author | Pierre-Yves David <pierre-yves.david@fb.com> |
Tue, 23 Jun 2015 16:49:24 -0700 | |
branch | mercurial-3.0 |
changeset 1436 | b9f8eaa49733 |
parent 980 | 64a2e940e1b2 |
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.. Copyright © 2014 Greg Ward <greg@gerg.ca> |
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================================== |
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Changeset Evolution with Mercurial |
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================================== |
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`evolve`_ is an experimental Mercurial extension for safe mutable history. |
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.. _`evolve`: http://mercurial.selenic.com/wiki/EvolveExtension |
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With core Mercurial, changesets are permanent and immutable. You can |
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commit new changesets to modify your source code, but you cannot |
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modify or remove old changesets—they are carved in stone for all |
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eternity. |
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For years, Mercurial has included various extensions that allow |
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history modification: ``rebase``, ``mq``, ``histedit``, and so forth. |
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These are useful and popular extensions, and in fact history |
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modification is one of the big reasons DVCSes (distributed version |
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control systems) like Mercurial took off. |
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But there's a catch: until now, Mercurial's various mechanisms for |
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modifying history have been *unsafe*, in that changesets were |
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destroyed (“stripped”) rather than simply made invisible. |
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``evolve`` makes things better in a couple of ways: |
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* It changes the behaviour of most existing history modification |
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extensions (``rebase``, ``histedit``, etc.) so they use a safer |
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mechanism (*changeset obsolescence*, covered below) rather than |
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the older, less safe *strip* operation. |
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* It provides a new way of modifying history that is roughly |
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equivalent to ``mq`` (but much nicer and safer). |
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It helps to understand that ``evolve`` builds on infrastructure |
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already in core Mercurial: |
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* *Phases* (starting in Mercurial 2.1) allow you to distinguish |
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mutable and immutable changesets. We'll cover phases early in the |
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user guide, since understanding phases is essential to |
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understanding ``evolve``. |
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* *Changeset obsolescence* (starting in Mercurial 2.3) is how |
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Mercurial knows how history has been modified, specifically when |
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one changeset replaces another. In the obsolescence model, a |
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changeset is neither removed nor modified, but is instead marked |
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*obsolete* and typically replaced by a *successor*. Obsolete |
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changesets usually become *hidden* as well. Obsolescence is an |
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invisible feature until you start using ``evolve``, so we'll cover |
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it in the user guide too. |
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Some of the things you can do with ``evolve`` are: |
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* Fix a mistake immediately: “Oops! I just committed a changeset |
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with a syntax error—I'll fix that and amend the changeset so no |
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one sees my mistake.” (While this is possible using existing |
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features of core Mercurial, ``evolve`` makes it safer.) |
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* Fix a mistake a little bit later: “Oops! I broke the tests three |
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commits back, but only noticed it now—I'll just update back to the |
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bad changeset, fix my mistake, amend the changeset, and evolve |
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history to update the affected changesets.” |
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* Remove unwanted changes: “I hacked in some debug output two |
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commits back; everything is working now, so I'll just prune that |
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unwanted changeset and evolve history before pushing.” |
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* Share mutable history with yourself: say you do most of your |
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programming work locally, but need to test on a big remote server |
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somewhere before you know everything is good. You can use |
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``evolve`` to share mutable history between two computers, pushing |
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finely polished changesets to a public repository only after |
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testing on the test server. |
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* Share mutable history for code review: you don't want to publish |
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unreviewed changesets, but you can't block every commit waiting |
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for code review. The solution is to share mutable history with |
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your reviewer, amending each changeset until it passes review. |
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``evolve`` is experimental! |
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--------------------------- |
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TODO |
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* unstable UI |
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* some corner cases not covered yet |
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Installation and setup |
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---------------------- |
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TODO |
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Next steps: |
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* For a practical guide to using ``evolve`` in a single repository, |
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see the `user guide`_. |
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* For more advanced tricks, see `sharing mutable history`_. |
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* To learn about the concepts underlying ``evolve``, see `concepts`_ |
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(incomplete). |
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* If you're coming from MQ, see the `MQ migration guide`_ (incomplete). |
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.. _`user guide`: user-guide.html |
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.. _`concepts`: concepts.html |
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.. _`sharing mutable history`: sharing.html |
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.. _`MQ migration guide`: from-mq.html |