[clangd-dev] Transport layer abstraction

Sam McCall via clangd-dev clangd-dev at lists.llvm.org
Thu Sep 13 07:15:14 PDT 2018

On Thu, Sep 13, 2018 at 3:21 PM Jan Korous <jkorous at apple.com> wrote:

> Hi Sam,
> Thanks for the response.
> I think I understand your intentions better now. The Transport abstraction
> looks good enough to be used to me so let me just explain what I though are
> imperfections that I know about (no big deals though). If these are ok with
> you I’d go ahead and work on it. Another thing is that you guys already
> have another transport layer implementation so if this abstraction fits
> nicely 2 out of 3 implementations with XPC being kind of little less
> aligned I’d consider it still pragmatic to use it.
So Transport isn't implemented or even really fully fleshed out. We should
design it (or something else) to bridge the JSON/XPC divide well, though
perfect is always hard.

Our "other transport layer" is really a different protocol, it doesn't use
ClangdLSPServer but wraps ClangdServer instead. The protocol happens to
follow LSP fairly closely (at least in spirit), but doesn't try to reuse
any of our LSP code.

Not sure whether this was the right call, but they're not likely to switch
to any Transport abstraction we add.
This is certainly an option for XCode too, but then the most obvious path
is write a wrapper around ClangdServer, and ignore

> I just wonder if this isn't actually the “reverse” of desired interface.
> It seems to me that MessageHandler is abstracting clangd implementation
> details when used in specific Transport implementation. For example it
> would help if we ever decide to swap clangd for another LSP implementation
> or clangd-ng. But it doesn’t abstract the transport layer itself. What
> benefit does it grant instead of calling for example
> ClangdLSPServer::notify() directly?
Yeah, this is mostly just ad-hoc decoupling, not real abstraction. It buys
us some things:
 - transports can be tested without involving the behavior of all of clangd
 - ClangdLSPServer's notify() can be private (using interface inheritance,
or a nested class, or something)
 - transports don't have to depend on ClangdLSPServer, if we decide to
start caring more about layering (interesting for out-of-tree transports!)

> ClangdLSPServer::run()
> IIUIC purpose of this method is to have abstraction over different
> Transport implementations with common strategies for program return value
> (ShutdownRequestReceived) and error handling (return value of
> Transport.loop()).

Well, the purpose of the *current* ClangdLSPServer::run() is set up the
dispatcher right, and to put the error handling code somewhere.
Maybe it's totally redundant and main should just call Transport.loop().

(FWIW I think the error handling on exit isn't an important part of the
protocol and we could take some shortcuts, especially in an XPC context).

> Yet since we can’t do this for XPC (as xpc_main doesn’t return) we’d have
> to duplicate the code (~10 LOC, no big deal) but it would mean that it
> wouldn’t be always possible to just blindly swap implementations of this
> interface.
> In other words it’s a violation of Liskov substitution principle which in
> itself doesn’t necessarily mean it’s a big deal - I am just wondering if it
> makes sense to have such abstraction.
So I'm not sure how big a deal this is. It would indeed be cleaner not to
have a loop() that sometimes returns and sometimes doesn't. Pragmatically,
I think we can arrange to handle it calling exit instead, I guess.

How do you want clangd shutdown to work in an XPC context? Reading the XPC
docs suggest services just run until they get SIGKILL. In which case a
Transport::loop() that never returns seems completely appropriate to me:
the transport dictates that the stream of events never ends.

This is basically what I meant that it’s difficult to have common
> abstraction over the “input direction” of transport layer (since
> Transport::MessageHandler looks more like an abstraction over
> ClangdLSPServer input handling) and because of that I’d consider different
> dispatchers instead of Transport::loop().
I'm not sure what distinction is being drawn here, I think we'll end up
with some function like loop() that returns in JSON and not with XPC.
Where is my logic confused?
 - JSONRPCDispatcher does some JSON stuff and some generic "message
handling" stuff
 - we want the "message handling" stuff to be common
 - the JSON stuff needs to be split from the message handling stuff, so it
can be replaced by XPC (in the sketch I called this part "Transport")
 - we want a common abstraction (e.g. interface) over the XPC and
JSON-specific code
 - XPC needs somewhere to call xpc_main(), which doesn't return
 - JSON needs every function to return, because it supports clean exit
 - therefore some XPC function doesn't return, when the corresponding JSON
one does
There are ways around this I guess (call xpc_main on a thread? have the
JSON code clean up and call exit()?) but they seem bad worse than just
having a transport that never stops looping...

> Cheers,
> Jan
> P. S. For the record (if it would matter in the future) XPC interface is
> callback-based:
> void XPCEntrypoint(xpc_connection_t connection) {
>   xpc_connection_set_event_handler(connection,
>     ^(xpc_object_t message) {/* C block handling the message */ }
>   );
>   xpc_connection_resume(connection);
> }
> int main() {
>   xpc_main(XPCEntrypoint);
> }
> > On Sep 12, 2018, at 11:17 PM, Sam McCall <sammccall at google.com> wrote:
> >
> > On Wed, Sep 12, 2018 at 6:20 PM Jan Korous via clangd-dev <
> clangd-dev at lists.llvm.org> wrote:
> > Hi all,
> >
> > We had an internal discussion about XPC use with clangd and the result
> is that XPC-based transport layer in clangd is a hard requirement. This
> effectively means that the adapter approach is not feasible and I am
> looking into the transport layer abstraction Sam proposed earlier (
> https://reviews.llvm.org/D49389). Sorry for the trouble!
> > No problem! (And sorry if I've been a bit unresponsive!)
> >
> > There is a difficulty with designing an abstraction over transport layer
> due to XPC API requiring client code to hand control over. It's necessary
> for XPC server to call xpc_main() with a message handling callback:
> > xpc_main(connection_handler);
> > But XPC is running it's own message loop inside xpc_main() which is
> supposed to never return (in case of failure exit() is called) which makes
> it difficult to have common interface for JSON RPC and XPC.
> > Unless I am mistaken that means that the message loop
> (Transport::loop(), ClangdLSPServer::run()) can't be part of generic code.
> >
> > I am curious what are other people's thoughts on this?
> > That makes sense to me, but I don't think a common interface is
> impossible.
> > I noticed this in your patch, and in D49389 the idea (which I didn't
> spell out) was that the XPC transport's loop() would call xpc_main and
> never return.
> > Transport::loop() is specific to a transport, it doesn't need to be part
> of generic code. ClangdLSPServer::run() ends up calling Transport::loop(),
> so it could be common code.
> >
> > It'd look something like:
> >
> > static Transport::MessageHandler *IncomingXPCMessages; // XPC -> clangd
> > static xpc_connection_t *XPCConnection; // clangd -> XPC
> >
> > XPCTransport::loop(MessageHandler &M) {
> >   IncomingXPCMessages = &M;
> >   xpc_main(&XPCEntrypoint);
> > }
> >
> > static void XPCEntrypoint(xpc_connection_t Conn) {
> >   // I don't remember how an XPC server looks
> >   // i guess you're reading editor->clangd messages in a loop?
> >   for (;;) {
> >     // lets say you parsed a notification
> >     std::string name = ...;
> >     json::Value args = xpcToJSON(...);
> >     IncomingXPCMessages->notify(name, args); // send the notification to
> clangd
> >   }
> > }
> >
> > XPCTransport::notify(StringRef name, json::Value args) {
> >   xpc_object_t XPCNotification = encodeNotificationForXPCSomehow(name,
> jsonToXPC(args))
> >   xpc_connection_send(*XPCConnection, XPCNotification);
> > }
> >
> > WDYT?
> >
> > I tried to come up with some reasonable abstraction but I am probably
> already anchored by my previous work (https://reviews.llvm.org/D48559) -
> all my design ideas tend to go in the direction of abstraction over
> JsonOutput and custom dispatchers (StdIoDispatcher/XPCDispatcher).
> >
> > Cheers,
> >
> > Jan
> > _______________________________________________
> > clangd-dev mailing list
> > clangd-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/clangd-dev
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