Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!CS.BROWN.EDU!cs126045
From: cs126045@CS.BROWN.EDU
Newsgroups: gnu.emacs.bug
Subject: File locking on shared filesystems
Message-ID: <8905070035.AA13356@transit.cs.brown.edu>
Date: 7 May 89 00:35:50 GMT
Sender: daemon@tut.cis.ohio-state.edu
Distribution: gnu
Organization: GNUs Not Usenet
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Problem: In the file locking code for GNU Emacs version 18.54, the
current_lock_owner routine in filelock.c checks to see if a lock is
obsolete by making a 'kill' system call with the process id found in
the lock to see if the process which created the lock is still active.
If the lock was put there by someone on another machine, the process
which locked the file will not be found whether it is still around or
not, and emacs will steal the lock without warning.

Easy solution: unconditionally return(owner) in current_lock_owner
before looking to see if the process is still around.  If the lock
actually is obsolete the user always has the option of stealing it.

Better solution: allow users to set a lisp variable to select how
cautious file locking should be.  If there's no risk of their files
being edited by someone on another machine, the current code is fine.

Best solution (?): store the hostname in the file along with the
process id, and rsh over to that host to see if the process is still
around.  This might be unacceptably slow... is there a better way?


    Nick Thompson
    cs126045@cs.brown.edu