LSP: reply MethodNotFound to unknown requests

Author: ciaran-matthew-dunne

src/lsp/lp_lsp.ml

@@ -817,7 +817,16 @@ let dispatch_message ofmt dict =
   | "initialized" ->
     ()
   | msg ->
-    LIO.log_error "no_handler" msg
+    (* Requests carry an id; notifications don't. For requests we must
+       reply with JSON-RPC MethodNotFound so the client doesn't wait
+       forever. Notifications get logged and dropped, matching the
+       spec's "no response" rule. [oint_field] defaults absent id to 0,
+       which we treat as the notification sentinel. *)
+    LIO.log_error "no_handler" msg;
+    if id <> 0 then
+      LIO.send_json ofmt
+        (LSP.mk_error_reply ~id ~code:(-32601)
+           ~msg:("Method not found: " ^ msg))
 
 let process_input ofmt (com : J.t) =
   try dispatch_message ofmt (U.to_assoc com)

src/lsp/lsp_base.ml

@@ -37,6 +37,13 @@ let _parse_uri str =
 let mk_reply ~id ~result =
   `Assoc [ "jsonrpc", `String "2.0"; "id",     `Int id;   "result", result ]
 
+(* JSON-RPC error reply. Codes follow the JSON-RPC 2.0 spec (see also
+   LSP §error-codes): -32601 MethodNotFound is the one we reach for. *)
+let mk_error_reply ~id ~code ~msg =
+  `Assoc [ "jsonrpc", `String "2.0";
+           "id",      `Int id;
+           "error",   `Assoc ["code", `Int code; "message", `String msg] ]
+
 let mk_event m p   =
   `Assoc [ "jsonrpc", `String "2.0"; "method", `String m; "params", `Assoc p ]
 

src/lsp/lsp_base.mli

@@ -35,6 +35,10 @@ val mk_range_of_interval : Range.t -> J.t
 
 val mk_reply : id:int -> result:J.t -> J.t
 
+val mk_error_reply : id:int -> code:int -> msg:string -> J.t
+(** JSON-RPC error reply. Codes follow the JSON-RPC 2.0 spec; [-32601]
+    is [MethodNotFound]. *)
+
 val mk_diagnostics
   : uri:string
   -> version: int