ikiwiki/ rcs/ details

A few bits about the RCS backends

## Terminology

``web-edit'' means that a page is edited by using the web (CGI) interface as opposed to using a editor and the RCS interface.

## svn

Subversion was the first RCS to be supported by ikiwiki.

### How does it work internally?

Master repository M.

RCS commits from the outside are installed into M.

There is a working copy of M (a checkout of M): W.

HTML is generated from W. rcs_update() will update from M to W.

CGI operates on W. rcs_commit() will commit from W to M.

For all the gory details of how ikiwiki handles this behind the scenes, see commit-internals.

You browse and web-edit the wiki on W.

W "belongs" to ikiwiki and should not be edited directly.

## darcs

Regarding the repository layout: There are two darcs repositories. One is the `srcdir`, the other we'll call `master`.

- HTML is generated from `srcdir`.
- CGI edits happen in `srcdir`.
- The backend pulls updates from `master` into `srcdir`, i.e. darcs commits should happen to `master`.

- `master` calls ikiwiki (through a wrapper) in its apply posthook, i.e. `master/_darcs/prefs/defaults` should look like this:

  apply posthook ikiwrap apply run-posthook

- The backend pushes CGI edits from `srcdir` back into `master` (triggering the apply hook).

- The working copies in `srcdir` and `master` should not be touched by the user, only by the CGI or darcs, respectively.

## Git

Regarding the Git support, Recai says:

I have been testing it for the past few days and it seems satisfactory. I haven't observed any race condition regarding the concurrent blog commits and it handles merge conflicts gracefully as far as I can see.

(After about a year, git support is nearly as solid as subversion support --Joey)

As you may notice from the patch size, GIT support is not so trivial to implement (for me, at least). It has some drawbacks (especially wrt merge which was the hard part). GIT doesn't have a similar functionality like 'svn merge -rOLD:NEW FILE' (please see the relevant comment in `_merge_past` for more details), so I had to invent an ugly hack just for the purpose.

> I was looking at this, and WRT the problem of uncommitted local changes, it seems to me you could just git-stash them now that git-stash exists. I think it didn't when you first added the git support.. --Joey

> > Yes, git-stash had not existed before. What about sth like below? It seems to work (I haven't given much thought on the specific implementation details). --roktas

> >      # create test files
> >      cd /tmp
> >      seq 6 >page
> >      cat page
> >      1
> >      2
> >      3
> >      4
> >      5
> >      6
> >      sed -e 's/2/2ME/' page >page.me # my changes
> >      cat page
> >      1
> >      2ME
> >      3
> >      4
> >      5
> >      6
> >      sed -e 's/5/5SOMEONE/' page >page.someone # someone's changes
> >      cat page
> >      1
> >      2
> >      3
> >      4
> >      5SOMEONE
> >      6

> >      # create a test repository
> >      mkdir t
> >      cd t
> >      cp ../page .
> >      git init
> >      git add .
> >      git commit -m init

> >      # save the current HEAD
> >      ME=$(git rev-list HEAD -- page)
> >      $EDITOR page # assume that I'm starting to edit page via web

> >      # simulates someone's concurrent commit
> >      cp ../page.someone page
> >      git commit -m someone -- page

> >      # My editing session ended, the resulting content is in page.me
> >      cp ../page.me page
> >      cat page
> >      1
> >      2ME
> >      3
> >      4
> >      5
> >      6

> >      # let's start to save my uncommitted changes
> >      git stash clear
> >      git stash save "changes by me"
> >      # we've reached a clean state
> >      cat page
> >      1
> >      2
> >      3
> >      4
> >      5SOMEONE
> >      6

> >      # roll-back to the $ME state
> >      git reset --soft $ME
> >      # now, the file is marked as modified
> >      git stash save "changes by someone"

> >      # now, we're at the $ME state
> >      cat page
> >      1
> >      2
> >      3
> >      4
> >      5
> >      6
> >      git stash list
> >      stash@{0}: On master: changes by someone
> >      stash@{1}: On master: changes by me

> >      # first apply my changes
> >      git stash apply stash@{1}
> >      cat page
> >      1
> >      2ME
> >      3
> >      4
> >      5
> >      6
> >      # ... and commit
> >      git commit -m me -- page

> >      # apply someone's changes
> >      git stash apply stash@{0}
> >      cat page
> >      1
> >      2ME
> >      3
> >      4
> >      5SOMEONE
> >      6
> >      # ... and commit
> >      git commit -m me+someone -- page

By design, Git backend uses a "master-clone" repository pair approach in contrast to the single repository approach (here, clone may be considered as the working copy of a fictious web user). Even though a single repository implementation is possible, it somewhat increases the code complexity of backend (I couldn't figure out a uniform method which doesn't depend on the prefered repository model, yet). By exploiting the fact that the master repo and web user's repo (`srcdir`) are all on the same local machine, I suggest to create the latter with the "`git clone -l -s`" command to save disk space.

Note that, as a rule of thumb, you should always put the rcs wrapper (`post-update`) into the master repository (`.git/hooks/`).

Here is how a web edit works with ikiwiki and git:

- ikiwiki cgi modifies the page source in the clone
- git-commit in the clone
- git push origin master, pushes the commit from the clone to the master repo
- the master repo's post-update hook notices this update, and runs ikiwiki
- ikiwiki notices the modifies page source, and compiles it

Here is a how a commit from a remote repository works:

- git-commit in the remote repository
- git-push, pushes the commit to the master repo on the server
- (Optionally, the master repo's pre-receive hook runs, and checks that the update only modifies files that the pushing user is allowed to update. If not, it aborts the receive.)
- the master repo's post-update hook notices this update, and runs ikiwiki
- ikiwiki notices the modifies page source, and compiles it

## Mercurial

The Mercurial backend is still in a early phase, so it may not be mature enough, but it should be simple to understand and use.

As Mercurial is a distributed RCS, it lacks the distinction between repository and working copy (every wc is a repo).

This means that the Mercurial backend uses directly the repository as working copy (the master M and the working copy W described in the svn example are the same thing).

You only need to specify 'srcdir' (the repository M) and 'destdir' (where the HTML will be generated).

Master repository M.

RCS commit from the outside are installed into M.

M is directly used as working copy (M is also W).

HTML is generated from the working copy in M. rcs_update() will update to the last committed revision in M (the same as 'hg update'). If you use an 'update' hook you can generate automatically the HTML in the destination directory each time 'hg update' is called.

CGI operates on M. rcs_commit() will commit directly in M.

If you have any question or suggestion about the Mercurial backend please refer to Emanuele

## tla

Nobody really understands how tla works.

## rcs

There is a patch that needs a bit of work linked to from rcs.

## Monotone

In normal use, monotone has a local database as well as a workspace/working copy. In ikiwiki terms, the local database takes the role of the master repository, and the srcdir is the workspace. As all monotone workspaces point to a default database, there is no need to tell ikiwiki explicitly about the "master" database. It will know.

The backend currently supports normal committing and getting the history of the page. To understand the parallel commit approach, you need to understand monotone's approach to conflicts:

Monotone allows multiple micro-branches in the database. There is a command, `mtn merge`, that takes the heads of all these branches and merges them back together (turning the tree of branches into a dag). Conflicts in monotone (at time of writing) need to be resolved interactively during this merge process. It is important to note that having multiple heads is not an error condition in a monotone database. This condition will occur in normal use. In this case 'update' will choose a head if it can, or complain and tell the user to merge.

For the ikiwiki plugin, the monotone ikiwiki plugin borrows some ideas from the svn ikiwiki plugin. On prepedit() we record the revision that this change is based on (I'll refer to this as the prepedit revision). When the web user saves the page, we check if that is still the current revision. If it is, then we commit. If it isn't then we check to see if there were any changes by anyone else to the file we're editing while we've been editing (a diff bewteen the prepedit revision and the current rev). If there were no changes to the file we're editing then we commit as normal.

It is only if there have been parallel changes to the file we're trying to commit that things get hairy. In this case the current approach is to commit the web changes as a branch from the prepedit revision. This will leave the repository with multiple heads. At this point, all data is saved. The system then tries to merge the heads with a merger that will fail if it cannot resolve the conflict. If the merge succeeds then everything is ok.

If that merge failed then there are conflicts. In this case, the current code calls merge again with a merger that inserts conflict markers. It commits this new revision with conflict markers to the repository. It then returns the text to the user for cleanup. This is less neat than it could be, in that a conflict marked revision gets committed to the repository.

## bzr

## cvs