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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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