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| </head> | |
| <body class="article"> | |
| <div id="header"> | |
| <h1>Keep authoritative canonical history correct with git pull</h1> | |
| </div> | |
| <div id="content"> | |
| <div id="preamble"> | |
| <div class="sectionbody"> | |
| <div class="paragraph"><p>Sometimes a new project integrator will end up with project history | |
| that appears to be "backwards" from what other project developers | |
| expect. This howto presents a suggested integration workflow for | |
| maintaining a central repository.</p></div> | |
| <div class="paragraph"><p>Suppose that that central repository has this history:</p></div> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre><code> ---o---o---A</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>which ends at commit <code>A</code> (time flows from left to right and each node | |
| in the graph is a commit, lines between them indicating parent-child | |
| relationship).</p></div> | |
| <div class="paragraph"><p>Then you clone it and work on your own commits, which leads you to | |
| have this history in <strong>your</strong> repository:</p></div> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre><code> ---o---o---A---B---C</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>Imagine your coworker did the same and built on top of <code>A</code> in <strong>his</strong> | |
| repository in the meantime, and then pushed it to the | |
| central repository:</p></div> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre><code> ---o---o---A---X---Y---Z</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>Now, if you <code>git push</code> at this point, because your history that leads | |
| to <code>C</code> lacks <code>X</code>, <code>Y</code> and <code>Z</code>, it will fail. You need to somehow make | |
| the tip of your history a descendant of <code>Z</code>.</p></div> | |
| <div class="paragraph"><p>One suggested way to solve the problem is "fetch and then merge", aka | |
| <code>git pull</code>. When you fetch, your repository will have a history like | |
| this:</p></div> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre><code> ---o---o---A---B---C | |
| \ | |
| X---Y---Z</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>Once you run merge after that, while still on <strong>your</strong> branch, i.e. <code>C</code>, | |
| you will create a merge <code>M</code> and make the history look like this:</p></div> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre><code> ---o---o---A---B---C---M | |
| \ / | |
| X---Y---Z</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p><code>M</code> is a descendant of <code>Z</code>, so you can push to update the central | |
| repository. Such a merge <code>M</code> does not lose any commit in both | |
| histories, so in that sense it may not be wrong, but when people want | |
| to talk about "the authoritative canonical history that is shared | |
| among the project participants", i.e. "the trunk", they often view | |
| it as "commits you see by following the first-parent chain", and use | |
| this command to view it:</p></div> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre><code> $ git log --first-parent</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>For all other people who observed the central repository after your | |
| coworker pushed <code>Z</code> but before you pushed <code>M</code>, the commit on the trunk | |
| used to be <code>o-o-A-X-Y-Z</code>. But because you made <code>M</code> while you were on | |
| <code>C</code>, <code>M</code>'s first parent is <code>C</code>, so by pushing <code>M</code> to advance the | |
| central repository, you made <code>X-Y-Z</code> a side branch, not on the trunk.</p></div> | |
| <div class="paragraph"><p>You would rather want to have a history of this shape:</p></div> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre><code> ---o---o---A---X---Y---Z---M' | |
| \ / | |
| B-----------C</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>so that in the first-parent chain, it is clear that the project first | |
| did <code>X</code> and then <code>Y</code> and then <code>Z</code> and merged a change that consists of | |
| two commits <code>B</code> and <code>C</code> that achieves a single goal. You may have | |
| worked on fixing the bug #12345 with these two patches, and the merge | |
| <code>M'</code> with swapped parents can say in its log message "Merge | |
| fix-bug-12345". Having a way to tell <code>git pull</code> to create a merge | |
| but record the parents in reverse order may be a way to do so.</p></div> | |
| <div class="paragraph"><p>Note that I said "achieves a single goal" above, because this is | |
| important. "Swapping the merge order" only covers a special case | |
| where the project does not care too much about having unrelated | |
| things done on a single merge but cares a lot about first-parent | |
| chain.</p></div> | |
| <div class="paragraph"><p>There are multiple schools of thought about the "trunk" management.</p></div> | |
| <div class="olist arabic"><ol class="arabic"> | |
| <li> | |
| <p> | |
| Some projects want to keep a completely linear history without any | |
| merges. Obviously, swapping the merge order would not match their | |
| taste. You would need to flatten your history on top of the | |
| updated upstream to result in a history of this shape instead: | |
| </p> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre><code> ---o---o---A---X---Y---Z---B---C</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>with <code>git pull --rebase</code> or something.</p></div> | |
| </li> | |
| <li> | |
| <p> | |
| Some projects tolerate merges in their history, but do not worry | |
| too much about the first-parent order, and allow fast-forward | |
| merges. To them, swapping the merge order does not hurt, but | |
| it is unnecessary. | |
| </p> | |
| </li> | |
| <li> | |
| <p> | |
| Some projects want each commit on the "trunk" to do one single | |
| thing. The output of <code>git log --first-parent</code> in such a project | |
| would show either a merge of a side branch that completes a single | |
| theme, or a single commit that completes a single theme by itself. | |
| If your two commits <code>B</code> and <code>C</code> (or they may even be two groups of | |
| commits) were solving two independent issues, then the merge <code>M'</code> | |
| we made in the earlier example by swapping the merge order is | |
| still not up to the project standard. It merges two unrelated | |
| efforts <code>B</code> and <code>C</code> at the same time. | |
| </p> | |
| </li> | |
| </ol></div> | |
| <div class="paragraph"><p>For projects in the last category (Git itself is one of them), | |
| individual developers would want to prepare a history more like | |
| this:</p></div> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre><code> C0--C1--C2 topic-c | |
| / | |
| ---o---o---A master | |
| \ | |
| B0--B1--B2 topic-b</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>That is, keeping separate topics on separate branches, perhaps like | |
| so:</p></div> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre><code> $ git clone $URL work && cd work | |
| $ git checkout -b topic-b master | |
| $ ... work to create B0, B1 and B2 to complete one theme | |
| $ git checkout -b topic-c master | |
| $ ... same for the theme of topic-c</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>And then</p></div> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre><code> $ git checkout master | |
| $ git pull --ff-only</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>would grab <code>X</code>, <code>Y</code> and <code>Z</code> from the upstream and advance your master | |
| branch:</p></div> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre><code> C0--C1--C2 topic-c | |
| / | |
| ---o---o---A---X---Y---Z master | |
| \ | |
| B0--B1--B2 topic-b</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>And then you would merge these two branches separately:</p></div> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre><code> $ git merge topic-b | |
| $ git merge topic-c</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>to result in</p></div> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre><code> C0--C1---------C2 | |
| / \ | |
| ---o---o---A---X---Y---Z---M---N | |
| \ / | |
| B0--B1-----B2</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>and push it back to the central repository.</p></div> | |
| <div class="paragraph"><p>It is very much possible that while you are merging topic-b and | |
| topic-c, somebody again advanced the history in the central repository | |
| to put <code>W</code> on top of <code>Z</code>, and make your <code>git push</code> fail.</p></div> | |
| <div class="paragraph"><p>In such a case, you would rewind to discard <code>M</code> and <code>N</code>, update the | |
| tip of your <em>master</em> again and redo the two merges:</p></div> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre><code> $ git reset --hard origin/master | |
| $ git pull --ff-only | |
| $ git merge topic-b | |
| $ git merge topic-c</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>The procedure will result in a history that looks like this:</p></div> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre><code> C0--C1--------------C2 | |
| / \ | |
| ---o---o---A---X---Y---Z---W---M'--N' | |
| \ / | |
| B0--B1---------B2</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>See also <a href="http://git-blame.blogspot.com/2013/09/fun-with-first-parent-history.html">http://git-blame.blogspot.com/2013/09/fun-with-first-parent-history.html</a></p></div> | |
| </div> | |
| </div> | |
| </div> | |
| <div id="footnotes"><hr /></div> | |
| <div id="footer"> | |
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| Last updated 2016-08-24 14:01:11 PDT | |
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