[Libwebsockets] How to set the "user" callback parameter for adopted sockets?

Andy Green andy at warmcat.com
Thu Nov 24 16:45:59 CET 2016



On November 24, 2016 11:10:15 PM GMT+08:00, Andy Green <andy at warmcat.com> wrote:

Forwarding to the list, since rhat mangles the list address in replies.

>On November 24, 2016 10:42:02 PM GMT+08:00, Alan Conway
><aconway at redhat.com> wrote:
>>On Wed, 2016-11-23 at 23:19 +0800, Andy Green wrote:
>>> 
>>> On November 23, 2016 10:53:57 PM GMT+08:00, Alan Conway
><aconway at redh
>>> at.com> wrote:
>>> > 
>>> > On Wed, 2016-11-23 at 09:46 -0500, Alan Conway wrote:
>>> > > 
>>> > > On Wed, 2016-11-23 at 08:50 +0800, Andy Green wrote:
>>> > > > 
>>> > > > 
>>> > > > On Tue, 2016-11-22 at 19:02 -0500, Alan Conway wrote:
>>> > > > > 
>>> > > > > 
>>> > > > > 
>>> > > > > I'm making progress! I can crash my router from a web brower
>>> > > > > :)
>>> > > > > 
>>> > > > > I can't figure out how to set the "user" parameter for lws
>>> > > > > callbacks
>>> > > > > for a socket adopted with lws_adopt_socket(). I can access
>>> > > > > the
>>> > > > 
>>> > > > That's not what *user_space is for.
>>> > > > 
>>> > > > It's for protocol-specific local data, per-connection.  When
>>> > > > the
>>> > > > protocol changes, which it may often do with a http/1.1
>>> > > > keepalive
>>> > > > connection visiting urls that map on to different plugins, the
>>> > > > user
>>> > > > data allocation is destroyed by lws and a new one sized
>>> > > > appropriately
>>> > > > for the new protocol allocated.
>>> > > 
>>> > > For me, the protocol starts as "http" and may change to "binary"
>>> > > or
>>> > > "amqp" (which I treat as both "amqp"). Does that mean that if I
>>> > > store
>>> > > context for the http protocol, it will be wiped out in the
>>> > > upgrade?
>>> > > Is
>>> > > there any way to preserve some per-connection data between the
>>> > > two or
>>> > > is each protocol session treated as entirely separate from
>>> > > previous
>>> > > ones?
>>> > > 
>>> > > > 
>>> > > > 
>>> > > > 
>>> > > > > 
>>> > > > > 
>>> > > > > 
>>> > > > > allocated space *after* the adopt call with lws_wsi_user(),
>>> > > > > but
>>> > > > > lws_adopt_socket() itself is firing my callbacks so that's
>>> > > > > too
>>> > > > > late.
>>> > > > > 
>>> > > > > The docs mention setting up your session data on the
>>> > > > > LWS_CALLBACK_ESTABLISHED event, but at that point I have
>>> > > > > nothing
>>> > > > > I
>>> > > > > can
>>> > > > > use to find data from my application.
>>> > > > 
>>> > > > LWS doesn't have this concept of external shadowed data per-
>>> > > > connection
>>> > > > built-in.
>>> > > > 
>>> > > > You have two ways to do it
>>> > > > 
>>> > > >  - add a new opaque void * in struct lws and set it at adopt-
>>> > > > time,
>>> > > > provide an accessor to get it from the wsi.  This is a bit of
>a
>>> > > > burden
>>> > > > for everyone who doesn't care about this then.
>>> > > 
>>> > > For current purposes I need to use a released, packaged version
>>> > > of
>>> > > lws
>>> > > so modifying it isn't an option - I do hope to make
>contributions
>>> > > later...
>>> > > 
>>> > > > 
>>> > > > 
>>> > > >  - Set the context user pointer at context creation time.  You
>>> > > > can
>>> > > > get
>>> > > > this from a wsi in the callback.  You can then use this
>>> > > > context-
>>> > > > wide
>>> > > > pointer to dereference the wsi pointer to your private
>>> > > > connection
>>> > > > pointer on your side, entirely outside of lws.
>>> > > 
>>> > > I could use either the fd or the wsi pointer to look up my
>>> > > application
>>> > > data, but it's an expensive lookup that requires a map of all
>the
>>> > > known
>>> > > fds/wsi-pointers. I could use thread-local storage as a hack to
>>> > > get
>>> > > my
>>> > > context into the initial "http" established callback, but that
>>> > > doesn't
>>> > > help if it will get wiped out in the upgrade to "amqp"... Hmmm.
>>> > > 
>>> > 
>>> > What if I use a separate context per connection? I know you said I
>>> 
>>> No, that is a very bad idea.
>>
>>Why? What will go wrong?
>
>Did you look at what goes on in the context + vhost structs you will be
>spawning one of per connection to see why it's a bad idea?
>
>If you still think it's a good idea, knock yourself out...
>
>-Andy




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