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pulling signed tag: add howto document

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Documentation/Makefile

 ARTICLES += git-tools
 ARTICLES += git-bisect-lk2009
 # with their own formatting rules.
-SP_ARTICLES = howto/revert-branch-rebase howto/using-merge-subtree user-manual
+SP_ARTICLES = user-manual
+SP_ARTICLES += howto/revert-branch-rebase
+SP_ARTICLES += howto/using-merge-subtree
+SP_ARTICLES += howto/using-signed-tag-in-pull-request
 API_DOCS = $(patsubst %.txt,%,$(filter-out technical/api-index-skel.txt technical/api-index.txt, $(wildcard technical/api-*.txt)))
 SP_ARTICLES += $(API_DOCS)
 SP_ARTICLES += technical/api-index

Documentation/howto/using-signed-tag-in-pull-request.txt

+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.