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POConsequences of using graft in Mercurial
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copied!<p>There've been several questions recently about skipping changes when maintaining release branches in Mercurial. For example:</p> <ul> <li><a href="https://stackoverflow.com/questions/9532823/mercurial-branch-specific-changes-keep-coming-back-after-dummy-merge">Mercurial: Branch specific changes keep coming back after dummy merge</a></li> <li><a href="https://stackoverflow.com/questions/9500399/why-are-mercurial-backouts-in-one-branch-affecting-other-branches">Why are Mercurial backouts in one branch affecting other branches?</a></li> </ul> <p>Since it was introduced in 2.0, I've wondered about using <code>graft</code> to avoid this problem. Given a revision tree like this:</p> <pre><code>A---B---C---D---E---F---G---H---I---J </code></pre> <p>Suppose we need to create a release branch that skips the Evil change <code>E</code>.</p> <pre><code>hg update -r D hg graft "F::J" </code></pre> <p>giving us:</p> <pre><code>A---B---C---D---E---F---G---H---I---J \ --F'--G'--H'--I'--J' </code></pre> <ul> <li>Q1: What just happened here? I can understand that <code>transplant</code> would have generated patches out of <code>F::J</code>, and then applied them onto <code>D</code>, but <code>graft</code> is said to use the 3-way merge rather than patches. So....... how does that work? Why is it better?</li> </ul> <p>Lets say I now fix <code>E</code>, and merge that into my release branch.</p> <pre><code> --E2----------------- / \ A---B---C---D---E---F---G---H---I---J---M1 \ \ --F'--G'--H'--I'--J'---------M2-- </code></pre> <p>M1 is a straight merge; nothing special there. M2 is merging branches which have "the same" (or at least equivalent) changes on.</p> <ul> <li>Q2: Is this merge just a normal 3-way merge using <code>D</code>, <code>J'</code> and <code>M1</code>?</li> <li>Q3: Has mercurial stored/used extra information about the graft operation to help it with the merge?</li> </ul> <p>And finally...</p> <ul> <li>Q4: What are the potential problems with a flow like this?</li> </ul>

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