howto/keep-canonical-history-correct.html

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   8 <title>Keep authoritative canonical history correct with git pull</title>
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 441 <body class="article">
 442 <div id="header">
 443 <h1>Keep authoritative canonical history correct with git pull</h1>
 444 <div class="details">
 445 <span id="revdate">2024-08-07</span>
 446 </div>
 447 </div>
 448 <div id="content">
 449 <div class="paragraph">
 450 <p>Sometimes a new project integrator will end up with project history
 451 that appears to be "backwards" from what other project developers
 452 expect. This howto presents a suggested integration workflow for
 453 maintaining a central repository.</p>
 454 </div>
 455 <div class="paragraph">
 456 <p>Suppose that that central repository has this history:</p>
 457 </div>
 458 <div class="listingblock">
 459 <div class="content">
 460 <pre>    ---o---o---A</pre>
 461 </div>
 462 </div>
 463 <div class="paragraph">
 464 <p>which ends at commit <code>A</code> (time flows from left to right and each node
 465 in the graph is a commit, lines between them indicating parent-child
 466 relationship).</p>
 467 </div>
 468 <div class="paragraph">
 469 <p>Then you clone it and work on your own commits, which leads you to
 470 have this history in <strong>your</strong> repository:</p>
 471 </div>
 472 <div class="listingblock">
 473 <div class="content">
 474 <pre>    ---o---o---A---B---C</pre>
 475 </div>
 476 </div>
 477 <div class="paragraph">
 478 <p>Imagine your coworker did the same and built on top of <code>A</code> in <strong>his</strong>
 479 repository in the meantime, and then pushed it to the
 480 central repository:</p>
 481 </div>
 482 <div class="listingblock">
 483 <div class="content">
 484 <pre>    ---o---o---A---X---Y---Z</pre>
 485 </div>
 486 </div>
 487 <div class="paragraph">
 488 <p>Now, if you <code>git push</code> at this point, because your history that leads
 489 to <code>C</code> lacks <code>X</code>, <code>Y</code> and <code>Z</code>, it will fail.  You need to somehow make
 490 the tip of your history a descendant of <code>Z</code>.</p>
 491 </div>
 492 <div class="paragraph">
 493 <p>One suggested way to solve the problem is "fetch and then merge", aka
 494 <code>git pull</code>. When you fetch, your repository will have a history like
 495 this:</p>
 496 </div>
 497 <div class="listingblock">
 498 <div class="content">
 499 <pre>    ---o---o---A---B---C
 500                 \
 501                  X---Y---Z</pre>
 502 </div>
 503 </div>
 504 <div class="paragraph">
 505 <p>Once you run merge after that, while still on <strong>your</strong> branch, i.e. <code>C</code>,
 506 you will create a merge <code>M</code> and make the history look like this:</p>
 507 </div>
 508 <div class="listingblock">
 509 <div class="content">
 510 <pre>    ---o---o---A---B---C---M
 511                 \         /
 512                  X---Y---Z</pre>
 513 </div>
 514 </div>
 515 <div class="paragraph">
 516 <p><code>M</code> is a descendant of <code>Z</code>, so you can push to update the central
 517 repository.  Such a merge <code>M</code> does not lose any commit in both
 518 histories, so in that sense it may not be wrong, but when people want
 519 to talk about "the authoritative canonical history that is shared
 520 among the project participants", i.e. "the trunk", they often view
 521 it as "commits you see by following the first-parent chain", and use
 522 this command to view it:</p>
 523 </div>
 524 <div class="listingblock">
 525 <div class="content">
 526 <pre>    $ git log --first-parent</pre>
 527 </div>
 528 </div>
 529 <div class="paragraph">
 530 <p>For all other people who observed the central repository after your
 531 coworker pushed <code>Z</code> but before you pushed <code>M</code>, the commit on the trunk
 532 used to be <code>o-o-A-X-Y-Z</code>.  But because you made <code>M</code> while you were on
 533 <code>C</code>, <code>M</code>'s first parent is <code>C</code>, so by pushing <code>M</code> to advance the
 534 central repository, you made <code>X-Y-Z</code> a side branch, not on the trunk.</p>
 535 </div>
 536 <div class="paragraph">
 537 <p>You would rather want to have a history of this shape:</p>
 538 </div>
 539 <div class="listingblock">
 540 <div class="content">
 541 <pre>    ---o---o---A---X---Y---Z---M'
 542                 \             /
 543                  B-----------C</pre>
 544 </div>
 545 </div>
 546 <div class="paragraph">
 547 <p>so that in the first-parent chain, it is clear that the project first
 548 did <code>X</code> and then <code>Y</code> and then <code>Z</code> and merged a change that consists of
 549 two commits <code>B</code> and <code>C</code> that achieves a single goal.  You may have
 550 worked on fixing the bug #12345 with these two patches, and the merge
 551 <code>M'</code> with swapped parents can say in its log message "Merge
 552 fix-bug-12345". Having a way to tell <code>git pull</code> to create a merge
 553 but record the parents in reverse order may be a way to do so.</p>
 554 </div>
 555 <div class="paragraph">
 556 <p>Note that I said "achieves a single goal" above, because this is
 557 important.  "Swapping the merge order" only covers a special case
 558 where the project does not care too much about having unrelated
 559 things done on a single merge but cares a lot about first-parent
 560 chain.</p>
 561 </div>
 562 <div class="paragraph">
 563 <p>There are multiple schools of thought about the "trunk" management.</p>
 564 </div>
 565 <div class="olist arabic">
 566 <ol class="arabic">
 567 <li>
 568 <p>Some projects want to keep a completely linear history without any
 569 merges.  Obviously, swapping the merge order would not match their
 570 taste.  You would need to flatten your history on top of the
 571 updated upstream to result in a history of this shape instead:</p>
 572 <div class="listingblock">
 573 <div class="content">
 574 <pre>    ---o---o---A---X---Y---Z---B---C</pre>
 575 </div>
 576 </div>
 577 <div class="paragraph">
 578 <p>with <code>git pull --rebase</code> or something.</p>
 579 </div>
 580 </li>
 581 <li>
 582 <p>Some projects tolerate merges in their history, but do not worry
 583 too much about the first-parent order, and allow fast-forward
 584 merges.  To them, swapping the merge order does not hurt, but
 585 it is unnecessary.</p>
 586 </li>
 587 <li>
 588 <p>Some projects want each commit on the "trunk" to do one single
 589 thing.  The output of <code>git log --first-parent</code> in such a project
 590 would show either a merge of a side branch that completes a single
 591 theme, or a single commit that completes a single theme by itself.
 592 If your two commits <code>B</code> and <code>C</code> (or they may even be two groups of
 593 commits) were solving two independent issues, then the merge <code>M'</code>
 594 we made in the earlier example by swapping the merge order is
 595 still not up to the project standard.  It merges two unrelated
 596 efforts <code>B</code> and <code>C</code> at the same time.</p>
 597 </li>
 598 </ol>
 599 </div>
 600 <div class="paragraph">
 601 <p>For projects in the last category (Git itself is one of them),
 602 individual developers would want to prepare a history more like
 603 this:</p>
 604 </div>
 605 <div class="listingblock">
 606 <div class="content">
 607 <pre>                 C0--C1--C2     topic-c
 608                 /
 609     ---o---o---A                master
 610                 \
 611                  B0--B1--B2     topic-b</pre>
 612 </div>
 613 </div>
 614 <div class="paragraph">
 615 <p>That is, keeping separate topics on separate branches, perhaps like
 616 so:</p>
 617 </div>
 618 <div class="listingblock">
 619 <div class="content">
 620 <pre>    $ git clone $URL work &amp;&amp; cd work
 621     $ git checkout -b topic-b master
 622     $ ... work to create B0, B1 and B2 to complete one theme
 623     $ git checkout -b topic-c master
 624     $ ... same for the theme of topic-c</pre>
 625 </div>
 626 </div>
 627 <div class="paragraph">
 628 <p>And then</p>
 629 </div>
 630 <div class="listingblock">
 631 <div class="content">
 632 <pre>    $ git checkout master
 633     $ git pull --ff-only</pre>
 634 </div>
 635 </div>
 636 <div class="paragraph">
 637 <p>would grab <code>X</code>, <code>Y</code> and <code>Z</code> from the upstream and advance your master
 638 branch:</p>
 639 </div>
 640 <div class="listingblock">
 641 <div class="content">
 642 <pre>                 C0--C1--C2     topic-c
 643                 /
 644     ---o---o---A---X---Y---Z    master
 645                 \
 646                  B0--B1--B2     topic-b</pre>
 647 </div>
 648 </div>
 649 <div class="paragraph">
 650 <p>And then you would merge these two branches separately:</p>
 651 </div>
 652 <div class="listingblock">
 653 <div class="content">
 654 <pre>    $ git merge topic-b
 655     $ git merge topic-c</pre>
 656 </div>
 657 </div>
 658 <div class="paragraph">
 659 <p>to result in</p>
 660 </div>
 661 <div class="listingblock">
 662 <div class="content">
 663 <pre>                 C0--C1---------C2
 664                 /                 \
 665     ---o---o---A---X---Y---Z---M---N
 666                 \             /
 667                  B0--B1-----B2</pre>
 668 </div>
 669 </div>
 670 <div class="paragraph">
 671 <p>and push it back to the central repository.</p>
 672 </div>
 673 <div class="paragraph">
 674 <p>It is very much possible that while you are merging topic-b and
 675 topic-c, somebody again advanced the history in the central repository
 676 to put <code>W</code> on top of <code>Z</code>, and make your <code>git push</code> fail.</p>
 677 </div>
 678 <div class="paragraph">
 679 <p>In such a case, you would rewind to discard <code>M</code> and <code>N</code>, update the
 680 tip of your <em>master</em> again and redo the two merges:</p>
 681 </div>
 682 <div class="listingblock">
 683 <div class="content">
 684 <pre>    $ git reset --hard origin/master
 685     $ git pull --ff-only
 686     $ git merge topic-b
 687     $ git merge topic-c</pre>
 688 </div>
 689 </div>
 690 <div class="paragraph">
 691 <p>The procedure will result in a history that looks like this:</p>
 692 </div>
 693 <div class="listingblock">
 694 <div class="content">
 695 <pre>                 C0--C1--------------C2
 696                 /                     \
 697     ---o---o---A---X---Y---Z---W---M'--N'
 698                 \                 /
 699                  B0--B1---------B2</pre>
 700 </div>
 701 </div>
 702 <div class="paragraph">
 703 <p>See also <a href="https://git-blame.blogspot.com/2013/09/fun-with-first-parent-history.html" class="bare">https://git-blame.blogspot.com/2013/09/fun-with-first-parent-history.html</a></p>
 704 </div>
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