[ProcessPool] Prevent workers starvation

Summary: The refactor from using 1 pipe for all worker-to-master communication to `n` (one per worker) introduced the possibility of starving workers because the master process read all the messages from one pipe (refreshing the file descriptors to read from with `Unix.select`) before moving to the next one. These changes aim to prevent that by reading one message from all available pipes before refreshing the file descriptors to read from.

Reviewed By: ngorogiannis

Differential Revision: D20194924

fbshipit-source-id: 91a0fbc47

infer/src/base/ProcessPool.ml

@@ -147,7 +147,7 @@ let wait_for_updates pool buffer =
     match read_fds with
     | [] ->
         (* no updates, break loop *) acc
-    | file_descr :: _ ->
+    | _ ->
         (* Read one OCaml value at a time. This is done by first reading the header of the marshalled
            value (fixed size), then get the total size of the data from that header, then request a
            read of the full OCaml value.
@@ -161,10 +161,15 @@ let wait_for_updates pool buffer =
            as much as possible eagerly. This can empty the pipe without us having a way to tell that
            there is more to read anymore since the [select] call will return that there is nothing to
            read. *)
+        let messages =
+          (* Read one message from each file descriptor for fairness *)
+          List.fold read_fds ~init:acc ~f:(fun msgs_acc file_descr ->
               really_read file_descr ~buf:buffer ~len:Marshal.header_size ;
               let data_size = Marshal.data_size buffer 0 in
               really_read file_descr ~buf:buffer ~pos:Marshal.header_size ~len:data_size ;
-        aux (Marshal.from_bytes buffer 0 :: acc) ~timeout:`Immediately
+              Marshal.from_bytes buffer 0 :: msgs_acc )
+        in
+        aux messages ~timeout:`Immediately
   in
   aux [] ~timeout:refresh_timeout |> List.rev