I recently completed a project that, while it seems to be working famously, worries me a little. I'm using Parallel::ForkManager and Data::Dumper/do(file) (rather than Storable) to run multiple comm sessions at the same time, and have each fork update a history file (that is really just a hash dump).

I wanted to avoid any possible race conditions when updating the file. The file's content may shrink, so I didn't want to open the file +<. I settled on creating .l files, establishing locks on those, and doing reads and updates on the history file only after establishing locks on the .l file.

My question is: Am I overlooking something dangerous? Access to the history file is restricted, so I'm not too concerned about using do(file) (although though I'll be happy when Storable is installed or Perl is upgraded to 5.8x). I don't see any race conditions that can occur in the code, but I'm still worried if I'm missing something that will occur when more sessions are run simultaneously.

Here is a trimmed down code snippet: 

#!/usr/bin/perl -w
use strict;
use Data::Dumper;
use Parallel::ForkManager;

my %sess_hist = ();           # Session history hash
my %hash = ();                # session flow information
my $session;                  # ID of current session
my $hist_file = '~/hist.dat'; # Session history file

## Check for sessions that are due/overdue and run them
for (keys %{$hash{Session}})    {
    $session = $_;
    my $pid = $pm->start($session) and next;
    if (&check_overdue) {
        if (open SESSLOCK, '>', "~/$session.l") {
            my $stime = time;
            flock SESSLOCK, 2;
            &parse_hist;
            if ($stime > $sess_hist{$session}{last}) { &run_session; }
            else { logit "Aborting to avoid simultaneous sessions"; }
            close SESSLOCK;
            unlink "~/$session.l";
        }
        else { logit "Can't open session lock file, aborting"; }      
+      
    }
    exit(0);
    $pm->finish($session);
}
$pm->wait_all_children;
unlink "$hist_file.l";


sub parse_hist  {   # Opens hist file and pulls info into %sess_hist
    if (-s $hist_file)  {                   # If the history file has 
+data,
        local $/ = undef;                   # set slurp mode,
        open HFL, '>', "$hist_file.l";      # History File Lock
        flock HFL, 2;                       # establish an exclusive l
+ock,
        %sess_hist = %{do($hist_file)};     # and load hist file as %s
+ess_hist
        close HFL;
    }
}


sub merge_hist_changes  {
    # Create copy of current session data
    my %temp_hash = %{$sess_hist{$session}};

    # Reload hist file after establishing lock on .l file
    open HFL, '>', "$hist_file.l";
    flock HFL, 2;
    local $/ = undef;
    %sess_hist = %{do($hist_file)} if -s $hist_file;

    # Change reference to point to copy made earlier
    $sess_hist{$session} = \%temp_hash;

    # Dump it
    local $Data::Dumper::Indent = 1;
    open  HF, '>', $hist_file;
    print HF Dumper \%sess_hist;
    close HF;
    close HFL;  # Release lock and let next child update $hist_file
}

[download]

&merge_hist_changes gets called at the end of &run_session to merge any changes already saved by other sessions into the current hash. The $session.l files are just to prevent the same session ID being run twice.

Thanks for any input.

In reply to Flocking .l file instead of file to be updated by delirium

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