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chromium / external / gitster / git-htmldocs / e6ab919354ec5366f2f6561dacbea23946788c0b / . / howto / revert-a-faulty-merge.html
blob: 7217a56b91375dd582f15b2916d58e54e987c677 [file] [log] [blame]
Junio C Hamano944ce252018-05-30 22:25:26[diff] [blame]1<?xml version="1.0" encoding="UTF-8"?>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]2<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
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Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]6<meta http-equiv="Content-Type" content="application/xhtml+xml; charset=UTF-8" />
Junio C Hamanoa85030a2022-07-27 16:48:21[diff] [blame]7<meta name="generator" content="AsciiDoc 10.2.0" />
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]8<title>How to revert a faulty merge</title>
9<style type="text/css">
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]10/* Shared CSS for AsciiDoc xhtml11 and html5 backends */
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]25}
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]49h1, h2, h3, h4, h5, h6 {
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]51 margin-top: 1.2em;
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Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]68h5 {
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]71
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Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]88ul > li { color: #aaa; }
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]90
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]91.monospaced, code, pre {
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]95 padding: 0;
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Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]98pre {
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]101
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]102#author {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]103 color: #527bbd;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]104 font-weight: bold;
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Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]107#email {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]108}
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]109#revnumber, #revdate, #revremark {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]110}
111
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]112#footer {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]113 font-size: small;
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Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]118#footer-text {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]119 float: left;
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Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]122#footer-badges {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]123 float: right;
124 padding-bottom: 0.5em;
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Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]127#preamble {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]128 margin-top: 1.5em;
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Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]131div.imageblock, div.exampleblock, div.verseblock,
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]132div.quoteblock, div.literalblock, div.listingblock, div.sidebarblock,
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]151 font-weight: bold;
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Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]172 border: 1px solid #dddddd;
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]174 padding: 0.5em;
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177div.listingblock > div.content {
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]178 border: 1px solid #dddddd;
179 border-left: 5px solid #f0f0f0;
180 background: #f8f8f8;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]181 padding: 0.5em;
182}
183
184div.quoteblock, div.verseblock {
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186 margin-left: 1.0em;
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Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]188 border-left: 5px solid #f0f0f0;
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]190}
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192div.quoteblock > div.attribution {
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196
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]197div.verseblock > pre.content {
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]200}
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Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]229span.image img { border-style: none; vertical-align: text-bottom; }
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]230a.image:visited { color: white; }
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245
246ul, ol {
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]272tfoot {
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]278
279div.hdlist {
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Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]334div.colist td {
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]341}
342
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]343@media print {
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347#toc {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]348 margin-bottom: 2.5em;
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Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]351#toctitle {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]352 color: #527bbd;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]353 font-size: 1.1em;
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Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]359div.toclevel0, div.toclevel1, div.toclevel2, div.toclevel3, div.toclevel4 {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]360 margin-top: 0;
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]375
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]376span.aqua { color: aqua; }
377span.black { color: black; }
378span.blue { color: blue; }
379span.fuchsia { color: fuchsia; }
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381span.green { color: green; }
382span.lime { color: lime; }
383span.maroon { color: maroon; }
384span.navy { color: navy; }
385span.olive { color: olive; }
386span.purple { color: purple; }
387span.red { color: red; }
388span.silver { color: silver; }
389span.teal { color: teal; }
390span.white { color: white; }
391span.yellow { color: yellow; }
392
393span.aqua-background { background: aqua; }
394span.black-background { background: black; }
395span.blue-background { background: blue; }
396span.fuchsia-background { background: fuchsia; }
397span.gray-background { background: gray; }
398span.green-background { background: green; }
399span.lime-background { background: lime; }
400span.maroon-background { background: maroon; }
401span.navy-background { background: navy; }
402span.olive-background { background: olive; }
403span.purple-background { background: purple; }
404span.red-background { background: red; }
405span.silver-background { background: silver; }
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416
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]417div.unbreakable { page-break-inside: avoid; }
418
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]419
420/*
421 * xhtml11 specific
422 *
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425div.tableblock {
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]428}
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]429div.tableblock > table {
430 border: 3px solid #527bbd;
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432thead, p.table.header {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]433 font-weight: bold;
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]434 color: #527bbd;
435}
436p.table {
437 margin-top: 0;
438}
Junio C Hamano725b0da2020-01-22 22:02:40[diff] [blame]439/* Because the table frame attribute is overridden by CSS in most browsers. */
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]440div.tableblock > table[frame="void"] {
441 border-style: none;
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443div.tableblock > table[frame="hsides"] {
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447div.tableblock > table[frame="vsides"] {
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]450}
451
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]452
453/*
454 * html5 specific
455 *
456 * */
457
458table.tableblock {
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462thead, p.tableblock.header {
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469table.tableblock {
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479 border-style: solid;
480 border-color: #527bbd;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]481}
482
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]483table.tableblock.frame-topbot {
484 border-left-style: hidden;
485 border-right-style: hidden;
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487table.tableblock.frame-sides {
488 border-top-style: hidden;
489 border-bottom-style: hidden;
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494
495th.tableblock.halign-left, td.tableblock.halign-left {
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]502 text-align: right;
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504
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]505th.tableblock.valign-top, td.tableblock.valign-top {
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]507}
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]508th.tableblock.valign-middle, td.tableblock.valign-middle {
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511th.tableblock.valign-bottom, td.tableblock.valign-bottom {
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]513}
514
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]515
516/*
517 * manpage specific
518 *
519 * */
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521body.manpage h1 {
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530body.manpage div.sectionbody {
531 margin-left: 3em;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]532}
533
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]534@media print {
535 body.manpage div#toc { display: none; }
536}
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]537
538
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]539</style>
540<script type="text/javascript">
541/*<![CDATA[*/
Junio C Hamano2b153182021-12-15 21:00:31[diff] [blame]542var asciidoc = { // Namespace.
543
544/////////////////////////////////////////////////////////////////////
545// Table Of Contents generator
546/////////////////////////////////////////////////////////////////////
547
548/* Author: Mihai Bazon, September 2002
549 * http://students.infoiasi.ro/~mishoo
550 *
551 * Table Of Content generator
552 * Version: 0.4
553 *
554 * Feel free to use this script under the terms of the GNU General Public
555 * License, as long as you do not remove or alter this notice.
556 */
557
558 /* modified by Troy D. Hanson, September 2006. License: GPL */
559 /* modified by Stuart Rackham, 2006, 2009. License: GPL */
560
561// toclevels = 1..4.
562toc: function (toclevels) {
563
564 function getText(el) {
565 var text = "";
566 for (var i = el.firstChild; i != null; i = i.nextSibling) {
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568 text += i.data;
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570 text += getText(i);
571 }
572 return text;
573 }
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575 function TocEntry(el, text, toclevel) {
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579 }
580
581 function tocEntries(el, toclevels) {
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583 var re = new RegExp('[hH]([1-'+(toclevels+1)+'])');
584 // Function that scans the DOM tree for header elements (the DOM2
585 // nodeIterator API would be a better technique but not supported by all
586 // browsers).
587 var iterate = function (el) {
588 for (var i = el.firstChild; i != null; i = i.nextSibling) {
589 if (i.nodeType == 1 /* Node.ELEMENT_NODE */) {
590 var mo = re.exec(i.tagName);
591 if (mo && (i.getAttribute("class") || i.getAttribute("className")) != "float") {
592 result[result.length] = new TocEntry(i, getText(i), mo[1]-1);
593 }
594 iterate(i);
595 }
596 }
597 }
598 iterate(el);
599 return result;
600 }
601
602 var toc = document.getElementById("toc");
603 if (!toc) {
604 return;
605 }
606
607 // Delete existing TOC entries in case we're reloading the TOC.
608 var tocEntriesToRemove = [];
609 var i;
610 for (i = 0; i < toc.childNodes.length; i++) {
611 var entry = toc.childNodes[i];
612 if (entry.nodeName.toLowerCase() == 'div'
613 && entry.getAttribute("class")
614 && entry.getAttribute("class").match(/^toclevel/))
615 tocEntriesToRemove.push(entry);
616 }
617 for (i = 0; i < tocEntriesToRemove.length; i++) {
618 toc.removeChild(tocEntriesToRemove[i]);
619 }
620
621 // Rebuild TOC entries.
622 var entries = tocEntries(document.getElementById("content"), toclevels);
623 for (var i = 0; i < entries.length; ++i) {
624 var entry = entries[i];
625 if (entry.element.id == "")
626 entry.element.id = "_toc_" + i;
627 var a = document.createElement("a");
628 a.href = "#" + entry.element.id;
629 a.appendChild(document.createTextNode(entry.text));
630 var div = document.createElement("div");
631 div.appendChild(a);
632 div.className = "toclevel" + entry.toclevel;
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Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]731asciidoc.install();
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Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]735<body class="article">
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]736<div id="header">
737<h1>How to revert a faulty merge</h1>
738</div>
739<div id="content">
740<div id="preamble">
741<div class="sectionbody">
742<div class="paragraph"><p>Alan &lt;<a href="mailto:alan@clueserver.org">alan@clueserver.org</a>&gt; said:</p></div>
743<div class="literalblock">
744<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]745<pre><code>I have a master branch. We have a branch off of that that some
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]746developers are doing work on. They claim it is ready. We merge it
747into the master branch. It breaks something so we revert the merge.
748They make changes to the code. they get it to a point where they say
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]749it is ok and we merge again.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]750</div></div>
751<div class="literalblock">
752<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]753<pre><code>When examined, we find that code changes made before the revert are
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]754not in the master branch, but code changes after are in the master
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]755branch.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]756</div></div>
757<div class="paragraph"><p>and asked for help recovering from this situation.</p></div>
758<div class="paragraph"><p>The history immediately after the "revert of the merge" would look like
759this:</p></div>
760<div class="literalblock">
761<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]762<pre><code>---o---o---o---M---x---x---W
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]763 /
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]764 ---A---B</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]765</div></div>
766<div class="paragraph"><p>where A and B are on the side development that was not so good, M is the
767merge that brings these premature changes into the mainline, x are changes
768unrelated to what the side branch did and already made on the mainline,
769and W is the "revert of the merge M" (doesn&#8217;t W look M upside down?).
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]770IOW, <code>"diff W^..W"</code> is similar to <code>"diff -R M^..M"</code>.</p></div>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]771<div class="paragraph"><p>Such a "revert" of a merge can be made with:</p></div>
772<div class="literalblock">
773<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]774<pre><code>$ git revert -m 1 M</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]775</div></div>
776<div class="paragraph"><p>After the developers of the side branch fix their mistakes, the history
777may look like this:</p></div>
778<div class="literalblock">
779<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]780<pre><code>---o---o---o---M---x---x---W---x
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]781 /
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]782 ---A---B-------------------C---D</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]783</div></div>
784<div class="paragraph"><p>where C and D are to fix what was broken in A and B, and you may already
785have some other changes on the mainline after W.</p></div>
786<div class="paragraph"><p>If you merge the updated side branch (with D at its tip), none of the
Junio C Hamanod75148a2014-04-08 19:48:38[diff] [blame]787changes made in A or B will be in the result, because they were reverted
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]788by W. That is what Alan saw.</p></div>
789<div class="paragraph"><p>Linus explains the situation:</p></div>
790<div class="literalblock">
791<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]792<pre><code>Reverting a regular commit just effectively undoes what that commit
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]793did, and is fairly straightforward. But reverting a merge commit also
794undoes the _data_ that the commit changed, but it does absolutely
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]795nothing to the effects on _history_ that the merge had.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]796</div></div>
797<div class="literalblock">
798<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]799<pre><code>So the merge will still exist, and it will still be seen as joining
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]800the two branches together, and future merges will see that merge as
801the last shared state - and the revert that reverted the merge brought
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]802in will not affect that at all.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]803</div></div>
804<div class="literalblock">
805<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]806<pre><code>So a "revert" undoes the data changes, but it's very much _not_ an
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]807"undo" in the sense that it doesn't undo the effects of a commit on
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]808the repository history.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]809</div></div>
810<div class="literalblock">
811<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]812<pre><code>So if you think of "revert" as "undo", then you're going to always
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]813miss this part of reverts. Yes, it undoes the data, but no, it doesn't
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]814undo history.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]815</div></div>
816<div class="paragraph"><p>In such a situation, you would want to first revert the previous revert,
817which would make the history look like this:</p></div>
818<div class="literalblock">
819<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]820<pre><code>---o---o---o---M---x---x---W---x---Y
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]821 /
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]822 ---A---B-------------------C---D</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]823</div></div>
824<div class="paragraph"><p>where Y is the revert of W. Such a "revert of the revert" can be done
825with:</p></div>
826<div class="literalblock">
827<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]828<pre><code>$ git revert W</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]829</div></div>
830<div class="paragraph"><p>This history would (ignoring possible conflicts between what W and W..Y
Junio C Hamanod75148a2014-04-08 19:48:38[diff] [blame]831changed) be equivalent to not having W or Y at all in the history:</p></div>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]832<div class="literalblock">
833<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]834<pre><code>---o---o---o---M---x---x-------x----
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]835 /
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]836 ---A---B-------------------C---D</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]837</div></div>
838<div class="paragraph"><p>and merging the side branch again will not have conflict arising from an
839earlier revert and revert of the revert.</p></div>
840<div class="literalblock">
841<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]842<pre><code>---o---o---o---M---x---x-------x-------*
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]843 / /
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]844 ---A---B-------------------C---D</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]845</div></div>
846<div class="paragraph"><p>Of course the changes made in C and D still can conflict with what was
847done by any of the x, but that is just a normal merge conflict.</p></div>
848<div class="paragraph"><p>On the other hand, if the developers of the side branch discarded their
849faulty A and B, and redone the changes on top of the updated mainline
850after the revert, the history would have looked like this:</p></div>
851<div class="literalblock">
852<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]853<pre><code>---o---o---o---M---x---x---W---x---x
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]854 / \
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]855 ---A---B A'--B'--C'</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]856</div></div>
857<div class="paragraph"><p>If you reverted the revert in such a case as in the previous example:</p></div>
858<div class="literalblock">
859<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]860<pre><code>---o---o---o---M---x---x---W---x---x---Y---*
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]861 / \ /
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]862 ---A---B A'--B'--C'</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]863</div></div>
864<div class="paragraph"><p>where Y is the revert of W, A' and B' are rerolled A and B, and there may
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]865also be a further fix-up C' on the side branch. <code>"diff Y^..Y"</code> is similar
866to <code>"diff -R W^..W"</code> (which in turn means it is similar to <code>"diff M^..M"</code>),
867and <code>"diff A'^..C'"</code> by definition would be similar but different from that,
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]868because it is a rerolled series of the earlier change. There will be a
869lot of overlapping changes that result in conflicts. So do not do "revert
870of revert" blindly without thinking..</p></div>
871<div class="literalblock">
872<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]873<pre><code>---o---o---o---M---x---x---W---x---x
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]874 / \
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]875 ---A---B A'--B'--C'</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]876</div></div>
877<div class="paragraph"><p>In the history with rebased side branch, W (and M) are behind the merge
878base of the updated branch and the tip of the mainline, and they should
879merge without the past faulty merge and its revert getting in the way.</p></div>
880<div class="paragraph"><p>To recap, these are two very different scenarios, and they want two very
881different resolution strategies:</p></div>
882<div class="ulist"><ul>
883<li>
884<p>
885If the faulty side branch was fixed by adding corrections on top, then
886 doing a revert of the previous revert would be the right thing to do.
887</p>
888</li>
889<li>
890<p>
891If the faulty side branch whose effects were discarded by an earlier
892 revert of a merge was rebuilt from scratch (i.e. rebasing and fixing,
893 as you seem to have interpreted), then re-merging the result without
894 doing anything else fancy would be the right thing to do.
895 (See the ADDENDUM below for how to rebuild a branch from scratch
896 without changing its original branching-off point.)
897</p>
898</li>
899</ul></div>
900<div class="paragraph"><p>However, there are things to keep in mind when reverting a merge (and
901reverting such a revert).</p></div>
902<div class="paragraph"><p>For example, think about what reverting a merge (and then reverting the
903revert) does to bisectability. Ignore the fact that the revert of a revert
904is undoing it - just think of it as a "single commit that does a lot".
905Because that is what it does.</p></div>
906<div class="paragraph"><p>When you have a problem you are chasing down, and you hit a "revert this
907merge", what you&#8217;re hitting is essentially a single commit that contains
908all the changes (but obviously in reverse) of all the commits that got
909merged. So it&#8217;s debugging hell, because now you don&#8217;t have lots of small
910changes that you can try to pinpoint which <em>part</em> of it changes.</p></div>
911<div class="paragraph"><p>But does it all work? Sure it does. You can revert a merge, and from a
Junio C Hamano076ffcc2013-02-06 05:13:21[diff] [blame]912purely technical angle, Git did it very naturally and had no real
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]913troubles. It just considered it a change from "state before merge" to
914"state after merge", and that was it. Nothing complicated, nothing odd,
915nothing really dangerous. Git will do it without even thinking about it.</p></div>
916<div class="paragraph"><p>So from a technical angle, there&#8217;s nothing wrong with reverting a merge,
917but from a workflow angle it&#8217;s something that you generally should try to
918avoid.</p></div>
919<div class="paragraph"><p>If at all possible, for example, if you find a problem that got merged
920into the main tree, rather than revert the merge, try <em>really</em> hard to
921bisect the problem down into the branch you merged, and just fix it, or
922try to revert the individual commit that caused it.</p></div>
923<div class="paragraph"><p>Yes, it&#8217;s more complex, and no, it&#8217;s not always going to work (sometimes
924the answer is: "oops, I really shouldn&#8217;t have merged it, because it wasn&#8217;t
925ready yet, and I really need to undo <em>all</em> of the merge"). So then you
926really should revert the merge, but when you want to re-do the merge, you
927now need to do it by reverting the revert.</p></div>
928<div class="paragraph"><p>ADDENDUM</p></div>
929<div class="paragraph"><p>Sometimes you have to rewrite one of a topic branch&#8217;s commits <strong>and</strong> you can&#8217;t
930change the topic&#8217;s branching-off point. Consider the following situation:</p></div>
931<div class="literalblock">
932<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]933<pre><code>P---o---o---M---x---x---W---x
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]934 \ /
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]935 A---B---C</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]936</div></div>
937<div class="paragraph"><p>where commit W reverted commit M because it turned out that commit B was wrong
938and needs to be rewritten, but you need the rewritten topic to still branch
939from commit P (perhaps P is a branching-off point for yet another branch, and
940you want be able to merge the topic into both branches).</p></div>
941<div class="paragraph"><p>The natural thing to do in this case is to checkout the A-B-C branch and use
942"rebase -i P" to change commit B. However this does not rewrite commit A,
943because "rebase -i" by default fast-forwards over any initial commits selected
944with the "pick" command. So you end up with this:</p></div>
945<div class="literalblock">
946<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]947<pre><code>P---o---o---M---x---x---W---x
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]948 \ /
949 A---B---C &lt;-- old branch
950 \
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]951 B'---C' &lt;-- naively rewritten branch</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]952</div></div>
953<div class="paragraph"><p>To merge A-B'-C' into the mainline branch you would still have to first revert
954commit W in order to pick up the changes in A, but then it&#8217;s likely that the
955changes in B' will conflict with the original B changes re-introduced by the
956reversion of W.</p></div>
957<div class="paragraph"><p>However, you can avoid these problems if you recreate the entire branch,
958including commit A:</p></div>
959<div class="literalblock">
960<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]961<pre><code> A'---B'---C' &lt;-- completely rewritten branch
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]962 /
963P---o---o---M---x---x---W---x
964 \ /
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]965 A---B---C</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]966</div></div>
967<div class="paragraph"><p>You can merge A'-B'-C' into the mainline branch without worrying about first
968reverting W. Mainline&#8217;s history would look like this:</p></div>
969<div class="literalblock">
970<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]971<pre><code> A'---B'---C'------------------
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]972 / \
973P---o---o---M---x---x---W---x---M2
974 \ /
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]975 A---B---C</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]976</div></div>
977<div class="paragraph"><p>But if you don&#8217;t actually need to change commit A, then you need some way to
978recreate it as a new commit with the same changes in it. The rebase command&#8217;s
979--no-ff option provides a way to do this:</p></div>
980<div class="literalblock">
981<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]982<pre><code>$ git rebase [-i] --no-ff P</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]983</div></div>
984<div class="paragraph"><p>The --no-ff option creates a new branch A'-B'-C' with all-new commits (all the
985SHA IDs will be different) even if in the interactive case you only actually
986modify commit B. You can then merge this new branch directly into the mainline
987branch and be sure you&#8217;ll get all of the branch&#8217;s changes.</p></div>
988<div class="paragraph"><p>You can also use --no-ff in cases where you just add extra commits to the topic
989to fix it up. Let&#8217;s revisit the situation discussed at the start of this howto:</p></div>
990<div class="literalblock">
991<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]992<pre><code>P---o---o---M---x---x---W---x
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]993 \ /
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]994 A---B---C----------------D---E &lt;-- fixed-up topic branch</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]995</div></div>
996<div class="paragraph"><p>At this point, you can use --no-ff to recreate the topic branch:</p></div>
997<div class="literalblock">
998<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]999<pre><code>$ git checkout E
1000$ git rebase --no-ff P</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1001</div></div>
1002<div class="paragraph"><p>yielding</p></div>
1003<div class="literalblock">
1004<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1005<pre><code> A'---B'---C'------------D'---E' &lt;-- recreated topic branch
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1006 /
1007P---o---o---M---x---x---W---x
1008 \ /
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1009 A---B---C----------------D---E</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1010</div></div>
1011<div class="paragraph"><p>You can merge the recreated branch into the mainline without reverting commit W,
1012and mainline&#8217;s history will look like this:</p></div>
1013<div class="literalblock">
1014<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1015<pre><code> A'---B'---C'------------D'---E'
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1016 / \
1017P---o---o---M---x---x---W---x---M2
1018 \ /
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1019 A---B---C</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1020</div></div>
1021</div>
1022</div>
1023</div>
1024<div id="footnotes"><hr /></div>
1025<div id="footer">
1026<div id="footer-text">
Junio C Hamano2ef0ba32018-01-26 23:13:53[diff] [blame]1027Last updated
Junio C Hamanoe6ab9192022-11-23 04:01:36[diff] [blame^]1028 2022-11-23 12:59:17 JST
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1029</div>
1030</div>
1031</body>
1032</html>