author | Durham Goode <durham@fb.com> |
Wed, 15 Oct 2014 15:41:26 -0700 | |
changeset 1226 | 901d2f4b21a9 |
parent 1186 | 0b66826f081c |
child 1259 | 0daf56a2032d |
permissions | -rw-r--r-- |
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.. Copyright © 2014 Greg Ward <greg@gerg.ca> |
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------------------------------ |
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Evolve: Shared Mutable History |
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------------------------------ |
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Once you have mastered the art of mutable history in a single |
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repository, you might want to move up to the next level: *shared* |
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mutable history. ``evolve`` lets you push and pull draft changesets |
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between repositories along with their obsolescence markers. This opens |
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up a number of interesting possibilities. |
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The most common scenario is a single developer working across two |
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computers. Say you're working on code that must be tested on a remote |
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test server, probably in a rack somewhere, only accessible by SSH, and |
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running an “enterprise-grade” (out-of-date) OS. But you probably |
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prefer to write code locally: everything is setup the way you like it, |
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and you can use your preferred editor, IDE, merge/diff tools, etc. |
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Traditionally, your options are limited: either |
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* (ab)use your source control system by committing half-working code |
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in order to get it onto the remote test server, or |
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* go behind source control's back by using ``rsync`` (or similar) to |
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transfer your code back-and-forth until it is ready to commit |
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The former is less bad with distributed version control systems like |
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Mercurial, but it's still far from ideal. (One important version |
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control “best practice” is that every commit should make things just a |
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little bit better, i.e. you should never commit code that is worse |
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than what came before.) The latter, avoiding version control entirely, |
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means that you're walking a tightrope without a safety net. One |
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accidental ``rsync`` in the wrong direction could destroy hours of |
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work. |
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Using Mercurial with ``evolve`` to share mutable history solves all of |
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these problems. As with single-repository ``evolve``, you can commit |
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whenever the code is demonstrably better, even if all the tests aren't |
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passing yet—just ``hg amend`` when they are. And you can transfer |
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those half-baked changesets between repositories to try things out on |
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your test server before anything is carved in stone. |
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A less common scenario is multiple developers sharing mutable history. |
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(This is in fact how Mercurial itself is developed.) We'll cover this |
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scenario later. But first, single-user sharing. |
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Publishing and non-publishing repositories |
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------------------------------------------ |
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The key to shared mutable history is to keep your changesets in |
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*draft* phase as you pass them around. Recall that by default, ``hg |
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push`` promotes changesets from *draft* to *public*, and public |
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changesets are immutable. You can change this behaviour by |
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reconfiguring the *target* repository so that it is non-publishing. |
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(Short version: set ``phases.publish`` to ``false``. Long version |
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follows.) |
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Setting things up |
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----------------- |
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We'll work an example with three local repositories, although in the |
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real world they'd most likely be on three different computers. First, |
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the public repository is where tested, polished changesets live, and |
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it is where you push/pull changesets to/from the rest of your team. :: |
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$ hg init public |
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We'll need two clones where work gets done:: |
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$ hg clone -q public test-repo |
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$ hg clone -q test-repo dev-repo |
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``dev-repo`` is your local machine, with GUI merge tools and IDEs and |
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everything configured just the way you like it. ``test-repo`` is the |
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test server in a rack somewhere behind SSH. So for the most part, |
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we'll develop in ``dev-repo``, push to ``test-repo``, test and polish |
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there, and push to ``public``. |
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The key to making this whole thing work is to make ``test-repo`` |
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non-publishing:: |
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$ cat >> test-repo/.hg/hgrc <<EOF |
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[phases] |
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publish = false |
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EOF |
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We also have to configure ``evolve`` in both ``test-repo`` and |
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``dev-repo``, so that we can amend and evolve in both of them. :: |
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$ cat >> test-repo/.hg/hgrc <<EOF |
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[extensions] |
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evolve = /path/to/mutable-history/hgext/evolve.py |
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EOF |
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$ cat >> dev-repo/.hg/hgrc <<EOF |
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[extensions] |
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evolve = /path/to/mutable-history/hgext/evolve.py |
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EOF |
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Keep in mind that in real life, these repositories would probably be |
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on separate computers, so you'd have to login to each one to configure |
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each repository. |
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To start things off, let's make one public, immutable changeset:: |
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$ cd test-repo |
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$ echo 'my new project' > file1 |
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$ hg add file1 |
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$ hg commit -m 'create new project' |
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$ hg push -q |
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and pull that into the development repository:: |
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$ cd ../dev-repo |
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$ hg pull -u |
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Amending a shared changeset |
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--------------------------- |
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Everything you learned in the `user guide`_ applies to work done in |
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``dev-repo``. You can commit, amend, uncommit, evolve, and so forth |
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just as before. |
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.. _`user guide`: user-guide.html |
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Things get different when you push changesets to ``test-repo``. Or |
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rather, things stay the same, which *is* different: because we |
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configured ``test-repo`` to be non-publishing, draft changesets stay |
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draft when we push them to ``test-repo``. Importantly, they're also |
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draft (mutable) in ``test-repo``. |
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Let's commit a preliminary change and push it to ``test-repo`` for |
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testing. :: |
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$ echo 'fix fix fix' > file1 |
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$ hg commit -m 'prelim change' |
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$ hg push ../test-repo |
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At this point, ``dev-repo`` and ``test-repo`` have the same changesets |
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in the same phases: |
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[figure SG01: rev 0:0dc9 public, rev 1:f649 draft, same on both repos] |
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(You may notice a change in notation from the user guide: now |
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changesets are labelled with their revision number and the first four |
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digits of the 40-digit hexadecimal changeset ID. Mercurial revision |
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numbers are never stable when working across repositories, especially |
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when obsolescence is involved. We'll see why shortly.) |
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Now let's switch to ``test-repo`` to test our change:: |
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$ cd ../test-repo |
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$ hg update |
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Don't forget to ``hg update``! Pushing only adds changesets to a |
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remote repository; it does not update the working directory (unless |
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you have a hook that updates for you). |
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Now let's imagine the tests failed because we didn't use proper |
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punctuation and capitalization (oops). Let's amend our preliminary fix |
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(and fix the lame commit message while we're at it):: |
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$ echo 'Fix fix fix.' > file1 |
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$ hg amend -m 'fix bug 37' |
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Now we're in a funny intermediate state (figure 2): revision 1:f649 is |
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obsolete in ``test-repo``, having been replaced by revision 3:60ff |
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(revision 2:2a03 is another one of those temporary amend commits that |
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we saw in the user guide)—but ``dev-repo`` knows nothing of these |
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recent developments. |
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[figure SG02: rev 0:0dc9 public, rev 1:f649, 2:2a03 obsolete, rev 3:60ff draft -- but dev-repo same as in SG01] |
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Let's resynchronize:: |
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$ cd ../dev-repo |
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$ hg pull -u |
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As seen in figure 3, this transfers the new changeset *and* the |
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obsolescence marker for revision 1. However, it does *not* transfer |
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the temporary amend commit, because it is obsolete. Push and pull |
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transfer obsolesence markers between repositories, but they do not |
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normally transfer obsolete changesets. |
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[figure SG03: dev-repo grows new rev 2:60ff, marks 1:f649 obsolete] |
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|
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Because of this deliberately incomplete synchronization, revision |
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numbers in ``test-repo`` and ``dev-repo`` are no longer consistent. We |
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*must* use changeset IDs. |
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Amend again, locally |
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-------------------- |
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This process can repeat. Perhaps you figure out a more elegant fix to |
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the bug, and want to mutate history so nobody ever knows you had a |
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less-than-perfect idea. We'll implement it locally in ``dev-repo`` and |
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push to ``test-repo``:: |
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$ echo 'Fix, fix, and fix.' > file1 |
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$ hg amend |
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$ hg push |
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This time around, the temporary amend commit is in ``dev-repo``, and |
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it is not transferred to ``test-repo``—the same as before, just in the |
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opposite direction. Figure 4 shows the two repositories after amending |
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in ``dev-repo`` and pushing to ``test-repo``. |
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[figure SG04: each repo has one temporary amend commit, but they're different in each one] |
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Let's hop over to ``test-repo`` to test the more elegant fix:: |
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|
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$ cd ../test-repo |
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$ hg update -q |
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This time, all the tests pass, so no further amendment is required. |
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This bug fix is finished, so we push it to the public repository:: |
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$ hg push |
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[...] |
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added 1 changesets with 1 changes to 1 files |
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Note that only one changeset—the final version, after two |
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amendments—was actually pushed. Again, Mercurial normally doesn't |
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transfer obsolete changesets on push and pull. (Specifically, it |
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doesn't transfer *hidden* changesets: roughly speaking, obsolete |
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changesets with no non-obsolete descendants. If you're curious, see |
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the `concept guide`_ for the precise definition of hidden.) |
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|
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.. _`concept guide`: concepts.html |
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|
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So the picture in ``public`` is much simpler than in either |
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``dev-repo`` or ``test-repo``. None of our missteps or amendments are |
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visible publicly, just the final, beautifully polished changeset: |
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[figure SG05: public repo with rev 0:0dc9, 1:de61, both public] |
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There is one important step left to do. Because we pushed from |
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``test-repo`` to ``public``, the pushed changeset is in *public* phase |
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in those two repositories. But ``dev-repo`` knows nothing of this: |
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that changeset is still *draft* there. If we're not careful, we might |
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mutate history in ``dev-repo``, obsoleting a changeset that is already |
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public. Let's avoid that situation for now by pulling from |
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``test-repo`` down to ``dev-repo``:: |
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|
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$ cd ../dev-repo |
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$ hg pull -u |
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|
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Getting into trouble |
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-------------------- |
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Mercurial with ``evolve`` is a powerful tool, and using powerful tools |
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can have consequences. (You can cut yourself badly with a sharp knife, |
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but every competent chef keeps several around. Ever try to chop onions |
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with a spoon?) |
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|
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In the user guide, we saw examples of *unstable* changesets, which are |
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the most common type of troubled changeset. (Recall that a |
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non-obsolete changeset with obsolete ancestors is unstable.) |
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|
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Two other types of trouble can crop up: *bumped* and *divergent* |
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changesets. Both are more likely with shared mutable history, |
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especially mutable history shared by multiple developers. |
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|
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To demonstrate, let's start with the ``public`` repository as we left |
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it in the last example, with two immutable changesets (figure 5 |
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above). Two developers, Alice and Bob, start working from this point:: |
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|
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$ hg clone -q public alice |
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$ hg clone -q public bob |
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We need to configure Alice's and Bob's working repositories similar to |
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``test-repo``, i.e. make them non-publishing and enable ``evolve``:: |
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|
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$ cat >> alice/.hg/hgrc <<EOF |
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[phases] |
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publish = false |
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[extensions] |
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evolve = /path/to/mutable-history/hgext/evolve.py |
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EOF |
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$ cp alice/.hg/hgrc bob/.hg/hgrc |
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Bumped changesets: only one gets on the plane |
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--------------------------------------------- |
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If two people show up at the airport with tickets for the same seat on |
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the same plane, only one of them gets on the plane. The would-be |
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traveller left behind in the airport terminal is said to have been |
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*bumped*. |
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|
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Similarly, if Alice and Bob are collaborating on some mutable |
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changesets, it's possible to get into a situation where an otherwise |
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worthwhile changeset cannot be pushed to the public repository; it is |
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bumped by an alternative changeset that happened to get there first. |
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Let's demonstrate one way this could happen. |
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It starts with Alice committing a bug fix. Right now, we don't yet |
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know if this bug fix is good enough to push to the public repository, |
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but it's good enough for Alice to commit. :: |
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$ cd alice |
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$ echo 'fix' > file2 |
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$ hg commit -q -A -m 'fix bug 15' |
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Now Bob has a bad idea: he decides to pull whatever Alice is working |
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on and tweak her bug fix to his taste:: |
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|
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$ cd ../bob |
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$ hg pull -q -u ../alice |
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$ echo 'Fix.' > file2 |
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$ hg amend -q -A -m 'fix bug 15 (amended)' |
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|
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(Note the lack of communication between Alice and Bob. Failing to |
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communicate with your colleagues is a good way to get into trouble. |
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Nevertheless, ``evolve`` can usually sort things out, as we will see.) |
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[figure SG06: Bob's repo with one amendment] |
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After some testing, Alice realizes her bug fix is just fine as it is: |
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no need for further polishing and amending, this changeset is ready to |
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publish. :: |
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$ cd ../alice |
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$ hg push -q |
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This introduces a contradiction: in Bob's repository, changeset 2:e011 |
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(his copy of Alice's fix) is obsolete, since Bob amended it. But in |
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Alice's repository (and ``public``), that changeset is public: it is |
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immutable, carved in stone for all eternity. No changeset can be both |
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obsolete and public, so Bob is in for a surprise the next time he |
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pulls from ``public``:: |
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|
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$ cd ../bob |
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$ hg pull -q -u |
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1 new bumped changesets |
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Figure 7 shows what just happened to Bob's repository: changeset |
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2:e011 is now public, so it can't be obsolete. When that changeset was |
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obsolete, it made perfect sense for it to have a successor, namely |
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Bob's amendment of Alice's fix (changeset 4:fe88). But it's illogical |
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for a public changeset to have a successor, so 4:fe88 is in trouble: |
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it has been *bumped*. |
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|
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[figure SG07: 2:e011 now public not obsolete, 4:fe88 now bumped] |
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|
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As usual when there's trouble in your repository, the solution is to |
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evolve it:: |
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|
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$ hg evolve --all |
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|
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Figure 8 illustrate's Bob's repository after evolving away the bumped |
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changeset. Ignoring the obsolete changesets, Bob now has a nice, |
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clean, simple history. His amendment of Alice's bug fix lives on, as |
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changeset 5:227d—albeit with a software-generated commit message. (Bob |
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should probably amend that changeset to improve the commit message.) |
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But the important thing is that his repository no longer has any |
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troubled changesets, thanks to ``evolve``. |
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|
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[figure SG08: 5:227d is new, formerly bumped changeset 4:fe88 now hidden] |
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|
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Divergent changesets |
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-------------------- |
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|
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In addition to *unstable* and *bumped*, there is a third kind of |
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troubled changeset: *divergent*. When an obsolete changeset has two |
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successors, those successors are divergent. |
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|
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To illustrate, let's start Alice and Bob at the same |
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point—specifically, the point where Alice's repository currently |
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stands. Bob's repository is a bit of a mess, so we'll throw it away |
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and start him off with a copy of Alice's repository:: |
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|
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$ cd .. |
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$ rm -rf bob |
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$ cp -rp alice bob |
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|
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Now we'll have Bob commit a bug fix that could still be improved:: |
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|
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$ cd bob |
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$ echo 'pretty good fix' >> file1 |
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$ hg commit -u bob -m 'fix bug 24 (v1)' |
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|
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This time, Alice meddles with her colleague's work (still a bad |
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idea):: |
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383 |
|
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$ cd ../alice |
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$ hg pull -q -u ../bob |
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$ echo 'better (alice)' >> file1 |
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$ hg amend -u alice -m 'fix bug 24 (v2 by alice)' |
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388 |
|
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Here's where things change from the "bumped" scenario above: this |
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time, the original author (Bob) decides to amend his changeset too. :: |
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391 |
|
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$ cd ../bob |
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$ echo 'better (bob)' >> file1 |
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394 |
$ hg amend -u bob -m 'fix bug 24 (v2 by bob)' |
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395 |
|
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396 |
At this point, the divergence exists, but only in theory: Bob's |
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original changeset, 3:fe81, is obsolete and has two successors. But |
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398 |
those successors are in different repositories, so the trouble is not |
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399 |
visible to anyone yet. It will be as soon as one of our players pulls |
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400 |
from the other's repository. Let's make Bob the victim again:: |
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401 |
|
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$ hg pull -q -u ../alice |
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not updating: not a linear update |
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(merge or update --check to force update) |
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2 new divergent changesets |
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|
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The “not a linear update” is our first hint that something is wrong, |
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but of course “2 new divergent changesets” is the real problem. Figure |
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409 |
9 shows both problems. |
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410 |
|
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[figure SG09: bob's repo with 2 heads for the 2 divergent changesets, 5:fc16 and 6:694f; wc is at 5:fc16, hence update refused; both are successors of obsolete 3:fe81, hence divergence] |
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|
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Now we need to get out of trouble. Unfortunately, a `bug`_ in |
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``evolve`` means that the usual answer (run ``hg evolve --all``) does |
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not work. Bob has to figure out the solution on his own: in this case, |
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merge. To avoid distractions, we'll set ``HGMERGE`` to make Mercurial |
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resolve any conflicts in favour of Bob. :: |
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418 |
|
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$ HGMERGE=internal:local hg merge |
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$ hg commit -m merge |
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421 |
|
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.. _`bug`: https://bitbucket.org/marmoute/mutable-history/issue/48/ |
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423 |
|
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This is approximately what ``hg evolve`` would do in this |
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circumstance, if not for that bug. One annoying difference is that |
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Mercurial thinks the two divergent changesets are still divergent, |
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which you can see with a simple revset query:: |
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428 |
|
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$ hg log -q -r 'divergent()' |
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430 |
5:fc16901f4d7a |
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431 |
6:694fd0f6b503 |
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432 |
|
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(That annoyance should go away when the bug is fixed.) |
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434 |
|
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435 |
Conclusion |
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436 |
---------- |
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437 |
|
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438 |
Mutable history is a powerful tool. Like a sharp knife, an experienced |
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439 |
user can do wonderful things with it, much more wonderful than with a |
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440 |
dull knife (never mind a rusty spoon). At the same time, an |
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441 |
inattentive or careless user can do harm to himself or others. |
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442 |
Mercurial with ``evolve`` goes to great lengths to limit the harm you |
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443 |
can do by trying to handle all possible types of “troubled” |
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444 |
changesets. But having a first-aid kit nearby does not excuse you from |
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445 |
being careful with sharp knives. |
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446 |
|
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447 |
Mutable history shared across multiple repositories by a single |
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448 |
developer is a natural extension of this model. Once you are used to |
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449 |
using a single sharp knife on its own, it's pretty straightforward to |
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450 |
chop onions and mushrooms using the same knife, or to alternate |
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451 |
between two chopping boards with different knives. |
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452 |
|
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453 |
Mutable history shared by multiple developers is a scary place to go. |
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454 |
Imagine a professional kitchen full of expert chefs tossing their |
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455 |
favourite knives back and forth, with the occasional axe or chainsaw |
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456 |
thrown in to spice things up. If you're confident that you *and your |
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457 |
colleagues* can do it without losing a limb, go for it. But be sure to |
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458 |
practice a lot first before you rely on it! |