[Libwebsockets] Odd "ERR: Ran out of header data space" error?

Bent Bisballe Nyeng deva at aasimon.org
Fri Jun 19 20:59:22 CEST 2020

On 06/19/20 20:10, Andy Green wrote:
> On 6/19/20 6:58 PM, Bent Bisballe Nyeng wrote:
>> Thank you very much for you reply :-)
>> On 06/19/20 18:01, andy at warmcat.com wrote:
>>> On June 19, 2020 3:20:47 PM UTC, Bent Bisballe Nyeng
>>> <deva at aasimon.org> wrote:
>>>> Hi List
>>>> I am making a server based on libwebsockets which serves textual
>>>> updates
>>>> to all connected user based on user input, akin to the drawing example
>>>> application; ie. everybody gets notificed about all changes.
>>>> My application uses a textual protocol and these messages can sometimes
>>>> be ~16kb and above.
>>>> The system on which I am developing is based on libwebsocket 2.1 and
>>>> everything seem to be working fine.
>>>> Recently, however, I deployed the service on a debian server for
>>>> borader
>>>> use, which has libwebsocket 2.0 installed and then I started seeing the
>>> 2.0... it's very old.
>> Yes I thought so as well - but this is for some reason the version that
>> Debian is currently shipping... ;)
> Hum.
>>>> error: "ERR: Ran out of header data space" a lot.
>>>> I traced it to the max_http_header_data parameter which in my
>>>> application was unset (ie. 0) and increasing it to 16kb seemed to
>>>> somewhat mitigate the issue.
>>>> I do not, however, understand why I am seeing that error in the first
>>>> place, shouldn't that parameter only affect http connections?
>>> I assume it's websockets... all ws connections start off as http. 
>>> And the server catches and stores any headers it understands, but it
>>> is not in control of, eg, how big the cookies are a browser will send
>>> it.  At some point I set the default size to 4KB it'd make sense if
>>> before v2.1 it was smaller.
>> So what you are saying is that it might be the size of my cookies which
>> is causing my problem and not as I initially thought the payload size of
>> my protocol ...
>> It does make sense since the number of cookies are growing linearly with
>> the size of my payload.
> It's the size of the header that is being sent from the client, that
> includes whatever cookies you have.  Since we discard headers we don't
> understand, usually the only big thing taking up space in there is
> cookies (whose header we do understand and capture the data).
>> Are there any limitations to how much data can be transmitted in one
>> lws_write call to a client?
> Basically we do not control how much of what you want to send the OS
> will accept on the connection, it will literally 'pick a number' and if
> you have more, it will just leave it unsent.
> For a long time (I am not sure if in v2.0) lws will notice it and copy
> it on the heap, and send it later in the background, concealing
> WRITEABLE until it's all gone.  So basically it will deal with this
> situation, but because it's relatively expensive in time and heap to do
> that, you generally want to avoid it... the times the kernel surprises
> you lws will conceal it, but you only want to be giving it chunks that
> it can immediately consume, generally.  That number is "a few MTU", it
> depends on the system and connection, a couple of KB is usually OK.
> You can send that and ask for another writeable callback if more to send
> until you sent everything.  For ws, there is a helper to help you
> calculate the flags to give lws_write when producing these message
> fragments
> https://libwebsockets.org/git/libwebsockets/tree/include/libwebsockets/lws-write.h#n221-246

Converting all the cookies to localStorage has solved the issue!

I will look into splitting the messages so they won't keep on growingin
size as well.

Thank you wery much for your help :-)

Cheers // Bent

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