sub merge_hist_changes  {   # Merges current session's history changes into %sess_hist.

	## Step 1 - Create copy of $session's hash tree
	my %temp_hash = %{$sess_hist{$session}};

	## Step 2 - Filter out files downloaded more than $hist_days days ago
	my $old = time - ( 86400 * $hist_days );    # 86400 seconds in a day
	@{$temp_hash{files}} = grep { /@(\d+)$/; $1 > $old } @{$temp_hash{files}};
	@{$temp_hash{uploads}} = grep { /@(\d+)$/; $1 > $old } @{$temp_hash{uploads}};

	## Step 3 - Get an flock on $hist_file.l. This is the critical step that prevents
	## other forks from updating until the current $session's info gets updated.
	open HFL, '>', "$hist_file.l";
	unless(flock HFL, 2) {
		my $failstr = "Can't get lock on $hist_file.l, changes to DB unsaved\n";
		$failstr .= "History tree for $session :\n" . Dumper \%temp_hash;
		&pager_alert($failstr);
		exit;
	}

	## Step 4 - Get new %sess_hist from disk (like &parse_hist)
	local $/ = undef;
	if (-s $hist_file)  {
		unless( %sess_hist = %{do($hist_file)} ) {
			my $failstr = "Can't parse history file, changes to DB unsaved\n";
			$failstr .= "History tree for $session :\n" . Dumper \%temp_hash;
			&pager_alert($failstr);
			exit;
		}
	}
	## Step 5 - Change $session's hash pointer to refer to %temp_hash
	$sess_hist{$session} = \%temp_hash;

	## Step 6 - Dump %sess_hist.
	local $Data::Dumper::Indent = 1;
	open  HF, '>', $hist_file;
	print HF Dumper \%sess_hist;
	close HF;
	close HFL;  # Releases flock and lets next child process update $hist_file