??? 123123123123 .....................................................................................................................................??? 123123123123 .....................................................................................................................................ó ¿œ^c@sôdZddlZddlZddlZddlZdZdZdZdZdZ ddd „ƒYZ d e fd „ƒYZ d dd „ƒYZ d„Z de fd„ƒYZde fd„ƒYZed„Zd„ZedkrðeƒndS(sûCVS locking algorithm. 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." 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." - 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." - To release a write lock: - remove the file "#cvs.wfl." - rmdir the meta-lock - To release a read lock: - remove the file "#cvs.rfl." 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. 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