1From: Junio C Hamano <gitster@pobox.com> 2Date: Tue, 17 Jan 2011 13:00:00 -0800 3Subject: Using signed tag in pull requests 4Abstract: Beginning v1.7.9, a contributor can push a signed tag to her 5 publishing repository and ask her integrator to pull it. This assures the 6 integrator that the pulled history is authentic and allows others to 7 later validate it. 8Content-type: text/asciidoc 9 10How to use a signed tag in pull requests 11======================================== 12 13A typical distributed workflow using Git is for a contributor to fork a 14project, build on it, publish the result to her public repository, and ask 15the "upstream" person (often the owner of the project where she forked 16from) to pull from her public repository. Requesting such a "pull" is made 17easy by the `git request-pull` command. 18 19Earlier, a typical pull request may have started like this: 20 21------------ 22 The following changes since commit 406da78032179...: 23 24 Froboz 3.2 (2011-09-30 14:20:57 -0700) 25 26 are available in the Git repository at: 27 28 example.com:/git/froboz.git for-xyzzy 29------------ 30 31followed by a shortlog of the changes and a diffstat. 32 33The request was for a branch name (e.g. `for-xyzzy`) in the public 34repository of the contributor, and even though it stated where the 35contributor forked her work from, the message did not say anything about 36the commit to expect at the tip of the for-xyzzy branch. If the site that 37hosts the public repository of the contributor cannot be fully trusted, it 38was unnecessarily hard to make sure what was pulled by the integrator was 39genuinely what the contributor had produced for the project. Also there 40was no easy way for third-party auditors to later verify the resulting 41history. 42 43Starting from Git release v1.7.9, a contributor can add a signed tag to 44the commit at the tip of the history and ask the integrator to pull that 45signed tag. When the integrator runs `git pull`, the signed tag is 46automatically verified to assure that the history is not tampered with. 47In addition, the resulting merge commit records the content of the signed 48tag, so that other people can verify that the branch merged by the 49integrator was signed by the contributor, without fetching the signed tag 50used to validate the pull request separately and keeping it in the refs 51namespace. 52 53This document describes the workflow between the contributor and the 54integrator, using Git v1.7.9 or later. 55 56 57A contributor or a lieutenant 58----------------------------- 59 60After preparing her work to be pulled, the contributor uses `git tag -s` 61to create a signed tag: 62 63------------ 64 $ git checkout work 65 $ ... "git pull" from sublieutenants, "git commit" your own work ... 66 $ git tag -s -m "Completed frotz feature" frotz-for-xyzzy work 67------------ 68 69Note that this example uses the `-m` option to create a signed tag with 70just a one-liner message, but this is for illustration purposes only. It 71is advisable to compose a well-written explanation of what the topic does 72to justify why it is worthwhile for the integrator to pull it, as this 73message will eventually become part of the final history after the 74integrator responds to the pull request (as we will see later). 75 76Then she pushes the tag out to her public repository: 77 78------------ 79 $ git push example.com:/git/froboz.git/ +frotz-for-xyzzy 80------------ 81 82There is no need to push the `work` branch or anything else. 83 84Note that the above command line used a plus sign at the beginning of 85`+frotz-for-xyzzy` to allow forcing the update of a tag, as the same 86contributor may want to reuse a signed tag with the same name after the 87previous pull request has already been responded to. 88 89The contributor then prepares a message to request a "pull": 90 91------------ 92 $ git request-pull v3.2 example.com:/git/froboz.git/ frotz-for-xyzzy >msg.txt 93------------ 94 95The arguments are: 96 97. the version of the integrator's commit the contributor based her work on; 98. the URL of the repository, to which the contributor has pushed what she 99 wants to get pulled; and 100. the name of the tag the contributor wants to get pulled (earlier, she could 101 write only a branch name here). 102 103The resulting msg.txt file begins like so: 104 105------------ 106 The following changes since commit 406da78032179...: 107 108 Froboz 3.2 (2011-09-30 14:20:57 -0700) 109 110 are available in the Git repository at: 111 112 example.com:/git/froboz.git tags/frotz-for-xyzzy 113 114 for you to fetch changes up to 703f05ad5835c...: 115 116 Add tests and documentation for frotz (2011-12-02 10:02:52 -0800) 117 118 ----------------------------------------------- 119 Completed frotz feature 120 ----------------------------------------------- 121------------ 122 123followed by a shortlog of the changes and a diffstat. Comparing this with 124the earlier illustration of the output from the traditional `git request-pull` 125command, the reader should notice that: 126 127. The tip commit to expect is shown to the integrator; and 128. The signed tag message is shown prominently between the dashed lines 129 before the shortlog. 130 131The latter is why the contributor would want to justify why pulling her 132work is worthwhile when creating the signed tag. The contributor then 133opens her favorite MUA, reads msg.txt, edits and sends it to her upstream 134integrator. 135 136 137Integrator 138---------- 139 140After receiving such a pull request message, the integrator fetches and 141integrates the tag named in the request, with: 142 143------------ 144 $ git pull example.com:/git/froboz.git/ tags/frotz-for-xyzzy 145------------ 146 147This operation will always open an editor to allow the integrator to fine 148tune the commit log message when merging a signed tag. Also, pulling a 149signed tag will always create a merge commit even when the integrator does 150not have any new commit since the contributor's work forked (i.e. 'fast 151forward'), so that the integrator can properly explain what the merge is 152about and why it was made. 153 154In the editor, the integrator will see something like this: 155 156------------ 157 Merge tag 'frotz-for-xyzzy' of example.com:/git/froboz.git/ 158 159 Completed frotz feature 160 # gpg: Signature made Fri 02 Dec 2011 10:03:01 AM PST using RSA key ID 96AFE6CB 161 # gpg: Good signature from "Con Tributor <nitfol@example.com>" 162------------ 163 164Notice that the message recorded in the signed tag "Completed frotz 165feature" appears here, and again that is why it is important for the 166contributor to explain her work well when creating the signed tag. 167 168As usual, the lines commented with `#` are stripped out. The resulting 169commit records the signed tag used for this validation in a hidden field 170so that it can later be used by others to audit the history. There is no 171need for the integrator to keep a separate copy of the tag in his 172repository (i.e. `git tag -l` won't list the `frotz-for-xyzzy` tag in the 173above example), and there is no need to publish the tag to his public 174repository, either. 175 176After the integrator responds to the pull request and her work becomes 177part of the permanent history, the contributor can remove the tag from 178her public repository, if she chooses, in order to keep the tag namespace 179of her public repository clean, with: 180 181------------ 182 $ git push example.com:/git/froboz.git :frotz-for-xyzzy 183------------ 184 185 186Auditors 187-------- 188 189The `--show-signature` option can be given to `git log` or `git show` and 190shows the verification status of the embedded signed tag in merge commits 191created when the integrator responded to a pull request of a signed tag. 192 193A typical output from `git show --show-signature` may look like this: 194 195------------ 196 $ git show --show-signature 197 commit 02306ef6a3498a39118aef9df7975bdb50091585 198 merged tag 'frotz-for-xyzzy' 199 gpg: Signature made Fri 06 Jan 2012 12:41:49 PM PST using RSA key ID 96AFE6CB 200 gpg: Good signature from "Con Tributor <nitfol@example.com>" 201 Merge: 406da78 703f05a 202 Author: Inte Grator <xyzzy@example.com> 203 Date: Tue Jan 17 13:49:41 2012 -0800 204 205 Merge tag 'frotz-for-xyzzy' of example.com:/git/froboz.git/ 206 207 Completed frotz feature 208 209 * tag 'frotz-for-xyzzy' (100 commits) 210 Add tests and documentation for frotz 211 ... 212------------ 213 214There is no need for the auditor to explicitly fetch the contributor's 215signature, or to even be aware of what tag(s) the contributor and integrator 216used to communicate the signature. All the required information is recorded 217as part of the merge commit.