API: Events: Return channel id from the API discover request

This refactors msgapck_rpc_{dipatch,call} to receive the channel id as
argument. Now the discovery request returns the [id, metadata] array.

scripts/msgpack-gen.lua

@@ -111,7 +111,7 @@ end
 output:write([[
 };
 
-void msgpack_rpc_dispatch(msgpack_object *req, msgpack_packer *res)
+void msgpack_rpc_dispatch(uint64_t id, msgpack_object *req, msgpack_packer *res)
 {
   Error error = { .set = false };
   uint64_t method_id = (uint32_t)req->via.array.ptr[2].via.u64;
@@ -119,7 +119,9 @@ void msgpack_rpc_dispatch(msgpack_object *req, msgpack_packer *res)
   switch (method_id) {
     case 0:
       msgpack_pack_nil(res);
-      // The result is the `msgpack_metadata` byte array
+      // The result is the [channel_id, metadata] array
+      msgpack_pack_array(res, 2);
+      msgpack_pack_uint64(res, id);
       msgpack_pack_raw(res, sizeof(msgpack_metadata));
       msgpack_pack_raw_body(res, msgpack_metadata, sizeof(msgpack_metadata));
       return;

src/nvim/os/channel.c

@@ -157,7 +157,7 @@ static void parse_msgpack(RStream *rstream, void *data, bool eof)
                         channel->proto.msgpack.sbuffer,
                         msgpack_sbuffer_write);
     // Perform the call
-    msgpack_rpc_call(&unpacked.data, &response);
+    msgpack_rpc_call(channel->id, &unpacked.data, &response);
     wstream_write(channel->data.streams.write,
                   xmemdup(channel->proto.msgpack.sbuffer->data,
                           channel->proto.msgpack.sbuffer->size),

src/nvim/os/msgpack_rpc.c

@@ -1,3 +1,6 @@
+#include <stdint.h>
+#include <stdbool.h>
+
 #include <msgpack.h>
 
 #include "nvim/os/msgpack_rpc.h"
@@ -52,7 +55,7 @@
     free(value.items);                                                      \
   }
 
-void msgpack_rpc_call(msgpack_object *req, msgpack_packer *res)
+void msgpack_rpc_call(uint64_t id, msgpack_object *req, msgpack_packer *res)
 {
   // The initial response structure is the same no matter what happens,
   // we set it up here
@@ -107,7 +110,7 @@ void msgpack_rpc_call(msgpack_object *req, msgpack_packer *res)
   }
 
   // dispatch the message
-  msgpack_rpc_dispatch(req, res);
+  msgpack_rpc_dispatch(id, req, res);
 }
 
 void msgpack_rpc_error(char *msg, msgpack_packer *res)

src/nvim/os/msgpack_rpc.h

@@ -11,9 +11,10 @@
 /// Validates the basic structure of the msgpack-rpc call and fills `res`
 /// with the basic response structure.
 ///
+/// @param id The channel id
 /// @param req The parsed request object
 /// @param res A packer that contains the response
-void msgpack_rpc_call(msgpack_object *req, msgpack_packer *res);
+void msgpack_rpc_call(uint64_t id, msgpack_object *req, msgpack_packer *res);
 
 /// Dispatches to the actual API function after basic payload validation by
 /// `msgpack_rpc_call`. It is responsible for validating/converting arguments
@@ -21,9 +22,12 @@ void msgpack_rpc_call(msgpack_object *req, msgpack_packer *res);
 /// The implementation is generated at compile time with metadata extracted
 /// from the api/*.h headers,
 ///
+/// @param id The channel id
 /// @param req The parsed request object
 /// @param res A packer that contains the response
-void msgpack_rpc_dispatch(msgpack_object *req, msgpack_packer *res);
+void msgpack_rpc_dispatch(uint64_t id,
+                          msgpack_object *req,
+                          msgpack_packer *res);
 
 /// Finishes the msgpack-rpc call with an error message.
 ///