wiki:DarcsEvaluation

Version 29 (modified by batterseapower, 6 years ago) (diff)

Tweak Mercurial output

Darcs retrospective, and the future

GHC has been using darcs for version control since the beginning of 2006. It has not been all plain sailing, so in this page we will record our experiences with darcs, and attempt to objectively evaluate whether we would be better off with a different version control system. In the event that we do switch, we need to track exactly what needs to change, so this page will also list those dependencies.

Problems we currently experience with darcs

  • Conflicts and merging. This is the biggest problem we encounter, and is also the #1 priority for Darcs development. Any non-trivial branch is affected, and essentially the workaround is to discard the history from the branch when merging, and use ordinary diff/patch tools. Keeping history is possible, but impractical for branches with more than a few patches.
  • Speed. many operations are impractical (annotate, darcs changes <file>), and many operations just take "too long" (i.e. long enough that you go and do something else rather than wait for it to finish, which incurs a context-switch cost). We can't use Trac's darcs integration or darcsweb, for example, because both rely on invoking darcs changes <file> (for that matter, that's not completely true for the trac darcs plugin as it does not execute that command on a per-file basis, but rather it loads and caches into its own database the result of darcs changes -v on the "not-yet-loaded" changesets, visiting every patch in the repository just once. It caches also the actual content of each file touched by any browsed changeset, to compute the unidiff.).
  • bugs: we run into darcs bugs other than the conflict/merging bug on a regular basis.
  • user interface issues: e.g. in a conflict there's no way to tell which two patches are conflicting with each other(!)
  • Windows support: is quite flaky still. (well, it's certainly better than it used to be, and at least some Windows users don't consider it to be bad).

Current status

On the 23rd July 2008 an IRC meeting on the #ghc channel decided to make a serious effort to replace Darcs, due to all the problems described above. The logs of that meeting are available in full at http://hackage.haskell.org/trac/ghc/attachment/wiki/IRC_Meetings/ghc-metting-2008-07-23.log, but the main conclusions were:

  • The GHC developers have sufficient problems with Darcs that a change would be beneficial

  • We want to stick with distributed version control, and have a widely-used and well-supported system, so Mercurial and Git are the only real contenders
  • Mercurial and Git and percived as being mostly feature-and-performance comparable, although git is more popular
  • More investigation of the Mercurial option for GHC is needed, especially in light of reported poor support for Windows with Git. This work is ongoing

Important workflows

Cherry-picking patches

This is how we maintain the stable GHC branch. Particular fixes are pulled from the HEAD. When the desired patches don't depend on undesired patches, darcs takes care of this automatically, as demonstrated below. Otherwise, with darcs, the patch has to be merged by hand.

# Make a repo with a single file with lines 1,3,5,7 in

mkdir repo1
cd repo1
darcs init
printf 'Line1\nLine3\nLine5\nLine7\n' > file
darcs record --all --look-for-adds -m patch1 -A igloo@earth.li
cd ..

# Check out 2 copies of the repo

darcs get repo1 repo2
darcs get repo1 repo3

# Add a patch that adds lines 2 and 6, then another that adds line 4

cd repo1
printf 'Line1\nLine2\nLine3\nLine5\nLine6\nLine7\n' > file
darcs record --all -m patch2
printf 'Line1\nLine2\nLine3\nLine4\nLine5\nLine6\nLine7\n' > file
darcs record --all -m patch3

# Pull the line 4 patch, but not the lines 2 and 6 patch, into the
# other repos non-interactively and interactively

cd ../repo2
darcs pull --all --patches patch3
cd ../repo3
darcs pull 
n
y

# repo2's and repo3's file now contains lines 1,3,4,5,7

Git:

mkdir testrepo
cd testrepo
git init
printf 'Line1\nLine3\nLine5\nLine7\n' > file
git add *
git commit -a -m patch1

# create the branch
git checkout -b branch1  # also switches to the branch
git checkout master      # switch back to master

printf 'Line1\nLine2\nLine3\nLine5\nLine6\nLine7\n' > file
git commit -a -m patch2
printf 'Line1\nLine2\nLine3\nLine4\nLine5\nLine6\nLine7\n' > file
git commit -a -m patch3

git log --grep=patch3   # prints the commit id for that change
git checkout branch1
git cherry-pick <commit-id from above>

# we can also cherry-pick using the GUI
git checkout master
git checkout -b branch2 master^^  # start a new branch from tree after patch1 
# we're in branch2 now
gitk master  # start the gui for master branch (which containts patch2 & patch3)
# in the gui you can rightclick on the desired patch and choose 
# "cherry-pick" this commit.  et voila, it's in branch2

Mercurial:

# Make a repo with a single file with lines 1,3,5,7 in

mkdir repo1
cd repo1
hg init
printf 'Line1\nLine3\nLine5\nLine7\n' > file
hg commit --addremove -m patch1 -u igloo@earth.li
cd ..

# Check out 2 copies of the repo

hg clone repo1 repo2
hg clone repo1 repo3

# Add a patch that adds lines 2 and 6, then another that adds line 4

cd repo1
printf 'Line1\nLine2\nLine3\nLine5\nLine6\nLine7\n' > file
hg commit -m patch2
printf 'Line1\nLine2\nLine3\nLine4\nLine5\nLine6\nLine7\n' > file
hg commit -m patch3

# Pull the line 4 patch, but not the lines 2 and 6 patch, into the
# other repos non-interactively and interactively

cd ../repo2
(cd ../repo1; hg log | grep patch3 -4) # note the changeset number of patch3, should be 2
hg transplant --source ../repo1 2
cd ../repo3
hg transplant --source ../repo1
# It doesn't appear to be possible to omit the source argument for transplant,
# though you can omit it from the non-cherrypicking hg pull
n <press enter>
y <press enter>

# repo2's and repo3's file now contains lines 1,3,4,5,7

Cherry-picking during record

In this example, we want to record just the fixes we have made, and not the debugging prints. We want something similar for:

  • reverting just the debugging prints, and not the fixes
  • reverting the parts of a change you are working on that turned out not to be right
  • in the middle of doing a large change, discovering a little bug and wanting to fix and record it
# Make a repo with a single file with lines 1,3,5,7,9 in

mkdir repo1
cd repo1
darcs init
printf 'Line1\nLine3\nLine5\nLine7\nLine9\n' > file
darcs record --all --look-for-adds -m patch1 -A igloo@earth.li

# Now we fix a bug, and in the process add some debugging prints.

printf 'Line1\nFix2\nLine3\nDebug4\nLine5\nFix6\nLine7\nDebug8\nLine9\n' > file

# We want to record our fix, but not the debugging prints.

darcs rec -m the_fix
y
n
y
n

# Get rid of the debug prints

darcs revert -a

# Now file contains lines 1,2,3,5,6,7,9

Git:

mkdir repo1
cd repo1
git init
printf 'Line1\nLine3\nLine5\nLine7\nLine9\n' > file
git add *
git commit -m patch1

printf 'Line1\nFix2\nLine3\nDebug4\nLine5\nFix6\nLine7\nDebug8\nLine9\n' > file

git add --patch
s  # split the diffs
y
n
y
n
git commit -m the_fix

git reset --hard   # delete all changes in working dir

Mercurial:

# Make a repo with a single file with lines 1,3,5,7,9 in

mkdir repo1
cd repo1
hg init
printf 'Line1\nLine3\nLine5\nLine7\nLine9\n' > file
hg commit --addremove -m patch1 -u igloo@earth.li

# Now we fix a bug, and in the process add some debugging prints.

printf 'Line1\nFix2\nLine3\nDebug4\nLine5\nFix6\nLine7\nDebug8\nLine9\n' > file

# We want to record our fix, but not the debugging prints.

hg record -m the_fix
Y <press enter>
y <press enter>
n <press enter>
y <press enter>
n <press enter>

# Get rid of the debug prints

hg revert -a

# Now file contains lines 1,2,3,5,6,7,9

amend-record

So, you make your lovely patch, it all looks good, so you record it. Then you do a build to make sure it works, and during the build or testsuite run you find that the patch wasn't quite right after all. You could just add a little 2-line patch, but that isn't very pleasant: It's nice if, as far as possible, all intermediate compiler states are buildable. Also, people might pull the first patch but not the second when cherry-picking, leading to head-scratching down the line. It's much nicer to be able to just amend-record the fix into your original patch.

The same is available for Git. The command is called git commit --amend. You usually checkout the commit you want to edit into a branch, do the changes, then rebase the remaining patches on top of this. Example coming soon...

I can't find a way to do this directly with Mercurial. You can of course do hg rollback and then add a new commit. The Mercurial Queues extension is also able to do this (hg qrefresh) but it is rather complicated to use.

Darcs vs Mercurial Overview

Commands somehow different in behaviour between Hg and Darcs:

darcs whatsnew -> hg diff / hg status
darcs record -> hg commit / hg record (record extension needed to allow cherrypicking)
darcs pull -> hg pull -u / hg pull && hg update (hg pull does not modify the working copy by default)
(automatic merging) -> hg merge && hg commit / hg fetch (fetch extension does pull/update/merge in one step, like Darcs)
darcs unrecord -> hg rollback (works for just the most recent record/push, confusingly different from Darcs equivalent command)

Commands that differ essentialy only in name:

darcs rollback -> hg backout --merge (records an inverse changeset, go back as far as you like)
darcs changes -> hg log
darcs move <FILE> -> hg rename <FILE>
darcs send -o <FILE> -> hg bundle <FILE>
darcs apply <FILE> -> hg unbundle <FILE>

Commands the same between Hg and Darcs:

darcs push -> hg push
darcs add/remove <FILE> -> hg add/remove <FILE>
darcs revert -> hg revert
darcs tag -> hg tag
darcs annotate -> hg annotate

hg addremove adds untracked files and marks missing files as removed. hg commit -A does a similar thing at commit time

Misc. differences from Darcs:

  • Don't have summary/message split: the first line of the message is the summary
  • Files are not automatically considered removed if you delete them. You need to run hg remove --after <FILE> to remove them from the repo as well

To be able to use all the commands in the example above, you should create a .hgrc file in your home directory, looking something like this:

[extensions]
hgext.record=
transplant=

[ui]
username = My Name <foo@bar.com>

Other issues:

  • The transplant command wiki page http://www.selenic.com/mercurial/wiki/index.cgi/TransplantExtension contains the text "Three-way merge doesn't cope particularly well with transplanted patches - it will tend to generate false conflicts", which doesn't fill me with confidence. However, we only want to use transplant to maintain a branch (e.g. 6.8) which we won't merge back into the one we are pulling from (e.g. HEAD), so this may be a non-issue

Setting up a Mercurial HTTP interface: http://hgbook.red-bean.com/hgbookch6.html#x10-1310006.6

Darcs / Git Command Comparison

darcs whatsnew -sgit status
darcs whatsnew git diff
darcs record git add --patch (goes through all changes)/git add -i (starts with a file-based view) Git add only marks changes for commit. This can be nicer if you want to check some things first before you commit them.
git commit (do the actual commit)
darcs record -a -m foogit commit -a -m foo
darcs pullgit pull then git cherry-pick/gitk + select patches using mouse. It's probably best to have one local branch correspond to the remote branch and then cherry-pick from that. You can also create local names for several remote repositories.

Darcs alternatives still in the running

Mercurial

Advantages:

  • Speed comparable to Git
  • Some operations become feasible (bisect, annotate)
  • Many helper tools
  • Good Windows support
  • HTTP and SSH sync possible, but unknown how this compares to Git native protocol sync speed

Disadvantages:

  • Similar problems with bisect support as Git
  • (Unknown: suitability of command set?)

Notes On Conversion

Currently using Tailor. Problems encountered:

  • Darcs outputs XML without an encoding header. Patched Tailor to append Latin-1 encoding to the XML output. This will be sent to the tailor author
  • In hg.py, replace the line self._hgCommand('tag', tag) with self._hgCommand('tag', tag, force=True) because we seem to be trying to apply duplicate tags at some points. Don't know quite how this is possible!
  • MUST USE the seperate-subdir mode of Tailor because we have some tricky Darcs patches. I've added my scripts to http://www.selenic.com/mercurial/wiki/index.cgi/Tailor#preview
  • No support for author remapping in Tailor yet. I've added it and I'm going to submit the patch to the Tailor author

Git

Advantages:

  • Speed
  • Very similar workflow possible: git add --patch, git cherry-pick, and others
  • Some operations become feasible (bisect, annotate)
  • Many helper tools

Disadvantages:

  • Complex command set? (Though, it should be possible to find replacements for the darcs commands and be happy.)
  • Lack of good Windows support?
  • bisect support would require git modules to also pick the correct version of libraries. Keeping this in sync is not easy, atm.
  • uses its own protocol for network transmission (http works but is slower, however, other hosting services are available, e.g., github)

Eliminated alternatives

Bzr

Advantages:

  • Fairly fast
  • Portable (as portable as python, anyhow)
  • Merging works correctly based on closest-common-ancestor
  • Tracking of renamed files / directories merges correctly
  • Revisions form a DAG (more like a tree with merge-points) rather than patchsets
  • Supports convenient "centralised-style" commit-remote-by-default as well as "distributed-style" commit-local-by-default. Just 'bind' or 'unbind' your branch whenever you want.
  • Simple clear UI

Disadvantages

  • Revisions form a DAG (more like a tree with merge-points) rather than patchsets (this is a subjective point, which is why it's in both lists. Which model do you believe in?)
  • Cherry-picking isn't very "native" to the data model.
  • UI is rather different from darcs (which current contributors are used to).

Reason for elimination: lack of uptake and hence more risk of Bzr becoming unmaintained.

Darcs

Advantages to staying with darcs:

  • Community consistency: essentially the Haskell community has standardised on darcs, so it would be an extra barrier for contributors if they had to learn another VC system.
  • Merging, when it works, is done right in darcs.

Disadvantages to staying with darcs:

  • Uncertain future: no critical mass of hackers/maintainers. The technical basis is not well enough understood by enough people.

Reason for elimination: persistent performance and algorithmic problems, see above.

Dependencies on darcs

The following is intended to be a complete list of the things that would need to change if we were to switch away from darcs, in addition to the conversion of the repository itself, which I am assuming can be automatically converted using available tools.

The following code/scripts would need to be adapted or replaced:

  • The darcs-all script
  • The push-all script
  • The aclocal.m4 code that attempts to determine the source tree date
  • .darcs-boring
  • The buildbot scripts
  • checkin email script: /home/darcs/bin/commit-messages-split.sh
  • Trac integration (the GHC Trac does not currently integrate with darcs, however)
  • darcsweb (use whatever alternative is available)

The following documentation would need to change:

External references

Posts/blogs:

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