CP-52708: Avoid making Unix read/write calls for internal API calls: forward the API call directly like we do with the CLI for calls to the coordinator when we are the coordinator

Signed-off-by: Edwin Török <edwin.torok@cloud.com>

Author: edwintorok

ocaml/xapi/helpers.ml

@@ -402,6 +402,11 @@ module TraceHelper = struct
     Tracing_propagator.Propagator.Http.inject_into trace_context
 end
 
+(** Once the server functor has been instantiated, xapi sets this reference to the appropriate
+    "fake_rpc" (loopback non-HTTP) rpc function. 
+    This way, internally the coordinator can short-circuit API calls without having to go over the network. *)
+let rpc_fun : (Http.Request.t -> Rpc.call -> Rpc.response) option ref = ref None
+
 (* Note that both this and `make_timeboxed_rpc` are almost always
  * partially applied, returning a function of type 'Rpc.request -> Rpc.response'.
  * The body is therefore not evaluated until the RPC call is actually being
@@ -418,18 +423,22 @@ let make_rpc ~__context rpc : Rpc.response =
   in
   let http = xmlrpc ~subtask_of ~version:"1.1" path in
   let http = TraceHelper.inject_span_into_req tracing http in
-  let transport =
-    if Pool_role.is_master () then
-      Unix Xapi_globs.unix_domain_socket
-    else
-      SSL
-        ( SSL.make ~use_stunnel_cache:true ~verify_cert:(Stunnel_client.pool ())
-            ()
-        , Pool_role.get_master_address ()
-        , !Constants.https_port
-        )
-  in
-  dorpc ~srcstr:"xapi" ~dststr:"xapi" ~transport ~http rpc
+  match !rpc_fun with
+  | Some rpcfun when Pool_role.is_master () ->
+      rpcfun http rpc
+  | _ ->
+      let transport =
+        if Pool_role.is_master () then
+          Unix Xapi_globs.unix_domain_socket
+        else
+          SSL
+            ( SSL.make ~use_stunnel_cache:true
+                ~verify_cert:(Stunnel_client.pool ()) ()
+            , Pool_role.get_master_address ()
+            , !Constants.https_port
+            )
+      in
+      dorpc ~srcstr:"xapi" ~dststr:"xapi" ~transport ~http rpc
 
 let make_timeboxed_rpc ~__context timeout rpc : Rpc.response =
   let subtask_of = Ref.string_of (Context.get_task_id __context) in

ocaml/xapi/xapi.ml

@@ -159,11 +159,14 @@ let random_setup () =
   finally (fun () -> really_input chan s 0 n) (fun () -> close_in chan) ;
   Random.full_init (Array.init n (fun i -> Char.code (Bytes.get s i)))
 
+let fake_rpc2 req rpc = Api_server.Server.dispatch_call req None rpc
+
 let register_callback_fns () =
   let fake_rpc req sock xml : Rpc.response =
     Api_server.callback1 false req sock xml
   in
   Xapi_cli.rpc_fun := Some fake_rpc ;
+  Helpers.rpc_fun := Some fake_rpc2 ;
   Message_forwarding.register_callback_fns ()
 
 let noevents = ref false