[Mesa-dev] [PATCH] automake: increase the MESA_GIT_SHA1 hash id length from 7 to 10 digits

[Emil Velikov]

On 15 June 2017 at 10:12, Eric Engestrom <eric.engestrom at imgtec.com> wrote:
> On Thursday, 2017-06-15 09:56:55 +0100, Emil Velikov wrote:
>> Hi Brian,
>>
>> On 15 June 2017 at 04:46, Brian Paul <brianp at vmware.com> wrote:
>> > The SCons build has been using 10 digits of the git hash id for the
>> > MESA_GIT_SHA1 string in git_sha1.h for about a year now.  I bumped it
>> > up after running into a case where a 7-digit hash ID was ambiguous.
>> >
>> > This patch makes the same change for the autotools build.
>> >
>> > The command "git log | grep "^commit" | cut -b 8-14 | sort | uniq -d"
>> > shows there are currently 17 cases where 7 digits of hash id are
>> > ambiguous on master (probably quite a few more if we'd consider other
>> > branches).
>> >
>> > Instead of using "git log -n 1 --oneline" use
>> > "git rev-parse --short=10 HEAD" to get the HEAD hash id.
>>
>> The current command produces 11 digit SHA
>
> That would depend on your git version (it used to be 7 until early 2.x
> and is 10 on my 2.13.1), and your core.abbrev config :)
>
Hmm that may explain some of it - using 2.13.0 here.
Local/global/system core.abbrev on the other hand is empty.

>> and size will grow automatically as needed.
>
> Ack, but only after the collision happens, I think the point is to be
> big enough that the hash stored will be unique compared to (reasonably)
> all the future versions. Being explicit about the min length does that.
>
Having it future proof sounds reasonable - thanks did not think of that.

Not sure about the growth algo. See the following:

$ for i in `seq 14 20`; do \
   echo -n "SHA digits $[$i-8+1] -- conflicts "; \
   git log | grep "^commit" | cut -b 8-$i | sort | uniq -d | wc -l; \
done

SHA digits 7 -- conflicts 17
SHA digits 8 -- conflicts 1
SHA digits 9 -- conflicts 0
SHA digits 10 -- conflicts 0
SHA digits 11 -- conflicts 0
SHA digits 12 -- conflicts 0
SHA digits 13 -- conflicts 0

$ git log -n 1 --oneline
18d4a6f964c (HEAD -> master) i965: gen4_blorp_exec.h to the sources list

-Emil