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With a fcntl lock, why this lsof output?

by welchavw (Pilgrim)
on Sep 23, 2005 at 21:28 UTC ( #494686=perlquestion: print w/replies, xml ) Need Help??
welchavw has asked for the wisdom of the Perl Monks concerning the following question:

I am writing a small program (below) to enable me to replace the /etc/sudoers file (via an RPM), while respecting visudo and its file lock. (First thing, please note that the visudo man page, at least for my distro is wrong - there is no lock on /etc/sudoers.tmp AFAICT). I am doing this on x86 FC4 (so that you may grok the pack()). Basically, this program works - when I do a visudo after acquiring the lock through this program, it behaves just like 2 visudos were contending. That is, I get a "visudo: sudoers file busy, try again later." So all is swell, except I am picky and ...

I look at the output of "lsof -p" for visudo and compare the same output for the perl program.

When visudo is running, I get...

visudo 13070 root 3uW REG 3,3 640 12835419 /etc/sudoers

When the perl program is running (having gotten lock and is looping), I get...

fcntl_loc 13091 root 3w REG 3,3 640 12835419 /etc/sudoers

See, the 3uW looks good for visudo, but where's the "W" to indicate write lock for the perl? Do I have a file lock, or is visudo failing to acquire the lock on /etc/sudoers for some other reason while this perl code is running?

So, I wonder if perhaps my pack is wrong and I am stomping on some memory. Or else something else is going on? Its too late on Friday for me to see what it is - maybe one of you monks can? (Pls don't kill me for the treatment of $rc - this code is just hacked together, thx).

Here's the program.

#!/usr/bin/perl use Fcntl qw(:DEFAULT :flock); use Errno qw(EAGAIN); use strict; use warnings; @ARGV == 1 or die "usage: $0 <filename>\n"; print "opening $ARGV[0]...\n"; open my $fh, ">>$ARGV[0]" or die "open:$!\n"; my $results = pack( "sslll", &F_WRLCK, 0, 0, 0, 0); print "locking $ARGV[0]...\n"; my $rc; while (1) { $rc = fcntl($fh, F_SETLK, $results); last if ($! != EAGAIN || (defined $rc && $rc == 0)); print "sleeping...\n"; sleep 5; print "trying again...\n"; } if ((!defined $rc) || $rc != 0) { die "fcntl:$!\n" }; print "looping forever...\n"; sleep 64*64;

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Re: With a fcntl lock, why this lsof output?
by traveler (Parson) on Sep 23, 2005 at 22:55 UTC
    The issue is that you use fcntl to get a byte-level write lock (the lowercase w) while visudo uses flock or lockf (depending on what you have) to lock the whole file (uppercase W). The lowercase u is because visudo opens the file read-write and you open it for append.

    HTH, --traveler

      Perfect. Thank you. I grabbed File::lockf and was indeed able to see that lockf with a "+<" mode open exactly replicates the visudo locking. I had never ventured into deeply enough into file-locking to learn of the multiple APIs and their differences.


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