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RFC: locking policy for Debian

Hi folks!

I'm just working on a new Policy manual and discovered that we don't have
a (working) locking policy right now.

AFAIK, this issue came up two times here on debian-devel (about end of Jan
97 and beginning Feb 97) and was about email folder locking and locking of

AFAIK, the only way to lock a file (even reliable over NFS) is to create
another file with a unique filename, e.g. foo.lock-ipaddress-processid,
and create a link say to foo.lock. The "link" function is atomic per
definition, even via NFS and will fail if the file already exists. Is this

I suggest that we

1) document the right way of locking (TM) in our Policy Manual,

2) force _all_ programs touching mail folders, /etc/passwd and alike to
support our way of locking

3) build a "libdebian" package with a shared library that contains a few
functions to lock, unlock, test a file (each program that needs to lock a
file should be linked against this library and call our functions)

4) provide examples for other languages than C (for example Perl)

I just had a look at the old discussion and discovered that Miquel van
Smoorenburg (miquels@cistron.nl) has already written such a library.
Perhaps we can use his (and adopt it if necessary). If I remember right
someone else posted a Perl example for right file locking--but I can't
find it. 

Anyways, it would be nice if someone more competent in these things could
provide me a section that I could include in the policy manual.

As always, any comments are appreciated.



--                 Christian Schwarz
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