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To: Greg Smith <greg@2ndquadrant.com>
cc: xsun <xu2002261@163.com>, pgsql-admin@postgresql.org
Subject: Re: The function of lock file named ".s.PGSQL.<port>.lock" on Linux? 
In-reply-to: <4C61F3FF.1080303@2ndquadrant.com> 
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	<4C61F3FF.1080303@2ndquadrant.com>
Comments: In-reply-to Greg Smith <greg@2ndquadrant.com>
	message dated "Tue, 10 Aug 2010 20:51:11 -0400"
Date: Tue, 10 Aug 2010 21:09:55 -0400
Message-ID: <23168.1281488995@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
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Greg Smith <greg@2ndquadrant.com> writes:
> Tom Lane wrote:
>> We can't portably lock the socket file itself, so we make a separate
>> ordinary file for locking purposes.

> It looks to me like the fact that the .lock file is created is only 
> documented in src/backend/utils/init/miscinit.c , and even there the 
> reason why (what you wrote above) isn't mentioned.  Seems like noting 
> this file is created in the unix_socket_directory entry of the docs, and 
> the rationale for why in the source code, would make a useful 
> improvement.  Anybody want to write a little documentation patch?

FWIW, the discussion about that was here:
http://archives.postgresql.org/pgsql-hackers/2000-11/msg01306.php

and the subsequent commit was

2000-11-29 15:59  tgl

	* configure, configure.in, src/backend/libpq/pqcomm.c,
	src/backend/postmaster/postmaster.c, src/backend/tcop/postgres.c,
	src/backend/utils/init/miscinit.c, src/backend/utils/misc/guc.c,
	src/bin/pg_ctl/pg_ctl.sh, src/include/config.h.in,
	src/include/miscadmin.h: Get rid of not-very-portable
	fcntl(F_SETLK) mechanism for locking the Unix socket file, in favor
	of having an ordinary lockfile beside the socket file.	Clean up a
	few robustness problems in the lockfile code.  If postmaster is
	going to reject a connection request based on database state, it
	will now tell you so before authentication exchange not after.	(Of
	course, a failure after is still possible if conditions change
	meanwhile, but this makes life easier for a yet-to-be-written
	pg_ping utility.)

(The pgsql-committers messages were pretty useless back then, so I won't
point you to that archive.)

			regards, tom lane