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Subject: Re: [PATCH] scripts/setlocalversion: make git describe output more
 reliable
To:     Brian Norris <briannorris@chromium.org>,
        Masahiro Yamada <masahiroy@kernel.org>
Cc:     Randy Dunlap <rdunlap@infradead.org>,
        Guenter Roeck <linux@roeck-us.net>,
        Bhaskar Chowdhury <unixbhaskar@gmail.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
References: <20200910112701.13853-1-linux@rasmusvillemoes.dk>
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 <CA+ASDXOuK=iCdzWbtc+aRhBy=8xy860XqxwJg+wFuQaXKfg3UQ@mail.gmail.com>
From:   Rasmus Villemoes <linux@rasmusvillemoes.dk>
Message-ID: <f3ce9b3e-d3c1-a96b-e14b-a8d3c4600b09@rasmusvillemoes.dk>
Date:   Fri, 11 Sep 2020 10:28:16 +0200
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On 10/09/2020 21.05, Brian Norris wrote:
> On Thu, Sep 10, 2020 at 7:35 AM Masahiro Yamada <masahiroy@kernel.org> wrote:
>> On Thu, Sep 10, 2020 at 8:57 PM Rasmus Villemoes
>> <linux@rasmusvillemoes.dk> wrote:
>>> So in order to avoid `uname -a` output relying on such random details
>>> of the build environment which are rather hard to ensure are
>>> consistent between developers and buildbots, use an explicit
>>> --abbrev=15 option (and for consistency, also use rev-parse --short=15
>>> for the unlikely case of no signed tags being usable).
> 
> For the patch:
> 
> Reviewed-by: Brian Norris <briannorris@chromium.org>
> 
>> I agree that any randomness should be avoided.
>>
>> My question is, do we need 15-digits?
> ...
>> So, I think the conflict happens
>> only when we have two commits that start with the same 7-digits
>> in the _same_ release. Is this correct?

No.

> For git-describe (the case where we have a tag to base off):
> "use <n> digits, or as many digits as needed to form a unique object name"

Yes, the abbreviated hash that `git describe` produces is unique among
all objects (and objects are more than just commits) in the current
repo, so what matters for probability-of-collision is the total number
of objects - linus.git itself probably grows ~60000 objects per release.

As for "do we need 15 digits", well, in theory the next time I merge the
next rt-stable tag into our kernel I could end up with a commit that
matches some existing object in the first 33 hex chars at which point I
should have chosen 34 - but of course that's so unlikely that it's
irrelevant.

But the upshot of that is that there really is no objective answer to
"how many digits do we need", so it becomes a tradeoff between "low
enough probability that anyone anywhere in the next few years would have
needed more than X when building their own kernel" and readability of
`uname -r` etc. So I decided somewhat arbitrarily that each time one
rolls a new release, there should be less than 1e-9 probability that
HEAD collides with some other object when abbreviated to X hexchars.
X=12 doesn't pass that criteria even when the repo has only 10M objects
(and, current linus.git already has objects that need 12 to be unique,
so such collisions do happen in practice, though of course very rarely).
13 and 14 are just weird numbers, so I ended with 15, which also allows
many many more objects in the repo before the probability crosses that
1e-9 threshold.

Rasmus

Side note 1: Note that there really isn't any such thing as "last
tag/previous tag" in a DAG; I think what git does is a best-effort
search for the eligible tag that minimizes #({objects reachable from
commit-to-be-described} - {objects reachable from tag}), where eligible
tag means at least reachable from commit-to-be-described (so that set
difference is a proper one), but there can be additional constraints
(e.g. --match=...).

Side note 2: Linus or Greg releases are actually not interesting here
(see the logic in setlocalversion that stops early if we're exactly at
an annotated tag) - the whole raison d'etre for setlocalversion is that
people do maintain and build non-mainline kernels, and it's extremely
useful for `uname -a` to accurately tell just which commit is running on
the target.