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diff --git a/Documentation/howto/using-signed-tag-in-pull-request.txt b/Documentation/howto/using-signed-tag-in-pull-request.txt new file mode 100644 index 0000000000..a1351c5bb8 --- /dev/null +++ b/Documentation/howto/using-signed-tag-in-pull-request.txt @@ -0,0 +1,217 @@ +From: Junio C Hamano <gitster@pobox.com> +Date: Tue, 17 Jan 2011 13:00:00 -0800 +Subject: Using signed tag in pull requests +Abstract: Beginning v1.7.9, a contributor can push a signed tag to her + publishing repository and ask her integrator to pull it. This assures the + integrator that the pulled history is authentic and allows others to + later validate it. +Content-type: text/asciidoc + +Using signed tag in pull requests +================================= + +A typical distributed workflow using Git is for a contributor to fork a +project, build on it, publish the result to her public repository, and ask +the "upstream" person (often the owner of the project where she forked +from) to pull from her public repository. Requesting such a "pull" is made +easy by the `git request-pull` command. + +Earlier, a typical pull request may have started like this: + +------------ + The following changes since commit 406da78032179...: + + Froboz 3.2 (2011-09-30 14:20:57 -0700) + + are available in the git repository at: + + example.com:/git/froboz.git for-xyzzy +------------ + +followed by a shortlog of the changes and a diffstat. + +The request was for a branch name (e.g. `for-xyzzy`) in the public +repository of the contributor, and even though it stated where the +contributor forked her work from, the message did not say anything about +the commit to expect at the tip of the for-xyzzy branch. If the site that +hosts the public repository of the contributor cannot be fully trusted, it +was unnecessarily hard to make sure what was pulled by the integrator was +genuinely what the contributor had produced for the project. Also there +was no easy way for third-party auditors to later verify the resulting +history. + +Starting from Git release v1.7.9, a contributor can add a signed tag to +the commit at the tip of the history and ask the integrator to pull that +signed tag. When the integrator runs `git pull`, the signed tag is +automatically verified to assure that the history is not tampered with. +In addition, the resulting merge commit records the content of the signed +tag, so that other people can verify that the branch merged by the +integrator was signed by the contributor, without fetching the signed tag +used to validate the pull request separately and keeping it in the refs +namespace. + +This document describes the workflow between the contributor and the +integrator, using Git v1.7.9 or later. + + +A contributor or a lieutenant +----------------------------- + +After preparing her work to be pulled, the contributor uses `git tag -s` +to create a signed tag: + +------------ + $ git checkout work + $ ... "git pull" from sublieutenants, "git commit" your own work ... + $ git tag -s -m "Completed frotz feature" frotz-for-xyzzy work +------------ + +Note that this example uses the `-m` option to create a signed tag with +just a one-liner message, but this is for illustration purposes only. It +is advisable to compose a well-written explanation of what the topic does +to justify why it is worthwhile for the integrator to pull it, as this +message will eventually become part of the final history after the +integrator responds to the pull request (as we will see later). + +Then she pushes the tag out to her public repository: + +------------ + $ git push example.com:/git/froboz.git/ +frotz-for-xyzzy +------------ + +There is no need to push the `work` branch or anything else. + +Note that the above command line used a plus sign at the beginning of +`+frotz-for-xyzzy` to allow forcing the update of a tag, as the same +contributor may want to reuse a signed tag with the same name after the +previous pull request has already been responded to. + +The contributor then prepares a message to request a "pull": + +------------ + $ git request-pull v3.2 example.com:/git/froboz.git/ frotz-for-xyzzy >msg.txt +------------ + +The arguments are: + +. the version of the integrator's commit the contributor based her work on; +. the URL of the repository, to which the contributor has pushed what she + wants to get pulled; and +. the name of the tag the contributor wants to get pulled (earlier, she could + write only a branch name here). + +The resulting msg.txt file begins like so: + +------------ + The following changes since commit 406da78032179...: + + Froboz 3.2 (2011-09-30 14:20:57 -0700) + + are available in the git repository at: + + example.com:/git/froboz.git frotz-for-xyzzy + + for you to fetch changes up to 703f05ad5835c...: + + Add tests and documentation for frotz (2011-12-02 10:02:52 -0800) + + ----------------------------------------------- + Completed frotz feature + ----------------------------------------------- +------------ + +followed by a shortlog of the changes and a diffstat. Comparing this with +the earlier illustration of the output from the traditional `git request-pull` +command, the reader should notice that: + +. The tip commit to expect is shown to the integrator; and +. The signed tag message is shown prominently between the dashed lines + before the shortlog. + +The latter is why the contributor would want to justify why pulling her +work is worthwhile when creating the signed tag. The contributor then +opens her favorite MUA, reads msg.txt, edits and sends it to her upstream +integrator. + + +Integrator +---------- + +After receiving such a pull request message, the integrator fetches and +integrates the tag named in the request, with: + +------------ + $ git pull example.com:/git/froboz.git/ frotz-for-xyzzy +------------ + +This operation will always open an editor to allow the integrator to fine +tune the commit log message when merging a signed tag. Also, pulling a +signed tag will always create a merge commit even when the integrator does +not have any new commit since the contributor's work forked (i.e. 'fast +forward'), so that the integrator can properly explain what the merge is +about and why it was made. + +In the editor, the integrator will see something like this: + +------------ + Merge tag 'frotz-for-xyzzy' of example.com:/git/froboz.git/ + + Completed frotz feature + # gpg: Signature made Fri 02 Dec 2011 10:03:01 AM PST using RSA key ID 96AFE6CB + # gpg: Good signature from "Con Tributor <nitfol@example.com>" +------------ + +Notice that the message recorded in the signed tag "Completed frotz +feature" appears here, and again that is why it is important for the +contributor to explain her work well when creating the signed tag. + +As usual, the lines commented with `#` are stripped out. The resulting +commit records the signed tag used for this validation in a hidden field +so that it can later be used by others to audit the history. There is no +need for the integrator to keep a separate copy of the tag in his +repository (i.e. `git tag -l` won't list the `frotz-for-xyzzy` tag in the +above example), and there is no need to publish the tag to his public +repository, either. + +After the integrator responds to the pull request and her work becomes +part of the permanent history, the contributor can remove the tag from +her public repository, if she chooses, in order to keep the tag namespace +of her public repository clean, with: + +------------ + $ git push example.com:/git/froboz.git :frotz-for-xyzzy +------------ + + +Auditors +-------- + +The `--show-signature` option can be given to `git log` or `git show` and +shows the verification status of the embedded signed tag in merge commits +created when the integrator responded to a pull request of a signed tag. + +A typical output from `git show --show-signature` may look like this: + +------------ + $ git show --show-signature + commit 02306ef6a3498a39118aef9df7975bdb50091585 + merged tag 'frotz-for-xyzzy' + gpg: Signature made Fri 06 Jan 2012 12:41:49 PM PST using RSA key ID 96AFE6CB + gpg: Good signature from "Con Tributor <nitfol@example.com>" + Merge: 406da78 703f05a + Author: Inte Grator <xyzzy@example.com> + Date: Tue Jan 17 13:49:41 2012 -0800 + + Merge tag 'frotz-for-xyzzy' of example.com:/git/froboz.git/ + + Completed frotz feature + + * tag 'frotz-for-xyzzy' (100 commits) + Add tests and documentation for frotz + ... +------------ + +There is no need for the auditor to explicitly fetch the contributor's +signature, or to even be aware of what tag(s) the contributor and integrator +used to communicate the signature. All the required information is recorded +as part of the merge commit. |