diff options
Diffstat (limited to '.html/dev/merging-structural-changes.html')
| -rw-r--r-- | .html/dev/merging-structural-changes.html | 156 |
1 files changed, 156 insertions, 0 deletions
diff --git a/.html/dev/merging-structural-changes.html b/.html/dev/merging-structural-changes.html new file mode 100644 index 0000000..e5c8795 --- /dev/null +++ b/.html/dev/merging-structural-changes.html @@ -0,0 +1,156 @@ +<!DOCTYPE html> +<html> +<head> + <title> + The Codex » + Merging Structural Changes + </title> + + <link + rel='stylesheet' + type='text/css' + href='http://fonts.googleapis.com/css?family=Buenard:400,700&subset=latin,latin-ext'> + <link + rel="stylesheet" + type="text/css" + href="../media/css/reset.css"> + <link + rel="stylesheet" + type="text/css" + href="../media/css/grimoire.css"> +</head> +<body> + +<div id="shell"> + + <ol id="breadcrumbs"> + + <li class="crumb-0 not-last"> + + <a href="../">index</a> + + </li> + + <li class="crumb-1 not-last"> + + <a href="./">dev</a> + + </li> + + <li class="crumb-2 last"> + + merging-structural-changes + + </li> + + </ol> + + + + <div id="article"> + <h1 id="merging-structural-changes">Merging Structural Changes</h1> +<p>In 2008, a project I was working on set out to reinvent their build process, +migrating from a mass of poorly-written Ant scripts to Maven and reorganizing +their source tree in the process. The development process was based on having +a branch per client, so there was a lot of ongoing development on the original +layout for clients that hadn't been migrated yet. We discovered that our +version control tool, <a href="http://subversion.tigris.org/">Subversion</a>, was unable +to merge the changes between client branches on the old structure and the +trunk on the new structure automatically.</p> +<p>Curiousity piqued, I cooked up a script that reproduces the problem and +performs the merge from various directions to examine the results. Subversion, +sadly, performed dismally: none of the merge scenarios tested retained content +changes when merging structural changes to the same files.</p> +<h2 id="the-preferred-outcome">The Preferred Outcome</h2> +<p><img alt="Both changes survive the +merge." src="/media/dev/merging-structural-changes/ideal-merge-results"></p> +<p>The diagram above shows a very simple source tree with one directory, <code>dir-a</code>, +containing one file with two lines in it. On one branch, the file is modified +to have a third line; on another branch, the directory is renamed to <code>dir-b</code>. +Then, both branches are merged, and the resulting tree contains both sets of +changes: the file has three lines, and the directory has a new name.</p> +<p>This is the preferred outcome, as no changes are lost or require manual +merging.</p> +<h2 id="subversion">Subversion</h2> +<p><img alt="Subversion loses the content +change." src="/media/dev/merging-structural-changes/subversion-merge-results"></p> +<p>There are two merge scenarios in this diagram, with almost the same outcome. +On the left, a working copy of the branch where the file's content changed is +checked out, then the changes from the branch where the structure changed are +merged in. On the right, a working copy of the branch where the structure +changed is checked out, then the changes from the branch where the content +changed are merged in. In both cases, the result of the merge has the new +directory name, and the original file contents. In one case, the merge +triggers a rather opaque warning about a “missing file”; in the other, the +merge silently ignores the content changes.</p> +<p>This is a consequence of the way Subversion implements renames and copies. +When Subversion assembles a changeset for committing to the repository, it +comes up with a list of primitive operations that reproduce the change. There +is no primitive that says “this object was moved,” only primitives which say +“this object was deleted” or “this object was added, as a copy of that +object.” When you move a file in Subversion, those two operations are +scheduled. Later, when Subversion goes to merge content changes to the +original file, all it sees is that the file has been deleted; it's completely +unaware that there is a new name for the same file.</p> +<p>This would be fairly easy to remedy by adding a “this object was moved to that +object” primitive to the changeset language, and <a href="http://subversion.tigris.org/issues/show_bug.cgi?id=898">a bug report for just such a +feature</a> was filed in +2002. However, by that time Subversion's repository and changeset formats had +essentially frozen, as Subversion was approaching a 1.0 release and more +important bugs <em>without</em> workarounds were a priority.</p> +<p>There is some work going on in Subversion 1.6 to handle tree conflicts (the +kind of conflicts that come from this kind of structural change) more +sensibly, which will cause the two merges above to generate a Conflict result, +which is not as good as automatically merging it but far better than silently +ignoring changes.</p> +<h2 id="mercurial">Mercurial</h2> +<p><img alt="Mercurial preserves the content +change." src="/media/dev/merging-structural-changes/mercurial-merge-results"></p> +<p>Interestingly, there are tools which get this merge scenario right: the +diagram above shows how <a href="http://www.selenic.com/mercurial/">Mercurial</a> handles +the same two tests. Since its changeset language does include an “object +moved” primitive, it's able to take a content change for <code>dir-a/file</code> and +apply it to <code>dir-b/file</code> if appropriate.</p> +<h2 id="git">Git</h2> +<p>Git also gets this scenario right, <em>usually</em>. Unlike Mercurial, Git does not +track file copies or renames in its commits at all, prefering to infer them by +content comparison every time it performs a move-aware operation, such as a +merge.</p> + </div> + + + +<div id="comments"> +<div id="disqus_thread"></div> +<script type="text/javascript"> + /* * * CONFIGURATION VARIABLES: EDIT BEFORE PASTING INTO YOUR WEBPAGE * * */ + var disqus_shortname = 'grimoire'; // required: replace example with your forum shortname + + /* * * DON'T EDIT BELOW THIS LINE * * */ + (function() { + var dsq = document.createElement('script'); dsq.type = 'text/javascript'; dsq.async = true; + dsq.src = 'http://' + disqus_shortname + '.disqus.com/embed.js'; + (document.getElementsByTagName('head')[0] || document.getElementsByTagName('body')[0]).appendChild(dsq); + })(); +</script> +<noscript>Please enable JavaScript to view the <a href="http://disqus.com/?ref_noscript">comments powered by Disqus.</a></noscript> +<a href="http://disqus.com" class="dsq-brlink">comments powered by <span class="logo-disqus">Disqus</span></a> +</div> + + + + <div id="footer"> + <p> + + The Codex — + + Powered by <a href="http://markdoc.org/">Markdoc</a>. + +<a href="https://bitbucket.org/ojacobson/grimoire.ca/src/master/wiki/dev/merging-structural-changes.md">See this page on Bitbucket</a> (<a href="https://bitbucket.org/ojacobson/grimoire.ca/history-node/master/wiki/dev/merging-structural-changes.md">history</a>). + + </p> + </div> + +</div> +</body> +</html>
\ No newline at end of file |