[Libwebsockets] i->pwsi value returned to user in H2 pipeline case

Andy Green andy at warmcat.com
Thu Nov 15 05:17:14 CET 2018



On 15/11/2018 11:43, Xi Chen wrote:
> Hi Andy,
> 
> I just want to understand the results I have with 2 children h2_wsi 
> sharing the same network wsi.
> 
> When I send the first request, it creates one network_wsi and then one 

The actual sequencing for the initial lws_http_client_connect_via_info() 
only goes as far as doing the name resolution and starting to make the 
tcp connection with connect() on a nonblocking socket... it returns to 
the caller without any error or conclusion at that time and i->pwsi is 
set.  Later, after returning to the event loop, it will get POLLOUT 
signalled on the socket to tell it retrying the connect() will now give 
a result.

Until then, the client doesn't actually know if it will find an 
h2-capable server or what, until after the connect() asynchronously 
completes and tls ALPN information is coming back.

> h2_wsi (say wsi_1), however I notice that the application gets the 
> i->pwsi value set to network_wsi instead of wsi_1. But later callbacks 
> such as HTTP_WRITE is called with wsi_1.

What happens there is the network wsi is wsi_1.  When it understands 
from ALPN the server offers h2, lws goes down that road and the initial 
client connection wsi becomes the h2 connection "network wsi" (after 
all, that wsi already directly owns the socket).  A new wsi is created 
to be the first stream wsi (SID 1), and the ah / client headers are 
migrated from what is becoming the network wsi to the new stream wsi.

https://libwebsockets.org/git/libwebsockets/tree/lib/roles/h2/http2.c#n1221-1300

So the meaning of wsi_1 splits... the original wsi as a wrapper for the 
socket stays the same and becomes the "network wsi" for the h2 
connection.  The original wsi as a container for a client HTTP request 
gets handed off to a newly created stream wsi attached as a child to the 
network wsi.

Also at that time any other queued client requests to the same host that 
had attached themselves to the first wsi are translated into h2 streams 
to be serviced concurrently.

> When I send the second request with LCCSCF_PIPELINE  set, it creates one 
> h2_wsi (say wsi_2), and the application gets i->pwsi = wsi_2.
> 
> Is this expected? I would expect the first application gets wsi_1 and 
> the second application gets wsi_2 ...

That does happen, but wsi_1 gets split as described, since h2 doesn't 
allow the network connection itself to have its own stream content... it 
has to create an explicit separate stream wsi with its own lifetime as a 
child of the network wsi.

-Andy

> Thanks
> -Xi
> 
> 
> 
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