make Watchman.call_on_instance callbacks 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.

this is starting to get to the meat of this stack. `call_on_instance` catches most of the exceptions raised by `raise_error` and marks the watchman instance as dead so it can be retried. this diff sets us up to get rid of the `catch` and instead just handle the `result`.

Reviewed By: nmote

Differential Revision: D28173783

fbshipit-source-id: 2817f87f3e84e121bc1f4de6bf9ddc1dbcf5569b

src/hack_forked/watchman/watchman.ml

@@ -760,13 +760,14 @@ let call_on_instance :
     watchman_instance ->
     string ->
     on_dead:(dead_env -> 'a) ->
-    on_alive:(env -> (env * 'a) Lwt.t) ->
+    on_alive:(env -> (env * 'a, error_severity) Result.t Lwt.t) ->
     (watchman_instance * 'a) Lwt.t =
   let on_alive' ~on_dead source f env =
     catch
       ~f:(fun () ->
-        let%lwt (env, result) = f env in
-        Lwt.return (Watchman_alive env, result))
+        match%lwt f env with
+        | Ok (env, result) -> Lwt.return (Watchman_alive env, result)
+        | Error err -> raise_error err)
       ~catch:(fun exn ->
         Hh_logger.exception_ ~prefix:("Watchman " ^ source ^ ": ") exn;
         let close_channel_on_instance' env =
@@ -850,31 +851,35 @@ let get_changes instance =
     ~on_alive:(fun env ->
       let debug_logging = env.settings.debug_logging in
       match%lwt blocking_read ~debug_logging ~conn:env.conn with
+      | Error _ as err -> Lwt.return err
       | Ok response ->
         (match transform_asynchronous_get_changes_response env response with
-        | Ok (env, result) -> Lwt.return (env, Watchman_pushed result)
-        | Error err -> raise_error err)
-      | Error err -> raise_error err)
+        | Error _ as err -> Lwt.return err
+        | Ok (env, result) -> Lwt.return (Ok (env, Watchman_pushed result))))
 
 let get_mergebase_and_changes instance =
   call_on_instance
     instance
     "get_mergebase_and_changes"
     ~on_dead:(fun _dead_env -> Error "Failed to connect to Watchman to get mergebase")
     ~on_alive:(fun env ->
-      let%lwt response =
-        request_exn
-          ~debug_logging:env.settings.debug_logging
-          (get_changes_since_mergebase_query env)
-      in
-      let result =
-        match extract_mergebase response with
-        | Some (_clock, mergebase) ->
-          let changes = extract_file_names env response in
-          (env, Ok (mergebase, changes))
-        | None -> (env, Error "Failed to extract mergebase from response")
-      in
-      Lwt.return result)
+      let debug_logging = env.settings.debug_logging in
+      let query = get_changes_since_mergebase_query env in
+      match%lwt request ~debug_logging query with
+      | Error _ as err -> Lwt.return err
+      | Ok response ->
+        let result =
+          match extract_mergebase response with
+          | Some (_clock, mergebase) ->
+            let changes = extract_file_names env response in
+            Ok (env, Ok (mergebase, changes))
+          | None ->
+            let msg = "Failed to extract mergebase" in
+            let request = Some (Hh_json.json_to_string query) in
+            let response = Hh_json.json_to_string response in
+            Error (Response_error { request; response; msg })
+        in
+        Lwt.return result)
 
 module Testing = struct
   type nonrec env = env