With a fcntl lock, why this lsof output?

[welchavw]

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;

[traveler]

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

[welchavw]

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.

Regards,
Adam