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Re: Tagging programs with git hash. [slightly OT]

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Ben Bacarisse <ben.usenet@bsb.me.uk> writes:

> Ben Bacarisse <ben.usenet@bsb.me.uk> writes:
>
>> Malcolm McLean <malcolm.arthur.mclean@gmail.com> writes:
>>
>>> Git and similar version control systems have an internal version tag,
>>> in git it is the sha-1 hash, which is used to identify the version
>>> within the control system.
>>>
>>> Now it would be nice to tag the executable with this hash, so that
>>> if a user reports a bug, you can ask them to run it in a mode that
>>> displays the hash, then go back to the correct version in the
>>> repository to find the bug.
>>>
>>> But here we have a problem.  The hash is created by applying a
>>> hash algorithm to the entire source, which is designed to change
>>> every digit of the hash if even a character of the source is
>>> changed.  So if we add the hash to the source, we've changed the
>>> hash.  So we can't tag an executable with its version number.
>>
>> Theoretically, this function -- the hash of a hash-containing
>> source -- will have a fixed point which would be the solution.
>> Unfortunately it will be impossible to find!
>
> It's a sad indictment of my state of mind, but I spent the time
> walking the dog thinking about this and I am not longer sure there
> will be a fixed point.

Right.  Certainly it is the case for some hash functions -- even
considering just those where every bit of output is potentially
affected by every bit of input -- that there may not be a fixed
point.  It seems likely (for good hash functions such as sha-1)
that there will be at least one fixed point for any given set of
source files, but like you say there is no good way to find one.
(And if there were a good way to find one then that would be an
argument for using a better hash function.)

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Thread

Tagging programs with git hash. [slightly OT] Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2019-05-31 07:02 -0700
  Re: Tagging programs with git hash. [slightly OT] queequeg@trust.no1 (Queequeg) - 2019-05-31 14:27 +0000
    Re: Tagging programs with git hash. [slightly OT] Mateusz Viste <mateusz@wont.tell> - 2019-05-31 16:42 +0000
      Re: Tagging programs with git hash. [slightly OT] Keith Thompson <kst-u@mib.org> - 2019-05-31 13:23 -0700
      Re: Tagging programs with git hash. [slightly OT] queequeg@trust.no1 (Queequeg) - 2019-05-31 21:10 +0000
        Re: Tagging programs with git hash. [slightly OT] Mateusz Viste <mateusz@wont.tell> - 2019-06-01 09:31 +0000
          Re: Tagging programs with git hash. [slightly OT] queequeg@trust.no1 (Queequeg) - 2019-06-01 12:39 +0000
  Re: Tagging programs with git hash. [slightly OT] David Brown <david.brown@hesbynett.no> - 2019-05-31 16:28 +0200
  Re: Tagging programs with git hash. [slightly OT] fir <profesor.fir@gmail.com> - 2019-05-31 07:51 -0700
  Re: Tagging programs with git hash. [slightly OT] Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-05-31 15:59 +0100
    Re: Tagging programs with git hash. [slightly OT] Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-05-31 22:56 +0100
      Re: Tagging programs with git hash. [slightly OT] John Forkosh <forkosh@panix.com> - 2019-06-01 02:55 +0000
        Re: Tagging programs with git hash. [slightly OT] fir <profesor.fir@gmail.com> - 2019-06-01 04:26 -0700
          Re: Tagging programs with git hash. [slightly OT] fir <profesor.fir@gmail.com> - 2019-06-01 04:35 -0700
      Re: Tagging programs with git hash. [slightly OT] Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-06-01 04:03 -0700
  Re: Tagging programs with git hash. [slightly OT] "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2019-05-31 13:30 -0700
  Re: Tagging programs with git hash. [slightly OT] Eli the Bearded <*@eli.users.panix.com> - 2019-05-31 21:46 +0000
  Re: Tagging programs with git hash. [slightly OT] Shuggie <shuggie@latepaul.com> - 2019-06-01 11:21 +0100
  Re: Tagging programs with git hash. [slightly OT] Ike Naar <ike@iceland.freeshell.org> - 2019-06-01 21:23 +0000
    Re: Tagging programs with git hash. [slightly OT] Keith Thompson <kst-u@mib.org> - 2019-06-01 14:34 -0700
    Re: Tagging programs with git hash. [slightly OT] Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2019-06-02 12:36 -0700
      Re: Tagging programs with git hash. [slightly OT] Keith Thompson <kst-u@mib.org> - 2019-06-02 14:35 -0700

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