Tools like `git filter-repo`[1] use `git fast-export` and
`git fast-import` to rewrite repository history. When rewriting
history using one such tool though, commit signatures might become
invalid because the commits they sign changed due to the changes
in the repository history made by the tool between the fast-export
and the fast-import steps.
Note that as far as signature handling goes:
* Since fast-export doesn't know what changes filter-repo may make
to the stream, it can't know whether the signatures will still be
valid.
* Since filter-repo doesn't know what history canonicalizations
fast-export performed (and it performs a few), it can't know whether
the signatures will still be valid.
* Therefore, fast-import is the only process in the pipeline that
can know whether a specified signature remains valid.
Having invalid signatures in a rewritten repository could be
confusing, so users rewritting history might prefer to simply
discard signatures that are invalid at the fast-import step.
For example a common use case is to rewrite only "recent" history.
While specifying commit ranges corresponding to "recent" commits
could work, users worry about getting it wrong and want to just
automatically rewrite everything, expecting older commit signatures
to be untouched.
To let them do that, let's add a new 'strip-if-invalid' mode to the
`--signed-commits=<mode>` option of `git fast-import`.
It would be interesting for the `--signed-tags=<mode>` option to
have this mode too, but we leave that for a future improvement.
It might also be possible for `git fast-export` to have such a mode
in its `--signed-commits=<mode>` and `--signed-tags=<mode>`
options, but the use cases for it are much less clear, so we also
leave that for possible future improvements.
For now let's just die() if 'strip-if-invalid' is passed to these
options where it hasn't been implemented yet.
[1]: https://github.com/newren/git-filter-repo
Helped-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
A '--signed-commits=<mode>' option is already available when using
`git fast-export` to decide what should be done at export time about
commit signatures. At import time though, there is no option, or
other way, in `git fast-import` to decide about commit signatures.
To remediate that, let's add a '--signed-commits=<mode>' option to
`git fast-import` too.
For now the supported <mode>s are the same as those supported by
`git fast-export`.
The code responsible for consuming a signature is refactored into
the import_one_signature() and discard_one_signature() functions,
which makes it easier to follow the logic and add new modes in the
future.
In the 'strip' and 'warn-strip' modes, we deliberately use
discard_one_signature() to discard the signature without parsing it.
This ensures that even malformed signatures, which would cause the
parser to fail, can be successfully stripped from a commit.
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
