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chromium / external / gitster / git-htmldocs / c756926d575a42433fb764a16b48327974208afb / . / howto / revert-a-faulty-merge.html
blob: d2d1dc993fbbb9ba378c6c80a721e7ff0afdcd00 [file] [log] [blame]
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
2 "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
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Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]5<meta http-equiv="Content-Type" content="application/xhtml+xml; charset=UTF-8" />
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]6<meta name="generator" content="AsciiDoc 8.6.9" />
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]7<title>How to revert a faulty merge</title>
8<style type="text/css">
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]9/* Shared CSS for AsciiDoc xhtml11 and html5 backends */
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]24}
25
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]48h1, h2, h3, h4, h5, h6 {
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]50 margin-top: 1.2em;
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Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]67h5 {
68 font-size: 1.0em;
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]70
71div.sectionbody {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]72 margin-left: 0;
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Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]87ul > li { color: #aaa; }
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]89
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]90.monospaced, code, pre {
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]94 padding: 0;
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Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]97pre {
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]100
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]101#author {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]102 color: #527bbd;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]103 font-weight: bold;
104 font-size: 1.1em;
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Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]106#email {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]107}
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]108#revnumber, #revdate, #revremark {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]109}
110
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]111#footer {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]112 font-size: small;
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Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]117#footer-text {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]118 float: left;
119 padding-bottom: 0.5em;
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Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]121#footer-badges {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]122 float: right;
123 padding-bottom: 0.5em;
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Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]126#preamble {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]127 margin-top: 1.5em;
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Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]130div.imageblock, div.exampleblock, div.verseblock,
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]131div.quoteblock, div.literalblock, div.listingblock, div.sidebarblock,
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]150 font-weight: bold;
151 text-align: left;
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Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]171 border: 1px solid #dddddd;
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]173 padding: 0.5em;
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175
176div.listingblock > div.content {
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]177 border: 1px solid #dddddd;
178 border-left: 5px solid #f0f0f0;
179 background: #f8f8f8;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]180 padding: 0.5em;
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182
183div.quoteblock, div.verseblock {
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Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]187 border-left: 5px solid #f0f0f0;
188 color: #888;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]189}
190
191div.quoteblock > div.attribution {
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Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]196div.verseblock > pre.content {
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]199}
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Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]228span.image img { border-style: none; vertical-align: text-bottom; }
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]229a.image:visited { color: white; }
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231dl {
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245ul, ol {
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]271tfoot {
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]277
278div.hdlist {
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Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]333div.colist td {
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]340}
341
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]342@media print {
343 #footer-badges { display: none; }
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345
346#toc {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]347 margin-bottom: 2.5em;
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349
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]350#toctitle {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]351 color: #527bbd;
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357
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]358div.toclevel0, div.toclevel1, div.toclevel2, div.toclevel3, div.toclevel4 {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]359 margin-top: 0;
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]374
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]375span.aqua { color: aqua; }
376span.black { color: black; }
377span.blue { color: blue; }
378span.fuchsia { color: fuchsia; }
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380span.green { color: green; }
381span.lime { color: lime; }
382span.maroon { color: maroon; }
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384span.olive { color: olive; }
385span.purple { color: purple; }
386span.red { color: red; }
387span.silver { color: silver; }
388span.teal { color: teal; }
389span.white { color: white; }
390span.yellow { color: yellow; }
391
392span.aqua-background { background: aqua; }
393span.black-background { background: black; }
394span.blue-background { background: blue; }
395span.fuchsia-background { background: fuchsia; }
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397span.green-background { background: green; }
398span.lime-background { background: lime; }
399span.maroon-background { background: maroon; }
400span.navy-background { background: navy; }
401span.olive-background { background: olive; }
402span.purple-background { background: purple; }
403span.red-background { background: red; }
404span.silver-background { background: silver; }
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414span.line-through { text-decoration: line-through; }
415
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]416div.unbreakable { page-break-inside: avoid; }
417
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]418
419/*
420 * xhtml11 specific
421 *
422 * */
423
424div.tableblock {
425 margin-top: 1.0em;
426 margin-bottom: 1.5em;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]427}
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]428div.tableblock > table {
429 border: 3px solid #527bbd;
430}
431thead, p.table.header {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]432 font-weight: bold;
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]433 color: #527bbd;
434}
435p.table {
436 margin-top: 0;
437}
438/* Because the table frame attribute is overriden by CSS in most browsers. */
439div.tableblock > table[frame="void"] {
440 border-style: none;
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442div.tableblock > table[frame="hsides"] {
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446div.tableblock > table[frame="vsides"] {
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]449}
450
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]451
452/*
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454 *
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456
457table.tableblock {
458 margin-top: 1.0em;
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461thead, p.tableblock.header {
462 font-weight: bold;
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468table.tableblock {
469 border-width: 3px;
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479 border-color: #527bbd;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]480}
481
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]482table.tableblock.frame-topbot {
483 border-left-style: hidden;
484 border-right-style: hidden;
485}
486table.tableblock.frame-sides {
487 border-top-style: hidden;
488 border-bottom-style: hidden;
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490table.tableblock.frame-none {
491 border-style: hidden;
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493
494th.tableblock.halign-left, td.tableblock.halign-left {
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497th.tableblock.halign-center, td.tableblock.halign-center {
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]501 text-align: right;
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503
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]504th.tableblock.valign-top, td.tableblock.valign-top {
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]506}
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]507th.tableblock.valign-middle, td.tableblock.valign-middle {
508 vertical-align: middle;
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510th.tableblock.valign-bottom, td.tableblock.valign-bottom {
511 vertical-align: bottom;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]512}
513
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]514
515/*
516 * manpage specific
517 *
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528}
529body.manpage div.sectionbody {
530 margin-left: 3em;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]531}
532
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]533@media print {
534 body.manpage div#toc { display: none; }
535}
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]536
537
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]538</style>
539<script type="text/javascript">
540/*<![CDATA[*/
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]541var asciidoc = { // Namespace.
542
543/////////////////////////////////////////////////////////////////////
544// Table Of Contents generator
545/////////////////////////////////////////////////////////////////////
546
547/* Author: Mihai Bazon, September 2002
548 * http://students.infoiasi.ro/~mishoo
549 *
550 * Table Of Content generator
551 * Version: 0.4
552 *
553 * Feel free to use this script under the terms of the GNU General Public
554 * License, as long as you do not remove or alter this notice.
555 */
556
557 /* modified by Troy D. Hanson, September 2006. License: GPL */
558 /* modified by Stuart Rackham, 2006, 2009. License: GPL */
559
560// toclevels = 1..4.
561toc: function (toclevels) {
562
563 function getText(el) {
564 var text = "";
565 for (var i = el.firstChild; i != null; i = i.nextSibling) {
566 if (i.nodeType == 3 /* Node.TEXT_NODE */) // IE doesn't speak constants.
567 text += i.data;
568 else if (i.firstChild != null)
569 text += getText(i);
570 }
571 return text;
572 }
573
574 function TocEntry(el, text, toclevel) {
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576 this.text = text;
577 this.toclevel = toclevel;
578 }
579
580 function tocEntries(el, toclevels) {
581 var result = new Array;
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]582 var re = new RegExp('[hH]([1-'+(toclevels+1)+'])');
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]583 // Function that scans the DOM tree for header elements (the DOM2
584 // nodeIterator API would be a better technique but not supported by all
585 // browsers).
586 var iterate = function (el) {
587 for (var i = el.firstChild; i != null; i = i.nextSibling) {
588 if (i.nodeType == 1 /* Node.ELEMENT_NODE */) {
589 var mo = re.exec(i.tagName);
590 if (mo && (i.getAttribute("class") || i.getAttribute("className")) != "float") {
591 result[result.length] = new TocEntry(i, getText(i), mo[1]-1);
592 }
593 iterate(i);
594 }
595 }
596 }
597 iterate(el);
598 return result;
599 }
600
601 var toc = document.getElementById("toc");
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]602 if (!toc) {
603 return;
604 }
605
606 // Delete existing TOC entries in case we're reloading the TOC.
607 var tocEntriesToRemove = [];
608 var i;
609 for (i = 0; i < toc.childNodes.length; i++) {
610 var entry = toc.childNodes[i];
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]611 if (entry.nodeName.toLowerCase() == 'div'
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]612 && entry.getAttribute("class")
613 && entry.getAttribute("class").match(/^toclevel/))
614 tocEntriesToRemove.push(entry);
615 }
616 for (i = 0; i < tocEntriesToRemove.length; i++) {
617 toc.removeChild(tocEntriesToRemove[i]);
618 }
619
620 // Rebuild TOC entries.
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]621 var entries = tocEntries(document.getElementById("content"), toclevels);
622 for (var i = 0; i < entries.length; ++i) {
623 var entry = entries[i];
624 if (entry.element.id == "")
625 entry.element.id = "_toc_" + i;
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Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]734<body class="article">
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]735<div id="header">
736<h1>How to revert a faulty merge</h1>
737</div>
738<div id="content">
739<div id="preamble">
740<div class="sectionbody">
741<div class="paragraph"><p>Alan &lt;<a href="mailto:alan@clueserver.org">alan@clueserver.org</a>&gt; said:</p></div>
742<div class="literalblock">
743<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]744<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]745developers are doing work on. They claim it is ready. We merge it
746into the master branch. It breaks something so we revert the merge.
747They make changes to the code. they get it to a point where they say
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]748it is ok and we merge again.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]749</div></div>
750<div class="literalblock">
751<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]752<pre><code>When examined, we find that code changes made before the revert are
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]753not in the master branch, but code changes after are in the master
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]754branch.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]755</div></div>
756<div class="paragraph"><p>and asked for help recovering from this situation.</p></div>
757<div class="paragraph"><p>The history immediately after the "revert of the merge" would look like
758this:</p></div>
759<div class="literalblock">
760<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]761<pre><code>---o---o---o---M---x---x---W
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]762 /
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]763 ---A---B</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]764</div></div>
765<div class="paragraph"><p>where A and B are on the side development that was not so good, M is the
766merge that brings these premature changes into the mainline, x are changes
767unrelated to what the side branch did and already made on the mainline,
768and 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]769IOW, <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]770<div class="paragraph"><p>Such a "revert" of a merge can be made with:</p></div>
771<div class="literalblock">
772<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]773<pre><code>$ git revert -m 1 M</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]774</div></div>
775<div class="paragraph"><p>After the developers of the side branch fix their mistakes, the history
776may look like this:</p></div>
777<div class="literalblock">
778<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]779<pre><code>---o---o---o---M---x---x---W---x
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]780 /
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]781 ---A---B-------------------C---D</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]782</div></div>
783<div class="paragraph"><p>where C and D are to fix what was broken in A and B, and you may already
784have some other changes on the mainline after W.</p></div>
785<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]786changes made in A or B will be in the result, because they were reverted
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]787by W. That is what Alan saw.</p></div>
788<div class="paragraph"><p>Linus explains the situation:</p></div>
789<div class="literalblock">
790<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]791<pre><code>Reverting a regular commit just effectively undoes what that commit
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]792did, and is fairly straightforward. But reverting a merge commit also
793undoes the _data_ that the commit changed, but it does absolutely
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]794nothing to the effects on _history_ that the merge had.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]795</div></div>
796<div class="literalblock">
797<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]798<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]799the two branches together, and future merges will see that merge as
800the last shared state - and the revert that reverted the merge brought
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]801in will not affect that at all.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]802</div></div>
803<div class="literalblock">
804<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]805<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]806"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]807the repository history.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]808</div></div>
809<div class="literalblock">
810<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]811<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]812miss this part of reverts. Yes, it undoes the data, but no, it doesn't
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]813undo history.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]814</div></div>
815<div class="paragraph"><p>In such a situation, you would want to first revert the previous revert,
816which would make the history look like this:</p></div>
817<div class="literalblock">
818<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]819<pre><code>---o---o---o---M---x---x---W---x---Y
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]820 /
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]821 ---A---B-------------------C---D</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]822</div></div>
823<div class="paragraph"><p>where Y is the revert of W. Such a "revert of the revert" can be done
824with:</p></div>
825<div class="literalblock">
826<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]827<pre><code>$ git revert W</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]828</div></div>
829<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]830changed) 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]831<div class="literalblock">
832<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]833<pre><code>---o---o---o---M---x---x-------x----
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]834 /
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]835 ---A---B-------------------C---D</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]836</div></div>
837<div class="paragraph"><p>and merging the side branch again will not have conflict arising from an
838earlier revert and revert of the revert.</p></div>
839<div class="literalblock">
840<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]841<pre><code>---o---o---o---M---x---x-------x-------*
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]842 / /
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]843 ---A---B-------------------C---D</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]844</div></div>
845<div class="paragraph"><p>Of course the changes made in C and D still can conflict with what was
846done by any of the x, but that is just a normal merge conflict.</p></div>
847<div class="paragraph"><p>On the other hand, if the developers of the side branch discarded their
848faulty A and B, and redone the changes on top of the updated mainline
849after the revert, the history would have looked like this:</p></div>
850<div class="literalblock">
851<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]852<pre><code>---o---o---o---M---x---x---W---x---x
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]853 / \
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]854 ---A---B A'--B'--C'</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]855</div></div>
856<div class="paragraph"><p>If you reverted the revert in such a case as in the previous example:</p></div>
857<div class="literalblock">
858<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]859<pre><code>---o---o---o---M---x---x---W---x---x---Y---*
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]860 / \ /
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]861 ---A---B A'--B'--C'</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]862</div></div>
863<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]864also be a further fix-up C' on the side branch. <code>"diff Y^..Y"</code> is similar
865to <code>"diff -R W^..W"</code> (which in turn means it is similar to <code>"diff M^..M"</code>),
866and <code>"diff A'^..C'"</code> by definition would be similar but different from that,
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]867because it is a rerolled series of the earlier change. There will be a
868lot of overlapping changes that result in conflicts. So do not do "revert
869of revert" blindly without thinking..</p></div>
870<div class="literalblock">
871<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]872<pre><code>---o---o---o---M---x---x---W---x---x
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]873 / \
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]874 ---A---B A'--B'--C'</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]875</div></div>
876<div class="paragraph"><p>In the history with rebased side branch, W (and M) are behind the merge
877base of the updated branch and the tip of the mainline, and they should
878merge without the past faulty merge and its revert getting in the way.</p></div>
879<div class="paragraph"><p>To recap, these are two very different scenarios, and they want two very
880different resolution strategies:</p></div>
881<div class="ulist"><ul>
882<li>
883<p>
884If the faulty side branch was fixed by adding corrections on top, then
885 doing a revert of the previous revert would be the right thing to do.
886</p>
887</li>
888<li>
889<p>
890If the faulty side branch whose effects were discarded by an earlier
891 revert of a merge was rebuilt from scratch (i.e. rebasing and fixing,
892 as you seem to have interpreted), then re-merging the result without
893 doing anything else fancy would be the right thing to do.
894 (See the ADDENDUM below for how to rebuild a branch from scratch
895 without changing its original branching-off point.)
896</p>
897</li>
898</ul></div>
899<div class="paragraph"><p>However, there are things to keep in mind when reverting a merge (and
900reverting such a revert).</p></div>
901<div class="paragraph"><p>For example, think about what reverting a merge (and then reverting the
902revert) does to bisectability. Ignore the fact that the revert of a revert
903is undoing it - just think of it as a "single commit that does a lot".
904Because that is what it does.</p></div>
905<div class="paragraph"><p>When you have a problem you are chasing down, and you hit a "revert this
906merge", what you&#8217;re hitting is essentially a single commit that contains
907all the changes (but obviously in reverse) of all the commits that got
908merged. So it&#8217;s debugging hell, because now you don&#8217;t have lots of small
909changes that you can try to pinpoint which <em>part</em> of it changes.</p></div>
910<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]911purely technical angle, Git did it very naturally and had no real
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]912troubles. It just considered it a change from "state before merge" to
913"state after merge", and that was it. Nothing complicated, nothing odd,
914nothing really dangerous. Git will do it without even thinking about it.</p></div>
915<div class="paragraph"><p>So from a technical angle, there&#8217;s nothing wrong with reverting a merge,
916but from a workflow angle it&#8217;s something that you generally should try to
917avoid.</p></div>
918<div class="paragraph"><p>If at all possible, for example, if you find a problem that got merged
919into the main tree, rather than revert the merge, try <em>really</em> hard to
920bisect the problem down into the branch you merged, and just fix it, or
921try to revert the individual commit that caused it.</p></div>
922<div class="paragraph"><p>Yes, it&#8217;s more complex, and no, it&#8217;s not always going to work (sometimes
923the answer is: "oops, I really shouldn&#8217;t have merged it, because it wasn&#8217;t
924ready yet, and I really need to undo <em>all</em> of the merge"). So then you
925really should revert the merge, but when you want to re-do the merge, you
926now need to do it by reverting the revert.</p></div>
927<div class="paragraph"><p>ADDENDUM</p></div>
928<div class="paragraph"><p>Sometimes you have to rewrite one of a topic branch&#8217;s commits <strong>and</strong> you can&#8217;t
929change the topic&#8217;s branching-off point. Consider the following situation:</p></div>
930<div class="literalblock">
931<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]932<pre><code>P---o---o---M---x---x---W---x
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]933 \ /
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]934 A---B---C</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]935</div></div>
936<div class="paragraph"><p>where commit W reverted commit M because it turned out that commit B was wrong
937and needs to be rewritten, but you need the rewritten topic to still branch
938from commit P (perhaps P is a branching-off point for yet another branch, and
939you want be able to merge the topic into both branches).</p></div>
940<div class="paragraph"><p>The natural thing to do in this case is to checkout the A-B-C branch and use
941"rebase -i P" to change commit B. However this does not rewrite commit A,
942because "rebase -i" by default fast-forwards over any initial commits selected
943with the "pick" command. So you end up with this:</p></div>
944<div class="literalblock">
945<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]946<pre><code>P---o---o---M---x---x---W---x
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]947 \ /
948 A---B---C &lt;-- old branch
949 \
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]950 B'---C' &lt;-- naively rewritten branch</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]951</div></div>
952<div class="paragraph"><p>To merge A-B'-C' into the mainline branch you would still have to first revert
953commit W in order to pick up the changes in A, but then it&#8217;s likely that the
954changes in B' will conflict with the original B changes re-introduced by the
955reversion of W.</p></div>
956<div class="paragraph"><p>However, you can avoid these problems if you recreate the entire branch,
957including commit A:</p></div>
958<div class="literalblock">
959<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]960<pre><code> A'---B'---C' &lt;-- completely rewritten branch
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]961 /
962P---o---o---M---x---x---W---x
963 \ /
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]964 A---B---C</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]965</div></div>
966<div class="paragraph"><p>You can merge A'-B'-C' into the mainline branch without worrying about first
967reverting W. Mainline&#8217;s history would look like this:</p></div>
968<div class="literalblock">
969<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]970<pre><code> A'---B'---C'------------------
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]971 / \
972P---o---o---M---x---x---W---x---M2
973 \ /
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]974 A---B---C</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]975</div></div>
976<div class="paragraph"><p>But if you don&#8217;t actually need to change commit A, then you need some way to
977recreate it as a new commit with the same changes in it. The rebase command&#8217;s
978--no-ff option provides a way to do this:</p></div>
979<div class="literalblock">
980<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]981<pre><code>$ git rebase [-i] --no-ff P</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]982</div></div>
983<div class="paragraph"><p>The --no-ff option creates a new branch A'-B'-C' with all-new commits (all the
984SHA IDs will be different) even if in the interactive case you only actually
985modify commit B. You can then merge this new branch directly into the mainline
986branch and be sure you&#8217;ll get all of the branch&#8217;s changes.</p></div>
987<div class="paragraph"><p>You can also use --no-ff in cases where you just add extra commits to the topic
988to fix it up. Let&#8217;s revisit the situation discussed at the start of this howto:</p></div>
989<div class="literalblock">
990<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]991<pre><code>P---o---o---M---x---x---W---x
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]992 \ /
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]993 A---B---C----------------D---E &lt;-- fixed-up topic branch</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]994</div></div>
995<div class="paragraph"><p>At this point, you can use --no-ff to recreate the topic branch:</p></div>
996<div class="literalblock">
997<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]998<pre><code>$ git checkout E
999$ git rebase --no-ff P</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1000</div></div>
1001<div class="paragraph"><p>yielding</p></div>
1002<div class="literalblock">
1003<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1004<pre><code> A'---B'---C'------------D'---E' &lt;-- recreated topic branch
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1005 /
1006P---o---o---M---x---x---W---x
1007 \ /
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1008 A---B---C----------------D---E</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1009</div></div>
1010<div class="paragraph"><p>You can merge the recreated branch into the mainline without reverting commit W,
1011and mainline&#8217;s history will look like this:</p></div>
1012<div class="literalblock">
1013<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1014<pre><code> A'---B'---C'------------D'---E'
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1015 / \
1016P---o---o---M---x---x---W---x---M2
1017 \ /
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1018 A---B---C</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1019</div></div>
1020</div>
1021</div>
1022</div>
1023<div id="footnotes"><hr /></div>
1024<div id="footer">
1025<div id="footer-text">
Junio C Hamano2ef0ba32018-01-26 23:13:53[diff] [blame]1026Last updated
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1028</div>
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