[Libwebsockets] Truncated send handled by user not working as expected

nil100 at ig.com.br nil100 at ig.com.br
Mon Aug 18 14:07:40 CEST 2014


Em 18/08/2014 08:25, Andy Green escreveu: 

> On 18 August 2014 19:06:42 GMT+08:00, nil100 at ig.com.brwrote:
> Em 17/08/2014 21:56, Andy Green escreveu: On 18 August 2014 07:34:47 GMT+08:00, nil100 at ig.com.brwrote: Hello everybody, I'm using the library and in my case I want to
 handle truncated send myself but I ran into a situation. Linux's send
function is returning -2 which in turn is causing libwebsockets to
disregard my wish to handle truncated sends and consequently causes an
assert failure later on. 

> I'm not sure what your plan is for dealing with it in user code. I'm usually sending a large amount of data at a time and so I want to
 know when the pipe chokes and how many truncated sends it takes to
completely send my whole chunk. 

>> Everything is happening in file output.c at line 130 which is the
 return from linux's native send (I'm not using SSL so
lws_ssl_capable_write is actually lws_ssl_capable_write_no_ssl). Now
when this returns LWS_SSL_CAPABLE_MORE_SERVICE (value -2) then the value
of variable "n" 

> So looking at the code, the actual send() didn't send anything just
 came back with -EAGAIN or similar. Then that function translates that
to a generic "I didn't send anything because I would have blocked" enum
return you mentioned. 

>> is set to 0 which causes the condition at line 172 (n &&
 wsi->u.ws.clean_buffer) to be false and continue execution as if 

> Yes I see. 
>> libwebsockets was handling truncates and consequent calls to fail
 assertion at line 112. 

> I don't see why it fails that assertion though, did you understand
 why during your debugging? Not having sent anything and buffer the
whole thing should have been okay. 

> So here the code enters the condition at line 107-110 because
 wsi->truncated_send_len && buf > (wsi->truncated_send_malloc +
wsi->truncated_send_len + wsi->truncated_send_offset). Now, the library
was not handling truncated sends in the first place (I configured it not
to)but it erroneously started handling after send returned an error 

I dunno what this "configured it not to" thing is. I don't see anything
like that in the code (maybe I missed it).

> and since I don't expect the library to do that I call libwebsocket_write again which makes the library believe I'm trying to send new data when in fact I'm sending the rest of the data too.

Yes -- that's exactly what's happening.

So if you were OK with the lws buffering (for sake of argument) you just
need to open your sending loop up to the service loop.

Because the service loop is the only guy who can regulate flushing the
partial buffer without blocking.

Actually if you want to handle it, you have the same issue. If you just
sit there spamming you will also block, which makes your code only
useful for one connection or one socket fd at a time.

> I'm a new libwebsockets user and wasn't sure how to proceed in
 getting this fixed. 

> There seems to be two separate issues here, one is you don't want it
 to buffer things if nothing got sent, and the other is you get an
assert firing when it does. 

> Correct. I don't want it to buffer things and the assert is firing
 because the library started handling things without me wanting it to. 

> If you remove the n && from line 172 it might do something towards
 what you want. But I am not sure if that leaves whatever blew the
assert just waiting to blow the assert another time. 

> What blew the assert was wsi->truncated_send_len not being 0 when it
 should have been. Sorry for the bad formatting. My webmail is crap. 

Yeah it's pretty broken but it's fine as a text mode reply actually so
no worries.

I think it's maybe a bit more complicated than you are thinking.

1) What's actually sent on the wire may include protocol prepending for
websocket. That's tricky to account for but not impossible. So the stuff
at the send() is a different length than what you sent in your user

2) If there's an extension like compression, which is commonly
negotiated by browsers, what gets sent on the wire - to the send() - is
totally unrelated in size or content to what you are sending from your
user code.

If you heard he managed to send 1KByte of the compressed version, that's
completely worthless information to you because you don't know how big
the remainder of the compressed version is or what it was, from the user
code with the uncompressed buffer.

These reasons are why we ended up with in-lws transparent buffering,
because especially the second one has no real solution otherwise.


> Cheers, Nilson N. da Silva

Yes... I understand that there's more to sending over websockets than
just wiring to a socket and for that libwebsockets is very good (ofc
it's more than just websockets and thanks for creating it BTW). 

So what I originally intended to do was something like the following: 

int m = 0;
 size_t len = mybuffer_length;
 unsigned char *msg = mybuffer + LWS_SEND_BUFFER_PRE_PADDING;
 do {
 int l = len - m;
 unsigned char *p = msg + m;
 m += libwebsocket_write(wsi, p, l, LWS_WRITE_TEXT);
 do something so that I know send was truncated and tell me
 how many iterations it took to complete the whole chunk
 and how much was sent on each
} while(m < len); 

Now with the help Roger's comment if I have "if
(lws_send_pipe_choked(wsi)) continue;" inside this loop I get rid of the
assertion failure, but it seems libwebsockets is still handling partials
(I can see it the lib's logs). 

Thank you guys for the discussion. 


Nilson N. da Silva 
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