[Bitcoin-development] PSA: Please sign your git commits

[Wladimir <laanwj@gmail.com>]

Hello all,

When you're contributing to Bitcoin Core development please sign your
git commits. This is easy to do and will help in assuring the
integrity of the tree.

How to sign your commits?
------------------------------------------

Provide the `-S` flag (or `--gpg-sign`) to git commit when you commit
your changes, for example

    git commit -m "Commit message" -S

Optionally you can provide a key id after the -S option to sign with a
specific key.

What if I forgot?
-------------------------

You can retroactively sign your previous commit using --amend, for example

    git commit -S --amend

If you need to go further back, you can use the interactive rebase
command with 'edit'. Replace HEAD~3 with the base commit from which
you want to start.

    git rebase -i HEAD~3

Replace 'pick' by 'edit' for the commit that you want to sign and the
rebasing will stop after that commit. Then you can amend the commit as
above. Afterwards, do

    git rebase --continue

As this will rewrite history, you cannot do this when your commit is
already merged. In that case, too bad, better luck next time.

If you rewrite history for another reason - for example when squashing
commits - make sure that you re-sign as the signatures will be lost.

How to check if commits are signed?
-------------------------------------------------------

Use git log with show-signature,

    git log --show-signature

    commit 6fcdad787f1fb381a3a0fe6b1a1e45477426dccb
    gpg: Signature made Wed 21 May 2014 12:27:55 PM CEST using RSA key
ID 2346C9A6
    gpg: Good signature from "Wladimir J. van der Laan <laanwj@gmail.com>"
    Author: Wladimir J. van der Laan <laanwj@gmail.com>
    Date:   Wed May 21 12:27:37 2014 +0200

        qt: Periodic language update
    ...

You can also pass the --show-signature option to `git show` to check a
single commit.

If you do this on the current repository you'll see that I'm almost
the only person signing commits. I would like more people to get into
this habit.

How to sign merges?
--------------------------------

When using the github interface to merge a pull request, the resulting
merge commit is not signed.

Pieter Wullie wrote a script that simplifies merging and signing. It
can be found in contrib/devtools. Setup instructions can be found in
the README.md in that directory. After setting it up for the
repository you can use the script in the following way:

    contrib/devtools/github-merge.sh 1234

Replace 1234 by the pull request number that you want to merge. It
will merge the pull request and drop you into a shell so you can
verify changes and test. Once satisfied, exit the shell and answer the
questions to merge and sign it and push upstream automatically (or
not).

Please use this script when possible for merging instead of the github
interface.

--------------------------

Wladimir