catch exceptions when reading watchman responses

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: D28169686

fbshipit-source-id: f647ca5c2daac7ec61af1163d3d66e8e3e280a03

src/hack_forked/watchman/watchman.ml

@@ -394,6 +394,21 @@ let with_watchman_conn f =
     Lwt.return result
   | Error _ as err -> Lwt.return err
 
+let read_line reader =
+  try%lwt
+    let%lwt line = Buffered_line_reader_lwt.get_next_line reader in
+    Lwt.return (Ok line)
+  with
+  | End_of_file ->
+    let msg = "Connection closed (end of file)" in
+    Lwt.return (Error (Socket_unavailable { msg }))
+  | Unix.Unix_error (Unix.EBADF, _, _) ->
+    let msg = "Connection closed (bad file descriptor)" in
+    Lwt.return (Error (Socket_unavailable { msg }))
+  | Unix.Unix_error (Unix.ECONNRESET, _, _) ->
+    let msg = "Connection closed (connection reset)" in
+    Lwt.return (Error (Socket_unavailable { msg }))
+
 (* 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 *)
 let rec request ~debug_logging ?conn json =
@@ -402,12 +417,14 @@ let rec request ~debug_logging ?conn json =
   | Some (reader, oc) ->
     (match%lwt send_request ~debug_logging oc json with
     | Ok () ->
-      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 ->
-        let request = Some (Hh_json.json_to_string json) in
-        Lwt.return (Error (Response_error { request; response; msg })))
+      (match%lwt read_line reader with
+      | Ok response ->
+        (match parse_response ~debug_logging response with
+        | Ok _ as result -> Lwt.return result
+        | Error msg ->
+          let request = Some (Hh_json.json_to_string json) in
+          Lwt.return (Error (Response_error { request; response; msg })))
+      | Error _ as err -> Lwt.return err)
     | Error _ as err -> Lwt.return err)
 
 let request_exn ~debug_logging ?conn json =
@@ -427,20 +444,17 @@ let blocking_read ~debug_logging ~conn:(reader, _) =
          promises. *)
       raise (Watchman_error "Connection closed")
   in
-  let%lwt output =
-    let read_timeout = 40.0 in
-    try%lwt
-      Lwt_unix.with_timeout read_timeout @@ fun () -> Buffered_line_reader_lwt.get_next_line reader
-    with
-    | Lwt_unix.Timeout ->
-      raise (Watchman_error (spf "Timed out reading payload after %f seconds" read_timeout))
-    | End_of_file -> raise (Watchman_error "Connection closed")
-  in
-  match parse_response ~debug_logging output with
-  | Ok response -> Lwt.return response
-  | Error msg ->
-    EventLogger.watchman_error ~response:(String_utils.truncate 100000 output) msg;
-    raise (Watchman_error msg)
+  let read_timeout = 40.0 in
+  match%lwt Lwt_unix.with_timeout read_timeout @@ fun () -> read_line reader with
+  | exception Lwt_unix.Timeout ->
+    raise (Watchman_error (spf "Timed out reading payload after %f seconds" read_timeout))
+  | Error err -> raise_error err
+  | Ok output ->
+    (match parse_response ~debug_logging output with
+    | Ok response -> Lwt.return response
+    | Error msg ->
+      EventLogger.watchman_error ~response:(String_utils.truncate 100000 output) msg;
+      raise (Watchman_error msg))
 
 (****************************************************************************)
 (* Initialization, reinitialization *)