Meanings of various return codes with non-blocking I/O?

[James Marshall]

I'm trying to write a secure embedded HTTP server using OpenSSL.  I'm using
non-blocking I/O, and the main functions I'm using are SSL_accept(),
SSL_read(), SSL_write(), and SSL_shutdown().   After each one, I want to
handle all possible return codes, but I can't find docs that describe the
meaning of them all.  I've read many pages under
http://www.openssl.org/docs/ssl/ and reviewed s_server.c .

For example, SSL_accept() sometimes returns 0, with SSL_get_error()
returning SSL_ERROR_SYSCALL .  No errors show up from ERR_get_error().  How
should I handle this?  Merely trying the SSL_accept() again results in a
busy loop with the same results.

Here's the block of code in question, in Perl and using the Net::SSLeay
module (which is basically wrappers around OpenSSL functions with the
"SSL_" prefixes removed).  $select_read and $select_write are IO::Select
objects, which are collections of handles to use in a select() call:

==================================
        my $rv= Net::SSLeay::accept($ssl) ;
        if ($rv<=0) {
            my $err= Net::SSLeay::get_error($ssl, $rv) ;
            if ($err==Net::SSLeay::ERROR_WANT_READ() or
$err==Net::SSLeay::ERROR_WANT_ACCEPT()) {
                $select_read->add($socket) ;
                $select_write->remove($socket) ;
            } elsif ($err==Net::SSLeay::ERROR_WANT_WRITE()) {
                $select_write->add($socket) ;
                $select_read->remove($socket) ;
            } elsif ($err==Net::SSLeay::ERROR_SYSCALL()) {
                warn Net::SSLeay::ERR_error_string($err) . "\n"  while
$err= Net::SSLeay::ERR_get_error() ;
            }
            return 0 ;

        # Upon success, do stuff.
        } else {
            # do stuff
               .
               .
               .
        }
==================================

The zero+SSL_ERROR_SYSCALL busy-loop result comes when using Chrome
19.0.1084.56, but not Firefox 13.0.  When I kill the server process, either
browser immediately displays the resulting page correctly.  Does this
behavior ring any bells?  This is all on Linux 3.1.10 (openSUSE).
SSL_read(), SSL_write(), and SSL_shutdown() are each handled with a similar
block.

That's one of several questions I have.  A more general question is:  Where
can I find descriptions of all return codes when using non-blocking I/O?

Thanks for any help!

Cheers,
James