Saving this irc transcript here, since it's a fairly in-depth discussion of how ikiwiki handles commits, locking, etc, and avoids some races while doing so.

<tschwinge> What happens if I edit a page and in the underground a new version is installed into the svn repository?
<tschwinge> The revision when I started editing was saved, right?
<joeyh> what happens, exactly is:
<joeyh> 1. the new version that was committed first get into svn, and ikiwiki updates its WC to have the new version
<joeyh> 2. When you save your edit, ikiwiki detects a conflict.
<joeyh> 3. It uses svn merge to try to resolve it; if it's resolved it adds your changes transparently
<joeyh> 4. If the conflict needs manual resolution, it displays the page with conflict markers in the editor for you to resolve
<tschwinge> Ok.
<joeyh> Note that in step 2, it detects the conflict by using svn info to get the current Revision of the page in the WC, and compares that to a revision that is stored when you start to edit the page
<joeyh> that's why rcs_prepedit exists, to get that revision info
<tschwinge> But isn't there a race condition?
<joeyh> well, there is locking going on too
<joeyh> ikiwiki won't update the WC in step 1. if another instance of itself is getting the Revision info
<tschwinge> Is that lockwiki()?
<joeyh> yeah
<joeyh> note that when it gets the current Revision info of a page during its conflict detection, svn could have changed the page in the repo, and the WC not been updated yet due to the lock, but this isn't a race since the commit will then fail due to a regular svn conflict and the conflict detction will still work.