http://nagoya.apache.org/bugzilla/show_bug.cgi?id=22030

 > Apache 2.0.47

 > SECURITY: 4097+ bytes of stderr from cgi script causes script to hang

Reported by Brandon Black

 > If a cgi script under mod_cgi outputs more than 4096 bytes of stderr before it
 > finishes writing to and closing its stdout, the write() inthe cgi script
 > containing the 4097th byte of stderr will hang indefinitely, hanging the
 > script's execution.
 >
 > This appears to be cause by the fact that mod_cgi reads all stdout output
 > first, and then begins reading stderr output.  APR's file_io which is handling
 > the streams will only buffer 4096 characters before further writes by the
 > script to stderr will hang, waiting for mod_cgi to read some of the data from
 > the stream via APR file_io.
 >
 > This occured for me where a perl cgi script was producing a large volume of
 > harmless warning messages to ssl_error_log before it got to the part of it's
 > execution where it actually wrote the stdout output, and causing the script to
 > hang and produce no output to the end user.  Below is a test script to
 > demonstrate:
 >
 > #!/usr/bin/perl
 > # 24x170 = 4080 bytes to stderr
 > foreach my $x (1..24) {
 >   print STDERR 'X' x 169 . "\n";
 > }
 > # + 17 more bytes, putting us at 4097
 > # Delete one char from the print below to make
 > # it work again
 > print STDERR "0123456789ABCDEF\n";
 > # Our actual script output, which never comes
 > print "Content-type: text/plain\n\nASDF\n";