[Vortex] "no data were waiting on this non-blocking connection" warning

[Jens Alfke]

On 19 Mar '08, at 10:02 AM, Jens Alfke wrote:
> If only a partial line has been received by the socket, readline  
> will read past the end of the available data and get an EWOULDBLOCK  
> error, thus returning -2 and triggering this warning.

Now that I've actually set a breakpoint in __vortex_frame_readline at  
the point where it returns -2, I see that this isn't actually what's  
happening. It hasn't read any characters at all yet (n==1) before  
getting EWOULDBLOCK; that implies that there were _no_ bytes available.

What happens is that vortex_tls_ssl_read is called, and its call to  
SSL_read fails with error SSL_ERROR_WANT_READ. This causes the debug  
message "SSL_read returned that isn't ready to read: retrying", and  
vortex_tls_ssl_read then returns -2.

So it looks as though this is a TLS-specific problem. Raw (encrypted)  
data has arrived on the socket, but OpenSSL can't decrypt it yet,  
maybe because it hasn't gotten a complete cipher block. (I don't know  
how SSL works internally, so this is guesswork.)

I found a good thread about this on the OpenSSL-Users list:
	http://www.mail-archive.com/openssl-users@openssl.org/msg39582.html
Here's the most relevant single quote:
>         If either an SSL_write or an SSL_read results in a WANT_READ  
> error, it
> means that neither call can progress until some data is read from the
> socket. You can retry the operation later, try another operation, or
> whatever you want to do. You can take the hint that 'select'ing on the
> socket for readability will likely tell you when the operation is  
> going to
> succeed.

It sounds, then, as though this particular situation I'm seeing is  
harmless.

But I still think there's in _general_ a potential for data loss if  
__vortex_frame_readline gets partway through reading a line and then  
gets the WANT_READ — the partial line will be ignored, probably  
causing BEEP syntax/framing errors on the next read, which will abort  
the connection.

—Jens
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