Implement completion request handler logic

Summary:
# Overview
Refer to D47564289

This stack of diff will be split into the following steps:
1. Setup `CompletionItem` and helper signatures
2. Implement attribute completion core logic given file and position and test
3. Update symbol traversal logic to be right inclusive if necessary (to handle cursor at end of line case)
4. Implement prefix filtering and test
5. Implement localBasedLookup completion public endpoint
6. **Extend Pyre `requestHandler.ml` with handler for completion request**

Reviewed By: kinto0

Differential Revision: D47636309

fbshipit-source-id: 8f65a2b4e45e6095786a410b1e2a613df9e547d9

source/code_navigation_server/requestHandler.ml

@@ -135,6 +135,46 @@ let handle_location_of_definition
   >>| fun definitions -> Response.(LocationOfDefinition { definitions })
 
 
+let get_completion_for_module ~overlay ~position module_reference =
+  let open Option in
+  let type_environment = ErrorsEnvironment.ReadOnly.type_environment overlay in
+  LocationBasedLookup.completion_info_for_position ~type_environment ~module_reference position
+  >>| Ast.Identifier.SerializableMap.bindings
+  >>| List.map ~f:(fun (identifier, attribute) ->
+          let attribute_kind =
+            match Ast.Node.value attribute with
+            | { ClassSummary.Attribute.kind = Simple _; _ } ->
+                Response.CompletionItem.CompletionItemKind.Simple
+            | { ClassSummary.Attribute.kind = Method _; _ } ->
+                Response.CompletionItem.CompletionItemKind.Method
+            | { ClassSummary.Attribute.kind = Property _; _ } ->
+                Response.CompletionItem.CompletionItemKind.Property
+          in
+          { Response.CompletionItem.label = identifier; kind = attribute_kind })
+
+
+let get_completion_in_overlay ~overlay ~build_system ~position path =
+  let open Result in
+  let module_tracker = ErrorsEnvironment.ReadOnly.module_tracker overlay in
+  get_modules ~module_tracker ~build_system path
+  >>| List.filter_map ~f:(get_completion_for_module ~overlay ~position)
+
+
+let handle_completion
+    ~path
+    ~position
+    ~client_id
+    { State.environment; build_system; client_states; _ }
+  =
+  let open Result in
+  get_overlay_id ~path ~client_id client_states
+  >>= fun overlay_id ->
+  let overlay = get_overlay ~environment overlay_id in
+  get_completion_in_overlay ~overlay ~build_system ~position path
+  >>| List.concat
+  >>| fun completions -> Response.(Completion { completions })
+
+
 let handle_superclasses
     ~class_:{ Request.ClassExpression.module_; qualified_name }
     { State.environment; _ }
@@ -468,11 +508,9 @@ let handle_query
         Lwt.return response
       in
       Server.ExclusiveLock.read state ~f
-  | Request.Query.Completion _ ->
-      let f _state =
-        let response =
-          Response.Error (Response.ErrorKind.InvalidRequest "Autocomplete not yet implemented")
-        in
+  | Request.Query.Completion { path; position; client_id } ->
+      let f state =
+        let response = handle_completion ~path ~position ~client_id state |> response_from_result in
         Lwt.return response
       in
       Server.ExclusiveLock.read state ~f