Книга: Learning GNU Emacs, 3rd Edition

12.8.10 When VC Gets Confused

12.8.10 When VC Gets Confused

The filesystem operations required to determine a file's version control state can be expensive and slow, especially in an NFS or other networked environment. VC goes to some pains to compensate (unless, as we'll see later on, you tell it not to).

It has two major methods: (1) caching per-file information (such as the locking user and current revision number) in memory rather than running version control utilities to parse it out of the relevant master every time, and (2) assuming that it can deduce a registered file's version control state from its write permissions. Specifically, VC assumes that a registered file that is writable is in the checked-out-and-locked state and that a registered file that is not writable is not a checked-out version being edited.

Multiuser environments being what they are, VC's cached information and assumptions about permissions occasionally lead it down the wrong path. This situation almost always occurs because someone has manually changed a file's permissions behind VC's back.

If you think that this situation has occurred, call vc-clear-context. This command forces VC to throw away all its cached-in-memory assumptions about the version control state of the files you are working with.

It is also theoretically possible for VC to get confused by a race condition between two or more VCs, or between VC and someone running the bare SCCS, RCS, or CVS utilities. This is not just a VC problem; the same sort of race is possible (though less likely) between two or more people running the bare utilities. However, this kind of race is very rare even in VC; the authors haven't heard of any instance in hundreds of thousands of programmer-hours in which it's known to have happened.

If you're concerned about this issue, the VC source code (vc.el in your Emacs Lisp source directory) includes a comment giving a careful and extensive analysis of potential multiuser conflict and race situations. VC is exactly as safe from them as the underlying utilities can be.

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