Junio C Hamano <gits...@pobox.com> writes:

> There are unbound number of kinds of trailers people would want to
> add, depending on their projects' needs.  We should not have to add
> a specific support for a tailer like this one, before thinking
> through to see if we can add generic support for adding arbitrary
> trailers to avoid code and interface bloat.
> Think of the existing --signoff as a historical mistake.  Such a
> generic "adding arbitrary trailers" support, when done properly,
> should be able to express what "--signoff" does, and we should be
> able to redo "--signoff" as a special case of that generic "adding
> arbitrary trailers" support, and at that point, "Fixes:" trailer the
> kernel project wants to use should fall out as a natural consequence.

Thinking aloud further, what I had in mind was along the lines of
the following.

 * The most generic external interface would be spelled as

    --trailer <token>[=<param>]

   where <token> can be things like "signoff", "closes", "acked-by",
   "change-id", "fixes", etc.; they can be taken from an unbounded
   set.  The historical "--signoff" can become a short-hand for
   "--trailer signoff".  More than one "--trailer" option can be
   given on a single command line.

 * The token is used to look into the configuration, e.g.,

   [commitTrailer "signoff"]
        style = append-norepeat
        trailer = Signed-off-by
        command = echo "$GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL>"'

   [commitTrailer "change-id"]
        style = append-only-if-missing
        trailer = Change-Id
        command = 'git hash-object -t commit --stdin <$GIT_PROTO_COMMIT'

   [commitTrailer "fixes"]
        style = overwrite
        trailer = Fixes
        command = 'git log -1 --oneline --format="%h (%s)" --abbrev-commit=14 


   - "commitTrailer.<token>.style" defines the interaction with
     existing trailer of the same kind (e.g. S-o-b: accumulates by
     appending, but we try not to repeat the same sign-off twice
     which would show you forwarding your own message you are the
     last person in the Sign-off chain; Fixes: if there is already
     one will remove the old one and replaces; etc.);

   - "commitTrailer.<token>.trailer" defines the trailer label at
     the beginning of the trailer line;

   - "commitTrailer.<token>.command" gives the command to run to
     obtain the payload after the "trailer" label.  A handful
     obvious and useful variables are exported for the command to
     use, and <param> is exported as $ARG, if present.

With the most generic syntax, with the above commitTrailer.fixes.*
configuration, I would imagine that you can say something like:

    git commit --trailer fixes="v2.6.12^{/^i386: tweak frobnitz}"

to say that the first commit you find traversing the history of
v2.6.12 whose title is "i386: tweak frobnitz" was faulty, and you
are creating a commit that corrects its mistake.

Giving some default configuration to often used trailer types
(e.g. configuration for "--trailer signoff") and promoting some
commonly used ones into a separate built-in option (e.g. an option
"--signoff" that does not have to say "--trailer signoff") are
entirely separate issues, and only time can nudge us into evaluating
individual types of trailers.
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