Subject: Re: [PATCH] diff: add a config option to control orderfile

From: Jonathan Nieder <[email-masked]>
Date: Mon, 23 Sep 2013 14:37:29 -0700

Hi,

Michael S. Tsirkin wrote:
>> On Tue, Sep 17, 2013 at 04:56:16PM -0400, Jeff King wrote:

>>>>> A problem with both schemes, though, is that they are not
>>>>> backwards-compatible with existing git-patch-id implementations.
[...]
>>> It may be esoteric enough not to worry about, though.

Yeah, I think it would be okay.  Details of the diff generation
algorithm have changed from time to time anyway (and broken things,
as you mentioned) and we make no guarantee about this.

[...]
>> patch-id: make it more stable
>>
>> Add a new patch-id algorithm making it stable against
>> hunk reodering:
>> 	- prepend header to each hunk (if not there)
>> 	- calculate SHA1 hash for each hunk separately
>> 	- sum all hashes to get patch id
>>
>> Add --order-sensitive to get historical unstable behaviour.

The --order-sensitive option seems confusing.  How do I use it to
replicate a historical patch-id?  If I record all options that might
have influenced ordering (which are those?) then am I guaranteed to
get a reproducible result?  

So I would prefer either of the following over the above:

 a) When asked to compute the patch-id of a seekable file, use the
    current streaming implementation until you notice a filename that
    is out of order.  Then start over with sorted hunks (for example
    building a table of offsets within the patch for each hunk to
    support this).

    When asked to compute the patch-id of an unseekable file, stream
    to a temporary file under $GIT_DIR to get a seekable file.

 b) Unconditionally use the new patch-id definition that is stable
    under permutation of hunks.  If and when someone complains that
    this invalidates their old patch-ids, they can work on adding a
    nice interface for getting the old-style patch-ids.  I suspect it
    just wouldn't come up.

Of course I can easily be wrong.  Thanks for a clear patch that makes
the choices easy to reasonable about.

Thoughts?
Jonathan