Summary -
On the server-side, we are changing the point at which we decide whether
a method is registered or not from the surface to the transport at the
point where we are done receiving initial metadata and before we invoke
the recv_initial_metadata_ready closures from the filters. The main
motivation for this is to allow filters to check whether the incoming
method is a registered or not. The exact use-case is for observability
where we only want to record the method if it is registered. We store
the information about the registered method in the initial metadata.
On the client-side, we also set information about whether the method is
registered or not in the outgoing initial metadata.
Since we are effectively changing the lookup point of the registered
method, there are slight concerns of this being a potentially breaking
change, so we are guarding this with an experiment to be safe.
Changes -
* Transport API changes -
* Along with `accept_stream_fn`, a new callback
`registered_method_matcher_cb` will be sent down as a transport op on
the server side. When initial metadata is received on the server side,
this callback is invoked. This happens before invoking the
`recv_initial_metadata_ready` closure.
* Metadata changes -
* We add a new non-serializable metadata trait `GrpcRegisteredMethod()`.
On the client-side, the value is a uintptr_t with a value of 1 if the
call has a registered/known method, or 0, if it's not known. On the
server side, the value is a (ChannelRegisteredMethod*). This metadata
information can be used throughout the stack to check whether a call is
registered or not.
* Server Changes -
* When a new transport connection is accepted, the server sets
`registered_method_matcher_cb` along with `accept_stream_fn`. This
function checks whether the method is registered or not and sets the
RegisteredMethod matcher in the metadata for use later.
* Client Changes -
* Set the metadata on call creation on whether the method is registered
or not.
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