302 Redirect from CGI script stopped working in Apache 2.4 - cgi

Question

I inherited maintenance of a self-written CGI application without
documentation and have never met the original author. The application
stopped working in Debian 8, but worked in Debian 7 and CentOS 5. The
main changes were the upgrade from Apache 2.2 (used by Debian 7 &
CentOS 5) to Apache 2.4 (used by Debian 8) and the upgrade from perl
5.8 (in CentOS 5) respectively perl 5.14 (in Debian 7) to perl 5.20.
The problematic part boils down to the following script (a
302-redirect):
#!/usr/bin/perl
$|=1; # activate auto-flushing of stdout
use strict;
use warnings;
my $CRLF = "\015\012";
print STDOUT "Status: 302 Moved Temporarily$CRLF" .
"Location: /does_not_matter$CRLF" .
"URI: /does_not_matter$CRLF" .
"Connection: close$CRLF" .
"Content-type: text/html; charset=UTF-8$CRLF$CRLF";
close STDOUT;
while(1) {
sleep 1;
}
The observed behavior is that the redirect never reaches the client
as long as the script is still running when used with Apache 2.4, but
there is no error message in Apache's error.log. I changed the client
(Firefox, Chromium, wget), the Apache module (mod_cgid & mod_cgi),
sent additional headers, removed the close STDOUT, removed the
$|=1, replaced the $CRLF with \n and made the script
fork and exit the parent process (so that Apache is no longer the
parent process), all to no avail. The only things that worked: use
Apache 2.2, turn the script into an NPH-CGI-script (which has to send
complete HTTP headers that Apache will not modify in any way, even if
they contain errors), make the script exit instead of entering an
endless loop. I confirmed via tcpdump that the packages with the
redirect indeed never leave the server before the script is killed.
And from the Date-line in the response and the time of the eventual
arrival I gather that Apache receives my output immediately (&
immediately adds the Date-line to the headers), but does not send the
response to the client.
Don't bother answering, I already figured the solution out by myself
and will write an answer. I just want to make the solution available
to others who might encounter the same problem.

Answer 1

The problem was the missing message body. A 302 redirect may contain
a message body (see RFC 2616, section 4.3 (Message Body): "All other
responses do include a message-body, although it MAY be of zero
length"), but a Content-Length-line is optional (section 4.4 of RFC
2616 says the message body length can be determined by closing the
connection if the Content-Length-line is missing). Since Apache can
not know whether I want to send a message body or not, it has to wait
until I close the connection or actually send the message body
(Apache 2.2 apparently behaved erroneously here by not waiting for
the message body - or maybe close STDOUT; does not do in perl
5.20 what it did in older perl versions). The correct script should
therefore look like this (verified to work both in Apache 2.2 and in
Apache 2.4) - the only difference is an additional $CRLF which
terminates a zero-length message body:
#!/usr/bin/perl
$|=1; # activate auto-flushing of stdout
use strict;
use warnings;
my $CRLF = "\015\012";
print STDOUT "Status: 302 Moved Temporarily$CRLF" .
"Location: /doesNotMatter$CRLF" .
"URI: /doesNotMatter$CRLF" .
"Connection: close$CRLF" .
"Content-type: text/html; charset=UTF-8$CRLF$CRLF$CRLF";
close STDOUT;
while(1) {
sleep 1;
}
It was https://stackoverflow.com/a/8062277/2845840 that pointed me in the right direction.