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| </head> |
| <body class="article"> |
| <div id="header"> |
| <h1>Rerere</h1> |
| </div> |
| <div id="content"> |
| <div id="preamble"> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>This document describes the rerere logic.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_conflict_normalization">Conflict normalization</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>To ensure recorded conflict resolutions can be looked up in the rerere |
| database, even when branches are merged in a different order, |
| different branches are merged that result in the same conflict, or |
| when different conflict style settings are used, rerere normalizes the |
| conflicts before writing them to the rerere database.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Different conflict styles and branch names are normalized by stripping |
| the labels from the conflict markers, and removing the common ancestor |
| version from the <code>diff3</code> or <code>zdiff3</code> conflict styles. Branches that |
| are merged in different order are normalized by sorting the conflict |
| hunks. More on each of those steps in the following sections.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Once these two normalization operations are applied, a conflict ID is |
| calculated based on the normalized conflict, which is later used by |
| rerere to look up the conflict in the rerere database.</p> |
| </div> |
| <div class="sect2"> |
| <h3 id="_removing_the_common_ancestor_version">Removing the common ancestor version</h3> |
| <div class="paragraph"> |
| <p>Say we have three branches AB, AC and AC2. The common ancestor of |
| these branches has a file with a line containing the string "A" (for |
| brevity this is called "line A" in the rest of the document). In |
| branch AB this line is changed to "B", in AC, this line is changed to |
| "C", and branch AC2 is forked off of AC, after the line was changed to |
| "C".</p> |
| </div> |
| <div class="paragraph"> |
| <p>Forking a branch ABAC off of branch AB and then merging AC into it, we |
| get a conflict like the following:</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre><<<<<<< HEAD |
| B |
| ======= |
| C |
| >>>>>>> AC</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>Doing the analogous with AC2 (forking a branch ABAC2 off of branch AB |
| and then merging branch AC2 into it), using the diff3 or zdiff3 |
| conflict style, we get a conflict like the following:</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre><<<<<<< HEAD |
| B |
| ||||||| merged common ancestors |
| A |
| ======= |
| C |
| >>>>>>> AC2</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>By resolving this conflict, to leave line D, the user declares:</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>After examining what branches AB and AC did, I believe that making |
| line A into line D is the best thing to do that is compatible with |
| what AB and AC wanted to do.</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>As branch AC2 refers to the same commit as AC, the above implies that |
| this is also compatible with what AB and AC2 wanted to do.</p> |
| </div> |
| <div class="paragraph"> |
| <p>By extension, this means that rerere should recognize that the above |
| conflicts are the same. To do this, the labels on the conflict |
| markers are stripped, and the common ancestor version is removed. The above |
| examples would both result in the following normalized conflict:</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre><<<<<<< |
| B |
| ======= |
| C |
| >>>>>>></pre> |
| </div> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="_sorting_hunks">Sorting hunks</h3> |
| <div class="paragraph"> |
| <p>As before, let’s imagine that a common ancestor had a file with line A |
| its early part, and line X in its late part. And then four branches |
| are forked that do these things:</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>AB: changes A to B</p> |
| </li> |
| <li> |
| <p>AC: changes A to C</p> |
| </li> |
| <li> |
| <p>XY: changes X to Y</p> |
| </li> |
| <li> |
| <p>XZ: changes X to Z</p> |
| </li> |
| </ul> |
| </div> |
| <div class="paragraph"> |
| <p>Now, forking a branch ABAC off of branch AB and then merging AC into |
| it, and forking a branch ACAB off of branch AC and then merging AB |
| into it, would yield the conflict in a different order. The former |
| would say "A became B or C, what now?" while the latter would say "A |
| became C or B, what now?"</p> |
| </div> |
| <div class="paragraph"> |
| <p>As a reminder, the act of merging AC into ABAC and resolving the |
| conflict to leave line D means that the user declares:</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>After examining what branches AB and AC did, I believe that |
| making line A into line D is the best thing to do that is |
| compatible with what AB and AC wanted to do.</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>So the conflict we would see when merging AB into ACAB should be |
| resolved the same way—​it is the resolution that is in line with that |
| declaration.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Imagine that similarly previously a branch XYXZ was forked from XY, |
| and XZ was merged into it, and resolved "X became Y or Z" into "X |
| became W".</p> |
| </div> |
| <div class="paragraph"> |
| <p>Now, if a branch ABXY was forked from AB and then merged XY, then ABXY |
| would have line B in its early part and line Y in its later part. |
| Such a merge would be quite clean. We can construct 4 combinations |
| using these four branches AB, AC) x (XY, XZ.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Merging ABXY and ACXZ would make "an early A became B or C, a late X |
| became Y or Z" conflict, while merging ACXY and ABXZ would make "an |
| early A became C or B, a late X became Y or Z". We can see there are |
| 4 combinations of ("B or C", "C or B") x ("X or Y", "Y or X").</p> |
| </div> |
| <div class="paragraph"> |
| <p>By sorting, the conflict is given its canonical name, namely, "an |
| early part became B or C, a late part became X or Y", and whenever |
| any of these four patterns appear, and we can get to the same conflict |
| and resolution that we saw earlier.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Without the sorting, we’d have to somehow find a previous resolution |
| from combinatorial explosion.</p> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="_conflict_id_calculation">Conflict ID calculation</h3> |
| <div class="paragraph"> |
| <p>Once the conflict normalization is done, the conflict ID is calculated |
| as the sha1 hash of the conflict hunks appended to each other, |
| separated by <NUL> characters. The conflict markers are stripped out |
| before the sha1 is calculated. So in the example above, where we |
| merge branch AC which changes line A to line C, into branch AB, which |
| changes line A to line C, the conflict ID would be |
| SHA1(<em>B<NUL>C<NUL></em>).</p> |
| </div> |
| <div class="paragraph"> |
| <p>If there are multiple conflicts in one file, the sha1 is calculated |
| the same way with all hunks appended to each other, in the order in |
| which they appear in the file, separated by a <NUL> character.</p> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="_nested_conflicts">Nested conflicts</h3> |
| <div class="paragraph"> |
| <p>Nested conflicts are handled very similarly to "simple" conflicts. |
| Similar to simple conflicts, the conflict is first normalized by |
| stripping the labels from conflict markers, stripping the common ancestor |
| version, and sorting the conflict hunks, both for the outer and the |
| inner conflict. This is done recursively, so any number of nested |
| conflicts can be handled.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Note that this only works for conflict markers that "cleanly nest". If |
| there are any unmatched conflict markers, rerere will fail to handle |
| the conflict and record a conflict resolution.</p> |
| </div> |
| <div class="paragraph"> |
| <p>The only difference is in how the conflict ID is calculated. For the |
| inner conflict, the conflict markers themselves are not stripped out |
| before calculating the sha1.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Say we have the following conflict for example:</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre><<<<<<< HEAD |
| 1 |
| ======= |
| <<<<<<< HEAD |
| 3 |
| ======= |
| 2 |
| >>>>>>> branch-2 |
| >>>>>>> branch-3~</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>After stripping out the labels of the conflict markers, and sorting |
| the hunks, the conflict would look as follows:</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre><<<<<<< |
| 1 |
| ======= |
| <<<<<<< |
| 2 |
| ======= |
| 3 |
| >>>>>>> |
| >>>>>>></pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>and finally the conflict ID would be calculated as: |
| <code>sha1</code>('1<em><NUL></em><<<<<<<\n3\n=======\n2\n>>>>>>><em><NUL></em>')</p> |
| </div> |
| </div> |
| </div> |
| </div> |
| </div> |
| <div id="footer"> |
| <div id="footer-text"> |
| Last updated 2025-06-20 18:10:42 -0700 |
| </div> |
| </div> |
| </body> |
| </html> |
