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Subsections

Best practices

This chapter is intended to review various scenarios and describe in each case effective ways of using darcs. There is no one ``best practice'', and darcs is a sufficiently low-level tool that there are many high-level ways one can use it, which can be confusing to new users.

Creating patches

This section will lay down the concepts around patch creation. The aim is to develop a way of thinking that corresponds well to how darcs is behaving -- even in complicated situations.

In a single darcs repository you can think of two ``versions'' of the source tree. They are called the working and pristine trees. Working is your normal source tree, with or without darcs alongside. The only thing that makes it part of a darcs repository is the _darcs directory in its root. Pristine is the recorded state of the source tree. The pristine tree is constructed from groups of changes, called patches (some other version control systems use the term changeset instead of patch).4.1 Darcs will create and store these patches based on the changes you make in working.

Changes

If working and pristine are the same, there are ``no changes'' in the working copy. Changes can be introduced (or removed) by editing the files in working. They can also be caused by darcs commands, which can modify both working and pristine. It is important to understand for each darcs command how it modifies working, pristine or both of them.

whatsnew (as well as diff) can show the difference between working and pristine to you. It will be shown as a difference in working. In advanced cases it need not be working that has changed; it can just as well have been pristine, or both. The important thing is the difference and what darcs can do with it.

Keeping or discarding changes

If you have a difference in working, you do two things with it: record it to keep it, or revert it to lose the changes.4.2

If you have a difference between working and pristine--for example after editing some files in working--whatsnew will show some ``unrecorded changes''. To save these changes, use record. It will create a new patch in pristine with the same changes, so working and pristine are no longer different. To instead undo the changes in working, use revert. It will modify the files in working to be the same as in pristine (where the changes do not exist).

Unrecording changes

unrecord is a command meant to be run only in private repositories. Its intended purpose is to allow developers the flexibility to undo patches that haven't been distributed yet.

If you unrecord a patch, that patch will be deleted from pristine. This will cause working to be different from pristine, and whatsnew to report unrecorded changes. The difference will be the same as just before that patch was recorded. Think about it. record examines what's different with working and constructs a patch with the same changes in pristine so they are no longer different. unrecord deletes this patch; the changes in pristine disappear and the difference is back.

If the recorded changes included an error, the resulting flawed patch can be unrecorded. When the changes have been fixed, they can be recorded again as a new--hopefully flawless--patch.

If the whole change was wrong it can be discarded from working too, with revert. revert will update working to the state of pristine, in which the changes do no longer exist after the patch was deleted.

Keep in mind that the patches are your history, so deleting them with unrecord makes it impossible to track what changes you really made. Redoing the patches is how you ``cover the tracks''. On the other hand, it can be a very convenient way to manage and organize changes while you try them out in your private repository. When all is ready for shipping, the changes can be reorganized in what seems as useful and impressive patches. Use it with care.

All patches are global, so don't ever replace an already ``shipped'' patch in this way! If an erroneous patch is deleted and replaced with a better one, you have to replace it in all repositories that have a copy of it. This may not be feasible, unless it's all private repositories. If other developers have already made patches or tags in their repositories that depend on the old patch, things will get complicated.

Special patches and pending

The patches described in the previous sections have mostly been hunks. A hunk is one of darcs' primitive patch types, and it is used to remove old lines and/or insert new lines. There are other types of primitive patches, such as adddir and addfile which add new directories and files, and replace which does a search-and-replace on tokens in files.

Hunks are always calculated in place with a diff algorithm just before whatsnew or record. But other types of primitive patches need to be explicitly created with a darcs command. They are kept in pending, ie, in the file _darcs/patches/pending, until they are either recorded or reverted.

Pending can be thought of as a special extension of working. When you issue, e.g., a darcs replace command, the replace is performed on the files in working and at the same time a replace patch is put in pending. Patches in pending describe special changes made in working. The diff algorithm will fictively apply these changes to pristine before it compares it to working, so all lines in working that are changed by a replace command will also be changed in pending and pristine when the hunks are calculated. That's why no hunks with the replaced lines will be shown by whatsnew; it only shows the replace patch in pending responsible for the change.

If a special patch is recorded, it will simply be moved to pristine. If it is instead reverted, it will be deleted from pending and the accompanying change will be removed from working.

Note that reverting a patch in pending is not the same as simply removing it from pending. It actually applies the inverse of the change to working. Most notable is that reverting an addfile patch will delete the file in working (the inverse of adding it). So if you add the wrong file to darcs by mistake, don't revert the addfile. Instead use remove, which cancels out the addfile in pending.

Using patches

This section will lay down the concepts around patch distribution and branches. The aim is to develop a way of thinking that corresponds well to how darcs is behaving -- even in complicated situations.

A repository is a collection of patches. Patches have no defined order, but patches can have dependencies on other patches. Patches can be added to a repository in any order as long as all patches depended upon are there. Patches can be removed from a repository in any order, as long as no remaining patches depend on them.

Repositories can be cloned to create branches. Patches created in different branches may conflict. A conflict is a valid state of a repository. A conflict makes the working tree ambiguous until the conflict is resolved.

Dependencies

There are two kinds of dependencies.

Implicit dependencies is the far most common kind. These are calculated automatically by darcs. If a patch removes a file or a line of code, it will have to depend on the patch that added that file or line of code. If a patch adds a line of code, it will usually have to depend on the patch or patches that added the adjacent lines.

Explicit dependencies can be created if you give the --ask-deps option to darcs record. This is good for assuring that logical dependencies hold between patches. It can also be used to group patches--a patch with explicit dependencies doesn't need to change anything--and pulling the patch also pulls all patches it was made to depend on.

Branches: just normal repositories

Darcs does not have branches--it doesn't need to. Any two repositories are ``branches'' in darcs, but it is not of much use unless they have a large portion of patches in common. If they are different projects they will have nothing in common, but darcs may still very well be able to merge them, although the result probably is nonsense. Therefore the word ``branch'' isn't a technical term in darcs; it's just the way we think of one repository in relation to another.

Branches are very useful in darcs. They are in fact necessary if you want to do more than only simple work. When you get someone's repository from the Internet, you are actually creating a branch of it. But darcs is designed this way, and it has means to make it efficient. The answer to many questions about how to do a thing with darcs is: ``use a branch''. It is a simple and elegant solution with great power and flexibility, which contributes to darcs' uncomplicated user interface.

You create new branches (i.e., clone repositories) with the get and put commands.

Moving patches around--no versions

Patches are global, and a copy of a patch either is or is not present in a branch. This way you can rig a branch almost any way you like, as long as dependencies are fulfilled--darcs won't let you break dependencies. If you suspect a certain feature from some time ago introduced a bug, you can remove the patch/patches that adds the feature, and try without it.4.3

Patches are added to a repository with pull and removed from the repositories with obliterate. Don't confuse these two commands with record and unrecord, which constructs and deconstructs patches.

It is important not to lose patches when (re)moving them around. pull needs a source repository to copy the patch from, whereas obliterate just erases the patch. Beware that if you obliterate all copies of a patch it is completely lost--forever. Therefore you should work with branches when you obliterate patches. The obliterate command can wisely be disabled in a dedicated main repository by adding obliterate disable to the repository's defaults file.

For convenience, there is a push command. It works like pull but in the other direction. It also differs from pull in an important way: it starts a second instance of darcs to apply the patch in the target repository, even if it's on the same computer. It can cause surprises if you have a ``wrong'' darcs in your PATH.

Tags--versions

While pull and obliterate can be used to construct different ``versions'' in a repository, it is often desirable to name specific configurations of patches so they can be identified and retrieved easily later. This is how darcs implements what is usually known as versions. The command for this is tag, and it records a tag in the current repository.

A tag is just a patch, but it only contains explicit dependencies. It will depend on all the patches in the current repository.4.4Darcs can recognize if a patch is as a tag; tags are sometimes treated specially by darcs commands.

While traditional revision control systems tag versions in the time line history, darcs lets you tag any configuration of patches at any time, and pass the tags around between branches.

With the option --tag to get you can easily get a named version in the repository as a new branch.

Conflicts

A conflict happens when two conflicting patches meet in the same repository. This is no problem for darcs; it can happily pull together just any patches. But it is a problem for the files in working (and pristine). The conflict can be thought of as two patches telling darcs different things about what a file should look like.

Darcs escapes this problem by ignoring those parts of the patches that conflict. They are ignored in both patches. If patch A changes the line ``FIXME'' to ``FIXED'', and patch B changes the same line to ``DONE'', the two patches together will produce the line ``FIXME''. Darcs doesn't care which one you pulled into the repository first, you still get the same result when the conflicting patches meet. All other changes made by A and B are performed as normal.

Darcs can mark a conflict for you in working. This is done with mark-conflicts. Conflicts are marked such that both conflicting changes are inserted with special delimiter lines around them. Then you can merge the two changes by hand, and remove the delimiters.

When you pull patches, darcs automatically performs a mark-conflicts for you if a conflict happens. You can remove the markup with revert, Remember that the result will be the lines from the previous version common to both conflicting patches. The conflict marking can be redone again with mark-conflicts.

A special case is when a pulled patch conflicts with unrecorded changes in the repository. The conflict will be automatically marked as usual, but since the markup is also an unrecorded change, it will get mixed in with your unrecorded changes. There is no guarantee you can revert only the markup after this, and mark-conflicts will not be able to redo this markup later if you remove it. It is good practice to record important changes before pulling.

mark-conflicts can't mark complicated conflicts. In that case you'll have to use darcs diff and other commands to understand what the conflict is all about. If for example two conflicting patches create the same file, mark-conflicts will pick just one of them, and no delimiters are inserted. So watch out if darcs tells you about a conflict.

mark-conflicts can also be used to check for unresolved conflicts. If there are none, darcs replies ``No conflicts to mark.''. While pull reports when a conflict happens, obliterate and get don't.

Resolving conflicts

A conflict is resolved (not marked, as with the command mark-conflicts) as soon as some new patch depends on the conflicting patches. This will usually be the resolve patch you record after manually putting together the pieces from the conflict markup produced by mark-conflicts (or pull). But it can just as well be a tag. So don't forget to fix conflicts before you accidentally ``resolve'' them by recording other patches.

If the conflict is with one of your not-yet-published patches, you may choose to amend that patch rather than creating a resolve patch.

If you want to back out and wait with the conflict, you can obliterate the conflicting patch you just pulled. Before you can do that you have to revert the conflict markups that pull inserted when the conflict happened.

Global and per-repository caches

Darcs uses a global cache, as this is one of its biggest performance enhancing tools. The global cache acts as a giant patch pool where darcs first looks for a patch when grabbing new patches. This saves time by not downloading the same patch twice from a remote server. It also saves space by storing the patch only once, if you ensure your cache and your repositories are on the same hardlink-supporting filesystem.

Darcs now enables a global patch cache under your home directory by default. Older darcs 2.x versions required this manual step:

$ mkdir -p $HOME/.darcs/cache
$ echo cache:$HOME/.darcs/cache > $HOME/.darcs/sources

On MS Windows [*], using cmd.exe (Command Prompt under Accessories):

> md "%APPDATA%\darcs\cache" (notice double quotes!)
> echo cache:%APPDATA%\darcs\cache > "%APPDATA%\darcs\sources"

Individual repositories also contain cache location information. Each time a repository is got, its location is added as an entry in _darcs/prefs/sources. If one of these repositories were to become totally or temporarily unreachable, it can cause darcs to hang for a long time trying to reach it. Fortunately darcs has a mechanism which helps us to deal with that problem: if an unreachable entry is discovered, darcs stops using it for the rest of the session and then notifies the user to take further action. It will display a message like the following:

> I could not reach the following repository:
> http://darcs.net/
> If you're not using it, you should probably delete
> the corresponding entry from _darcs/prefs/sources.

There are some other advanced things you can do in _darcs/prefs/sources, such as create read-only caches and even set a primary source repository above any used in a darcs get or darcs pull command.


Distributed development with one primary developer

This is how a project with many contributors, but every contribution is reviewed and manually applied by the project leader, can be run. For this sort of a situation, darcs send is ideal, since the barrier for contributions is very low, which helps encourage contributors.

One could simply set the _darcs/prefs/email value to the project mailing list, but you may also want to use darcs send to send your changes to the main server, so instead the email address could be set to something like ``Darcs Repo <maintainer@host.com>''. Then the .procmailrc file on the server should have the following rule:

:0
* ^TODarcs Repo
|(umask 022; darcs apply --reply project-mailing-list@host.com \
             --repodir /path/to/repo --verify /path/to/allowed_keys)
This causes darcs apply to be run on any email sent to ``Darcs Repo''. apply actually applies them only if they are signed by an authorized key.

The central darcs repository contains the following values in its _darcs/prefs/defaults:

apply test
apply verbose
apply happy-forwarding
The first line tells apply to always run the test suite. The test suite can be a reason to use send rather than push, since it allows to easily continue working (or put one's computer to sleep) while the tests are being run on the main server. The second line is just there to improve the email response that the maintainer gets when a patch has either been applied or failed the tests. The third line makes darcs not complain about unsigned patches, but just to forward them to darcs-devel.

On one's development computer, one can have in their .muttrc the following alias, which allows to easily apply patches received via email directly to one's darcs working directory:

macro pager A "<pipe-entry>(umask 022; darcs apply --no-test -v \
        --repodir ~/darcs)"

Sending signed patches by email

Darcs send can be configured to send a cryptographically signed patch by email. You can then set up your mail system to have darcs verify that patches were signed by an authorized user and apply them when a patch is received by email. The results of the apply can be returned to the user by email. Unsigned patches (or patches signed by unauthorized users) will be forwarded to the repository owner (or whoever you configure them to be forwarded to...).

This method is especially nice when combined with the --test option of darcs apply, since it allows you to run the test suite (assuming you have one) and reject patches that fail--and it's all done on the server, so you can happily go on working on your development machine without slowdown while the server runs the tests.

Setting up darcs to run automatically in response to email is by far the most complicated way to get patches from one repository to another... so it'll take a few sections to explain how to go about it.

Security considerations

When you set up darcs to run apply on signed patches, you should assume that a user with write access can write to any file or directory that is writable by the user under which the apply process runs. Unless you specify the --no-test flag to darcs apply (and this is not the default), you are also allowing anyone with write access to that repository to run arbitrary code on your machine (since they can run a test suite--which they can modify however they like). This is quite a potential security hole.

For these reasons, if you don't implicitly trust your users, it is recommended that you create a user for each repository to limit the damage an attacker can do with access to your repository. When considering who to trust, keep in mind that a security breach on any developer's machine could give an attacker access to their private key and passphrase, and thus to your repository.

Installing necessary programs

You also must install the following programs: gnupg, a mailer configured to receive mail (e.g. exim, sendmail or postfix), and a web server (usually apache).

Granting access to a repository

You create your gpg key by running (as your normal user):

$ gpg --gen-key
You will be prompted for your name and email address, among other options. Of course, you can skip this step if you already have a gpg key you wish to use.

You now need to export the public key so we can tell the patcher about it. You can do this with the following command (again as your normal user):

$ gpg --export "email@address" > /tmp/exported_key
And now we can add your key to the allowed_keys:
(as root)> gpg --keyring /var/lib/darcs/repos/myproject/allowed_keys \
               --no-default-keyring --import /tmp/exported_key
You can repeat this process any number of times to authorize multiple users to send patches to the repository.

You should now be able to send a patch to the repository by running as your normal user, in a working copy of the repository:

$ darcs send --sign http://your.computer/repos/myproject
You may want to add ``send sign'' to the file _darcs/prefs/defaults so that you won't need to type --sign every time you want to send...

If your gpg key is protected by a passphrase, then executing send with the --sign option might give you the following error:

darcs failed:  Error running external program 'gpg'
The most likely cause of this error is that you have a misconfigured gpg that tries to automatically use a non-existent gpg-agent program. GnuPG will still work without gpg-agent when you try to sign or encrypt your data with a passphrase protected key. However, it will exit with an error code 2 (ENOENT) causing darcs to fail. To fix this, you will need to edit your ~/.gnupg/gpg.conf file and comment out or remove the line that says:
use-agent
If after commenting out or removing the use-agent line in your gpg configuration file you still get the same error, then you probably have a modified GnuPG with use-agent as a hard-coded option. In that case, you should change use-agent to no-use-agent to disable it explicitly.

Setting up a sendable repository using procmail

If you don't have root access on your machine, or perhaps simply don't want to bother creating a separate user, you can set up a darcs repository using procmail to filter your mail. Let us assume that you already use procmail to filter your email. If not, you will need to read up on it, or perhaps should use a different method for routing the email to darcs.

To begin with, you must configure your repository so that a darcs send to your repository will know where to send the email. Do this by creating a file in /path/to/your/repo/_darcs/prefs called email containing your email address. As a trick (to be explained below), we will create the email address with ``darcs repo'' as your name, in an email address of the form ``User Name 1#1user@host.com2#2.''

$ echo 'my darcs repo <user@host.com>' \
      > /path/to/your/repo/_darcs/prefs/email

The next step is to set up a gnupg keyring containing the public keys of people authorized to send to your repository. Here is a second way of going about this (see above for the first). This time let us assume you want to give someone write access to your repository. You can do this by:

gpg --no-default-keyring \
    --keyring /path/to/the/allowed_keys --recv-keys FFFFFFFF
With ``FFFFFFFF'' being the ID of the gpg key of the person to be authorised, if this person has uploaded their key to the gpg keyservers. Actually, this also requires that you have configured gpg to access a valid keyserver. You can, of course, repeat this command for all keys you want to allow access to.

Finally, we add a few lines to your .procmailrc:

:0
* ^TOmy darcs repo
|(umask 022; darcs apply --reply user@host.com \
    --repodir /path/to/your/repo --verify /path/to/the/allowed_keys)
The purpose for the ``my darcs repo'' trick is partially to make it easier to recognize patches sent to the repository, but is even more crucial to avoid nasty bounce loops by making the --reply option have an email address that won't go back to the repository. This means that unsigned patches that are sent to your repository will be forwarded to your ordinary email.

Like most mail-processing programs, Procmail by default sets a tight umask. However, this will prevent the repository from remaining world-readable; thus, the ``umask 022'' is required to relax the umask. (Alternatively, you could set Procmail's global UMASK variable to a more suitable value.)

Checking if your e-mail patch was applied

After sending a patch with darcs send, you may not receive any feedback, even if the patch is applied. You can confirm whether or not your patch was applied to the remote repository by pointing darcs changes at a remote repository:

darcs changes --last=10 --repo=http://darcs.net/

That shows you the last 10 changes in the remote repository. You can adjust the options given to changes if a more advanced query is needed.


next up previous contents
Next: Darcs commands Up: Darcs User Manual Previous: Configuring darcs   Contents
Daniil 2014-02-06