Review commit: https://reviewboard.mozilla.org/r/171412/diff/#index_header See other reviews: https://reviewboard.mozilla.org/r/171412/

Mercurial 4.3 has an experimental sparse checkout feature. It allows the working directory to consist of a subset of files from a revision. There are a number of different ways to specify what to include in the sparse checkout. But the method we care about supporting for Firefox is "profiles." Essentially, an in-tree file can define patterns of files to include or exclude in the working directory. You "activate" a profile and Mercurial reads that file and applies the patterns to the working directory. In this commit, we teach robustcheckout to perform sparse checkouts using sparse profile files. While Mercurial supports multiple active profiles, we make the arbitrary decision to only allow a single profile. And, when we change profiles, we deactivate the old one and enable the new one rather than union them. This avoids all kinds of non-determinism and potential wonkiness around combinations of profiles that don't play well together. This should therefore eliminate a whole class of bugs. Another area where we've made arbitrary decisions is around enabling and disabling sparse on existing checkouts. This isn't strictly needed. But I want robustcheckout to be very conservative in its sparse support initially. I don't want CI mixing sparse with non sparse. And this restriction forces that. The code in robustcheckout for supporting sparse is somewhat hacky and some things are sub-optimal. For example, when changing profiles it does a working directory update to the new profile and then potentially a separate working directory update to the new revision. Unfortunately, Mercurial's internal APIs don't allow you to write out the new profile without the working directory update. We /could/ copy some code to facilitate this. But my inclination is to go with the simple (albeit sub-optimal) solution and then add complexity only if needed. Another example is having to monkeypatch the dirstate object so purge purges files not in the sparse config. Because of the level of the hacks and the experimental nature of sparse in Mercurial, the feature will only run on Mercurial 4.3. When Mercurial 4.4 is out, we'll have to explicitly tell robustcheckout to support it. Review commit: https://reviewboard.mozilla.org/r/171414/diff/#index_header See other reviews: https://reviewboard.mozilla.org/r/171414/

Comment on attachment 8900070 [details] robustcheckout: support sparse profiles (bug 1391394); Review request updated; see interdiff: https://reviewboard.mozilla.org/r/171414/diff/1-2/