Reader/writer locking in BK.

This is single repository wide locking, not individual file locking.

Writer lock

Lock files are maintained in BitKeeper/writer. Currently there are two files hardlinked to each other: BitKeeper/writer/${pid}@${host}.lock (AKA - uniq) BitKeeper/writer/lock

Content: pid <space> host <space> time(0)

On unix, create uniq, then hard link to lock. On windows, use win32 api to create lock. Either way, should get some peace of mind that the lock has been grabbed. See sccs_lockfile() for more.

Reader lock

Lock files are maintained in BitKeeper/readers. There can be many files of the form: BitKeeper/readers/${pid}@${host}.lock (AKA - uniq)


The RESYNC directory gets created by takepatch and by various operations when hitting an error. The RESYNC directory acts like a read lock, blocking write operations, and allowing readlocks to be taken.

Locking stuff

When trying to get a writer lock, walk the reader list and kill 0 each of them that are on the same host. If they are all gone, then remove them and let the lock succeed.

This means that the process who created the lock can’t exit until all of the child processes are done.

Places we need to lock:

resolve (both its own, and in pass4, grabs a repo write lock)