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CVS locking strategy
====================
As reverse engineered from the CVS 1.3 sources (file lock.c):
- Locking is done on a per repository basis (but a process can hold
write locks for multiple directories); all lock files are placed in
the repository and have names beginning with "#cvs.".
- Before even attempting to lock, a file "#cvs.tfl.<pid>" is created
(and removed again), to test that we can write the repository. [The
algorithm can still be fooled (1) if the repository's mode is changed
while attempting to lock; (2) if this file exists and is writable but
the directory is not.]
- While creating the actual read/write lock files (which may exist for
a long time), a "meta-lock" is held. The meta-lock is a directory
named "#cvs.lock" in the repository. The meta-lock is also held while
a write lock is held.
- To set a read lock:
- acquire the meta-lock
- create the file "#cvs.rfl.<pid>"
- release the meta-lock
- To set a write lock:
- acquire the meta-lock
- check that there are no files called "#cvs.rfl.*"
- if there are, release the meta-lock, sleep, try again
- create the file "#cvs.wfl.<pid>"
- To release a write lock:
- remove the file "#cvs.wfl.<pid>"
- rmdir the meta-lock
- To release a read lock:
- remove the file "#cvs.rfl.<pid>"
Additional notes
----------------
- A process should read-lock at most one repository at a time.
- A process may write-lock as many repositories as it wishes (to avoid
deadlocks, I presume it should always lock them top-down in the
directory hierarchy).
- A process should make sure it removes all its lock files and
directories when it crashes.
- Limitation: one user id should not be committing files into the same
repository at the same time.
Turn this into Python code
--------------------------
rl = ReadLock(repository, waittime)
wl = WriteLock(repository, waittime)
list = MultipleWriteLock([repository1, repository2, ...], waittime)
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