make Watchman.request return a result

Summary:
this stack converts all of the functions in watchman.ml to return results instead of throwing exceptions, so that we can ensure we handle all the error cases.

during the transition, we will call `raise_error` to maintain exception behavior, but eventually thread the `result` further and further up and eliminate `try/with`'s.

Reviewed By: nmote

Differential Revision: D28169687

fbshipit-source-id: 668e6a43460f4eba3afdbaea398d62fc20276289

src/hack_forked/watchman/watchman.ml

@@ -27,8 +27,14 @@ exception Watchman_restarted
 type error_severity =
   | Not_installed of { path: string }
   | Socket_unavailable of { msg: string }
+  | Response_error of {
+      request: string option;
+      response: string;
+      msg: string;
+    }
 
 let log_error ?request ?response msg =
+  let response = Option.map ~f:(String_utils.truncate 100000) response in
   let () = EventLogger.watchman_error ?request ?response msg in
   let () = Hh_logger.error "%s" msg in
   ()
@@ -41,6 +47,9 @@ let raise_error = function
   | Socket_unavailable { msg } ->
     let () = log_error msg in
     raise (Watchman_error msg)
+  | Response_error { request; response; msg } ->
+    let () = log_error ?request ~response msg in
+    raise (Watchman_error msg)
 
 type subscribe_mode =
   | All_changes
@@ -378,7 +387,7 @@ let with_watchman_conn f =
     in
     let%lwt () = close_connection conn in
     Lwt.return result
-  | Error err -> raise_error err
+  | Error _ as err -> Lwt.return err
 
 (* Sends a request to watchman and returns the response. If we don't have a connection,
  * a new connection will be created before the request and destroyed after the response *)
@@ -387,12 +396,17 @@ let rec request ~debug_logging ?conn json =
   | None -> with_watchman_conn (fun conn -> request ~debug_logging ~conn json)
   | Some (reader, oc) ->
     let%lwt () = send_request ~debug_logging oc json in
-    let%lwt line = Buffered_line_reader_lwt.get_next_line reader in
-    (match parse_response ~debug_logging line with
-    | Ok response -> Lwt.return response
+    let%lwt response = Buffered_line_reader_lwt.get_next_line reader in
+    (match parse_response ~debug_logging response with
+    | Ok _ as result -> Lwt.return result
     | Error msg ->
-      EventLogger.watchman_error ~request:(Hh_json.json_to_string json) ~response:line msg;
-      raise (Watchman_error msg))
+      let request = Some (Hh_json.json_to_string json) in
+      Lwt.return (Error (Response_error { request; response; msg })))
+
+let request_exn ~debug_logging ?conn json =
+  match%lwt request ~debug_logging ?conn json with
+  | Ok result -> Lwt.return result
+  | Error err -> raise_error err
 
 let blocking_read ~debug_logging ~conn:(reader, _) =
   let%lwt () =
@@ -449,7 +463,7 @@ let has_watchman_restarted_since ~debug_logging ~conn ~watch_root ~clockspec =
             ];
         ])
   in
-  let%lwt response = request ~debug_logging ~conn query in
+  let%lwt response = request_exn ~debug_logging ~conn query in
   let result =
     match Hh_json_helpers.Jget.bool_opt (Some response) "is_fresh_instance" with
     | Some has_restarted -> Ok has_restarted
@@ -495,7 +509,7 @@ let get_clockspec ~debug_logging ~conn ~watch_root prior_clockspec =
       Hh_logger.error "%s" err;
       raise Exit.(Exit_with Watchman_failed))
   | None ->
-    let%lwt response = request ~debug_logging ~conn (clock watch_root) in
+    let%lwt response = request_exn ~debug_logging ~conn (clock watch_root) in
     Lwt.return (J.get_string_val "clock" response)
 
 (** Watch this root
@@ -510,7 +524,7 @@ let watch_project ~debug_logging ~conn root =
   let%lwt response =
     catch
       ~f:(fun () ->
-        let%lwt response = request ~debug_logging ~conn query in
+        let%lwt response = request_exn ~debug_logging ~conn query in
         Lwt.return (Some response))
       ~catch:(fun exn ->
         (match Exception.to_exn exn with
@@ -613,7 +627,7 @@ let re_init
     }
   in
   let%lwt response =
-    request ~debug_logging ~conn (subscribe ~mode:subscribe_mode ~states:defer_states env)
+    request_exn ~debug_logging ~conn (subscribe ~mode:subscribe_mode ~states:defer_states env)
   in
   ignore response;
   Lwt.return env
@@ -813,7 +827,9 @@ let get_mergebase_and_changes instance =
     ~on_dead:(fun _dead_env -> Error "Failed to connect to Watchman to get mergebase")
     ~on_alive:(fun env ->
       let%lwt response =
-        request ~debug_logging:env.settings.debug_logging (get_changes_since_mergebase_query env)
+        request_exn
+          ~debug_logging:env.settings.debug_logging
+          (get_changes_since_mergebase_query env)
       in
       let result =
         match extract_mergebase response with