[Python-Dev] Mercurial workflow question... (original) (raw)

Stephen J. Turnbull stephen at xemacs.org
Mon Dec 17 07:22:00 CET 2012

Raymond Hettinger writes:

Does hg's ability to "make merges easier than svn" depend on having all the intermediate commits? I thought the theory was that the smaller changesets provided extra information that made it possible to merge two expansive groups of changes.

Tim Delaney's explanation is correct as far as it goes.

But I would give a pretty firm "No" as the answer to your question.

The big difference between svn (and CVS) and hg (and git and bzr) at the time of migrating the Python repository was that svn didn't track merges, only branches. So in svn you get a 3-way merge with the branch point as the base version. This meant that you could not track progress of the mainline while working on a branch. svn tends to report the merge of recent mainline changes back into the mainline as conflicts when merging your branch into the mainline[1][2], all too often resulting in a big mess.

hg, because it records merges as well as branches, can use the most recent common version (typically the mainline parent of the most recent "catch-up" merge) as the base version. This means that (1) there are somewhat fewer divergences because your branch already contains most changes to the mainline, and (2) you don't get "spurious" conflicts. On the other hand, more frequent intermediate committing is mostly helpful in bisection, and so the usefulness depends on very disciplined committing (only commit build- and test-able code).

Summary: only the frequency of intermediate merge commits really matters. Because in hg it's possible to have frequent "catch-up" merges from mainline, you get smaller merges with fewer conflicts both at "catch-up" time and at merge-to-mainline time.

Footnotes: [1] Not the whole story, but OK for this purpose. Technical details available on request.

[2] I have paid almost no attention to svn since Python migrated to hg, so perhaps svn has improved merge support in the meantime. But that doesn't really matter since svn is merely being used to help explain why commit granularity doesn't matter much to hg's merge capabilities.

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