Re: [PATCH] technical doc: add a design doc for the evolve command

[Jonathan Nieder <jrnieder@gmail.com>]

Ævar Arnfjörð Bjarmason wrote:
> On Thu, Nov 15 2018, sxenos@google.com wrote:

>> +Parent-type
>> +-----------
>> +The “parent-type” field in the commit header identifies a commit as a
>> +meta-commit and indicates the meaning for each of its parents. It is never
>> +present for normal commits.
[...]
> I think it's worth pointing out for those that are rusty on commit
> object details (but I checked) is that the reason for it not being:
>
>     tree 4b825dc642cb6eb9a060e54bf8d69288fbee4904
>     parent aa7ce55545bf2c14bef48db91af1a74e2347539a
>     parent-type content
>     parent d64309ee51d0af12723b6cb027fc9f195b15a5e9
>     parent-type obsolete
>     parent 7e1bbcd3a0fa854a7a9eac9bf1eea6465de98136
>     parent-type origin
>     author Stefan Xenos <sxenos@gmail.com> 1540841596 -0700
>     committer Stefan Xenos <sxenos@gmail.com> 1540841596 -0700
>
> Which would be easier to read, is that we're very sensitive to the order
> of the first few fields (tree -> parent -> author -> committer) and fsck
> will error out if we interjected a new field.

By the way, in the spirit of limiting the initial scope, I wonder
whether the parent-type fields can be stored in the commit message
initially.

Elsewhere in this thread it was mentioned that the parent-type is a
field to allow tools like "git fsck" to understand the meaning of
these parent relationships (for example, to forbid a commit
referencing a meta-commit).  The same could be done using special
commit message text, though.

The advantage of such an approach would be that we could experiment
without changing the official object format at all.  If experiments
revealed a different set of information to store, we could update the
format without having to maintain the memory of the older format in
"git fsck"'s understanding of commit object fields.  So even though I
think that in the end we would want to put this information in the
commit object header, I'm tempted to suspect that the benefits of
putting it in the commit message to start outweigh the costs (in
particular, of having to migrate to another format later).

Thanks,
Jonathan