How do I do nested reads from <STDIN> in perl? - perl

Question

I'm writing a script to parse thread dumps from Java. For some reason when I try to read from within the subroutine, or inside a nest loop, it doesn't enter the nested loop at all. Ideally I want to be able to operate on STDIN on nested loops otherwise you'll have to write some ugly state transition code.
Before I was using STDIN, but just to make sure that my subroutine didn't have an independent pointer to STDIN, I opened it into $in.
When I run it, it looks like below. You can see that it never enters the nested loop despite the outer loop having more files from STDIN to read.
~/$ cat catalina.out-20160* | thread.dump.find.all.pl
in is GLOB(0x7f8d440054e8)
found start of thread dump at 2016-06-17 13:38:23 saving to tdump.2016.06.17.13.38.23.txt
in is GLOB(0x7f8d440054e8)
BEFORE NESTED STDIN
BUG!!!!
found start of thread dump at 2016-06-17 13:43:05 saving to tdump.2016.06.17.13.43.05.txt
in is GLOB(0x7f8d440054e8)
BEFORE NESTED STDIN
BUG!!!!
...
The code:
#!/usr/bin/perl
use strict;
use warnings;
use Getopt::Long;
use DateTime::Format::Strptime;
use DateTime::Format::Duration;
use Data::Dumper;
# DO NOT touch ARGV!
Getopt::Long::Configure("pass_through");
# cat catalina.out-* | thread.dump.find.all.pl
sub processThreadDump {
my $in=$_[0];
my $currentLine=$_[1];
my $prevLine=$_[2];
my $parsedDatetime=$_[2];
# 2016-09-28 09:27:34
$parsedDatetime=~ s/[ \-\:]/./g;
my $outfile="tdump.$parsedDatetime.txt";
print " saving to $outfile\n";
print " in is $in\n";
open(my $out, '>', $outfile);
print $out "$prevLine\n";
print $out "$currentLine\n";
print "BEFORE NESTED STDIN\n";
foreach my $line ( <$in> ) {
print "INSIDE NESTED STDIN\n";
$line =~ s/\R//g; #remove newlines
print $out "$line\n";
if( $line =~ m/JNI global references:/ ) {
print "PROPERLY LEFT NESTED STDIN\n";
close($out);
return;
} elsif( $line =~ m/Found \d+ deadlock\./ ) {
print "PROPERLY LEFT NESTED STDIN\n";
close($out);
return;
}
}
print "BUG!!!!\n";
close($out);
}
open(my $in, '<-');
print "in is $in\n";
my $prevLine;
# read from standard in
foreach my $line ( <$in> ) {
$line =~ s/\R//g; #remove newlines
if( $line =~ m/Full thread dump OpenJDK 64-Bit Server VM/ ) {
# we found the start of a thread dump
print "found start of thread dump at ${prevLine}";
processThreadDump($in, $line, $prevLine);
} else {
#print "setting prev line to $line\n";
$prevLine=$line;
}
}
close($in);

Answer 1

The foreach iterates over a list, so <> is in the list context and thus it reads everything from the filehandle. So when you pass $in to the sub there's no input left on it. See I/O Operators in perlop.
You can read a line at a time, while (my $line = <$in>), but I am not sure whether that may affect the rest of your algorithm.
Alternatively, if you do read all input ahead of time why not just work with an array of lines then.

Answer 2

When you say foreach my $line ( <$in> ), this causes perl to read the entire $in filehandle before starting the loop. What you probably want is more like this:
while (defined(my $line = <$in>))
This will only read one line at a time, discarding it as you finish with it.