Wait for file to exist and have non-zero size.

Author: tmcgilchrist

lib/olly_common/launch.ml

@@ -56,14 +56,21 @@ let exec_process (config : runtime_events_config) (argsl : string list) :
       raise
         (Fail (Printf.sprintf "executable %s not found" executable_filename))
   in
-  (* Poll until the child's .events ring buffer file appears.
-     The file is created by the OCaml runtime at startup, but under
-     load (e.g. parallel dune builds) process creation can be slow. *)
+  (* Poll until the child's .events ring buffer is ready.
+     The OCaml runtime creates the file then writes the ring buffer header,
+     so we must wait for the file to exist AND have non-zero size.
+     Under load (e.g. parallel dune builds) process creation can be slow. *)
   let ring_file = Filename.concat dir (string_of_int child_pid ^ ".events") in
   let timeout = 5.0 in
   let poll_interval = 0.05 in
   let deadline = Unix.gettimeofday () +. timeout in
-  while (not (Sys.file_exists ring_file)) && Unix.gettimeofday () < deadline do
+  let ring_ready () =
+    Sys.file_exists ring_file
+    &&
+      try (Unix.stat ring_file).Unix.st_size > 0
+      with Unix.Unix_error _ -> false
+  in
+  while (not (ring_ready ())) && Unix.gettimeofday () < deadline do
     Unix.sleepf poll_interval
   done;
   let cursor =