Obtain an exclusive lock on log files when writing logs

More is probably needed with this as there are probably other files that
multiple server processes could be attempting to write to at the same
time.  Finding these would take careful review of the code.

This is to address issue openwebwork#2016.

lib/WeBWorK/Utils.pm

@@ -32,6 +32,7 @@ use Date::Parse;
 use Date::Format;
 use File::Copy;
 use File::Spec::Functions qw(canonpath);
+use Fcntl qw(:flock);
 use Time::Zone;
 use MIME::Base64 qw(encode_base64 decode_base64);
 use Errno;
@@ -828,43 +829,46 @@ sub between { my $t = (@_ == 3) ? $_[2] : time; return $t >= $_[0] && $t <= $_[1
 # Logging
 ################################################################################
 
-sub writeLog($$@) {
+sub writeLog {
 	my ($ce, $facility, @message) = @_;
-	unless ($ce->{webworkFiles}->{logs}->{$facility}) {
+	unless ($ce->{webworkFiles}{logs}{$facility}) {
 		warn "There is no log file for the $facility facility defined.\n";
 		return;
 	}
-	my $logFile = $ce->{webworkFiles}->{logs}->{$facility};
-	surePathToFile($ce->{webworkDirs}->{root}, $logFile);
-	local *LOG;
-	if (open LOG, ">>", $logFile) {
-		print LOG "[", time2str("%a %b %d %H:%M:%S %Y", time), "] @message\n";
-		close LOG;
+	my $logFile = $ce->{webworkFiles}{logs}{$facility};
+	surePathToFile($ce->{webworkDirs}{root}, $logFile);
+	if (open my $LOG, '>>:encoding(UTF-8)', $logFile) {
+		flock $LOG, LOCK_EX;
+		print $LOG "[", time2str("%a %b %d %H:%M:%S %Y", time), "] @message\n";
+		close $LOG;
 	} else {
 		warn "failed to open $logFile for writing: $!";
 	}
+	return;
 }
 
-sub writeCourseLog($$@) {
+sub writeCourseLog {
 	my ($ce, $facility, @message) = @_;
 	writeCourseLogGivenTime($ce, $facility, time, @message);
+	return;
 }
 
-sub writeCourseLogGivenTime($$@) {
+sub writeCourseLogGivenTime {
 	my ($ce, $facility, $myTime, @message) = @_;
-	unless ($ce->{courseFiles}->{logs}->{$facility}) {
+	unless ($ce->{courseFiles}{logs}{$facility}) {
 		warn "There is no course log file for the $facility facility defined.\n";
 		return;
 	}
-	my $logFile = $ce->{courseFiles}->{logs}->{$facility};
-	surePathToFile($ce->{courseDirs}->{root}, $logFile);
-	local *LOG;
-	if (open LOG, ">>:utf8", $logFile) {
-		print LOG "[", time2str("%a %b %d %H:%M:%S %Y", $myTime), "] @message\n";
-		close LOG;
+	my $logFile = $ce->{courseFiles}{logs}{$facility};
+	surePathToFile($ce->{courseDirs}{root}, $logFile);
+	if (open my $LOG, '>>:encoding(UTF-8)', $logFile) {
+		flock $LOG, LOCK_EX;
+		print $LOG "[", time2str("%a %b %d %H:%M:%S %Y", $myTime), "] @message\n";
+		close $LOG;
 	} else {
 		warn "failed to open $logFile for writing: $!";
 	}
+	return;
 }
 
 # $ce - a WeBWork::CourseEnvironment object
@@ -875,10 +879,11 @@ sub writeCourseLogGivenTime($$@) {
 # use an empty string for $details when calling for END
 # Information printed in format:
 # [formatted date & time ] processID unixTime BeginEnd $function  $details
-sub writeTimingLogEntry($$$$) {
+sub writeTimingLogEntry {
 	my ($ce, $function, $details, $beginEnd) = @_;
 	$beginEnd = ($beginEnd eq "begin") ? ">" : ($beginEnd eq "end") ? "<" : "-";
 	writeLog($ce, "timing", "$$ " . time . " $beginEnd $function [$details]");
+	return;
 }
 
 ################################################################################