Re: [Libevent-users] out-of-order writes in an evbuffer

[Adrian Chadd <adrian@xxxxxxxxxxxxxxx>]

... implement iovec writing to evbuffers.

That way he can assemble a set of iovecs, then hand them all off to
evbuffer when it's ready.







adrian

On 21 May 2013 16:25, Nick Mathewson <nickm@xxxxxxxxxxxxx> wrote:
> On Tue, May 21, 2013 at 4:58 PM, Adrian Chadd <adrian@xxxxxxxxxxxxxxx> wrote:
>> You can do a couple of things:
>>
>> * you can hack up the serialisation and buffering stuff so yes, it
>> injects that length value into an already-queued buffer;
>
> I think some kind of hack like this is what you're going to have to
> do, if you truly can't calculate the length in advance: reserve the
> four bytes, fill them with 0s, but keep a pointer to them that you
> populate later.
>
> This is *not allowed* according to the current evbuffer
> specifications, though it should likely work in practice. You'll need
> to skim the evbuffer code to see what can resize or remove an existing
> chunk.  I can't promise that this will work in all scenarios, or that
> it will work with future versions of Libevent: you'll need to make
> sure that your tests can handle checking all the assumptions you're
> depending on.
>
> If you can come up with any interface improvements to Libevent that
> might help with this, that would totally be another viable approach to
> take.  There is currently a "pinning" feature internally in evbuffers
> that can be used to tell Libevent not to reallocate or free a given
> chunk until it is unpinned -- but that feature is only designed for
> making iocp function correctly, and probably needs more work before it
> can be a general mechanism for applications to use.
>
>   You'll also need to do something to keep the four 0 bytes from
> getting flushed to the network before you replace them with the real
> values.
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