fork and HTTP::Daemon freezes on windows

[patcat88]

I am trying to make a HTTP::Daemon test server that can service 2 clients simultaneously on Windows. Ithreads fork is the easiest thing to use I suppose for a test server. I am using "v5.10.1 built for MSWin32-x86-multi-thread" ActivePerl build 1007. If I comment out the fork and uncomment " #$pid = 0;" (remember the "exit(0);" too) the script works fine, but can service only 1 client at a time. I also tried the forking version of this script on Linux with perl 5.8.8, it worked perfectly. According to Visual Studio debugger, it tells me perl is deadlocked when I break it. The callstack on the child thread is

     ntdll.dll!_KiFastSystemCallRet@0()     
     ntdll.dll!_ZwWaitForSingleObject@12()  + 0xc    
     ntdll.dll!_RtlpWaitForCriticalSection@4()  + 0x8c    
     ntdll.dll!_RtlEnterCriticalSection@4()  + 0x46    
     mswsock.dll!_WSPGetSockName@16()  + 0x8e    
     ws2_32.dll!_getsockname@12()  + 0x4d    
>    perl510.dll!win32_getsockname(unsigned int s=, sockaddr * addr=, 
+int * addrlen=)  Line 197 + 0x29    C

The callstack on the main perl thread is

     ntdll.dll!_KiFastSystemCallRet@0()     
     ntdll.dll!_ZwDeviceIoControlFile@40()  + 0xc    
     mswsock.dll!_WSPAccept@24()  + 0x1f5    
     ws2_32.dll!_WSAAccept@20()  + 0x85    
     ws2_32.dll!_accept@12()  + 0x17    
>    perl510.dll!win32_accept(unsigned int s=2382008, sockaddr * addr=
+0x00000003, int * addrlen=0x0140f6e4)  Line 160 + 0x29    C
     perl510.dll!Perl_pp_accept(interpreter * my_perl=0x01c75b3c)  Lin
+e 2588    C
     perl510.dll!Perl_runops_standard(interpreter * my_perl=0x00243fbc
+)  Line 38 + 0xc    C
     perl510.dll!S_run_body(interpreter * my_perl=0x00243fbc, long old
+scope=1)  Line 2432 + 0x7    C
     perl510.dll!perl_run(interpreter * my_perl=0x00243fbc)  Line 2350
+ + 0xa    C
     perl510.dll!RunPerl(int argc=2, char * * argv=0x00243f08, char * 
+* env=0x01242c10)  Line 270 + 0x6    C++
     perl.exe!main(int argc=2, char * * argv=0x00243f08, char * * env=
+0x00242c10)  Line 22 + 0x12    C
     perl.exe!_mainCRTStartup()  + 0xe3    
     kernel32.dll!_BaseProcessStart@4()  + 0x23

Callstack in Perl before the freeze is

Socket.pm, line 246, in IO::Socket::sockname
INET.pm, line 270, in IO::Socket::sockaddr
Daemon.pm, line 42, in HTTP::Daemon::url
Daemon.pm, line 147, in HTTP::Daemon::ClientConn::get_request
httpserverfork.pl, line 24, in main

The script is

#!/usr/bin/perl -w

use HTTP::Daemon;
use warnings;
use strict;

$HTTP::Daemon::DEBUG =1;

my $page = "Hello World!";
my $httpresponsestr = "HTTP/1.1 200 OK\nConnection: close\nDate: Sat, 
+16 Oct 2010 04:41:51 GMT\nCache-Control: no-store, no-cache, must-rev
+alidate\nContent-Length: ".length($page)."\n\n".$page;
$| = 1;
my $d = HTTP::Daemon->new (LocalPort => 80) || die "server died";
print "Please contact me at: <URL:", $d->url, ">\n";
while (my $c = $d->accept) {
     my $pid = fork();
     #my $pid = 0;
     if(!defined($pid))
          {die "Fork failed."}
     elsif ($pid == 0)
     {
          my $sn;
          $sn = $c->sockaddr;
          $sn = $c->sockname;
          my $r = $c->get_request;
          print "after get_request r\n";
          if ($r->method eq 'GET') {
               print "is a get\n";
               $c->print($httpresponsestr);
          }
          else {
               print "forbidden";
              $c->send_error(RC_FORBIDDEN());
          }
         $c->close;
         print "closed\n";
         #comment this out when fork disabled
         exit(0);
     }
     print "fork parent out\n";
}

Whats wrong with my code? Does perl's fork not support sockets?

[BrowserUk]

Whats wrong with my code?

Probably nothing on systems that have native fork.

Does perl's fork not support sockets?

You are probably aware that fork on windows is emulated using threads. You should also be aware that it is a very poor emulation. The simple answer is don't use it.

The more complicated answer is: spend hours days and weeks trying to understand where it is falling short, then realise that there is probably nothing you (or anyone?) can do to correct it; and stop using it.

[tye]

Looks like Win32 "get sock name" can't be done by one thread (by which I mean "Win32 thread" not iThread) while another thread is in the middle of "accept" (on a different socket). It doesn't look like it is Perl locks that are causing the deadlock based on Perl's "accept" and "get sock name" being in the middle of calling Win32's "accept" and "get sock name" (respectively).

So the problem should go away if you replace emulated fork() with spawning, taking care to inherit the parent's file handle in the child. And this method works on Win32 (Apache uses it).

But in trying to quickly throw together the pieces needed to do this in Perl, I have my doubts that Perl can currently support it well (at least in the case of sockets and even more so when trying to use something like HTTP::Daemon).

So, rather than inherit a socket created in the parent, you might just spawn and create separate sockets in each process using

my $d= HTTP::Daemon->new( ... ReuseAddr=>1, ... );

I'm not certain that such "separate but equal" sockets will work as desired, but it seems worth trying.

"Spawn" in Win32 Perl can be spelled something like

system( 1, $^X, $0, @ARGV );

but probably add something to let the child know it is a child. For example, set some %ENV variable or add some extra argument to the child's command line.