag is return value of
>> g_source_attach(GSource *source, GMainContext *context), the return
>> value is unique only in the same context, so we may get the same
>> values with different 'context' parameters.
>> It is no problem to find the right fd_in_tag by using
>>  g_main_context_find_source_by_id(GMainContext *context, guint source_id)
>> while there is only one default main context.
>> But colo-compare tries to create/use its own context, and if we pass wrong
>> 'context' parameter with right fd_in_tag, we will find a wrong GSource
>> to handle. We tied to fix the related codes in commit b43dec, but it didn't
> tied->tried
> Please use a bit longer commit sha1, or full sha1, it will likely conflict 
> otherwise in the future.

6 chars is indeed short, 7 is git's default as usually long enough,
although I've encountered collisions that require 8 chars.  [And google
has proved that you can have a collision across the entire hash,
although that is harder to generate.] I generally use 8 or so when
writing commit messages.  Fortunately, even if a collision is introduces
later, someone that is motivated enough can still resolve the collision
by filtering out any collisions that resolve to non-commits, and among
the remaining colliding SHA1 focus on the one that has a commit date
which predates the message with the reference.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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