The escape hatch says which hatch it is when asked for a re-run

--two-process has no parked thread to wake. Its program is a child, and a
child that finishes is gone, so running main again is not something that shape
can do — and saying "the program is already running" would send somebody back
to try again after it had exited. It names itself instead.

--x86 arrives here too and is the reason this is worth a round trip rather
than none: --x86 refuses the merged daemon, for the -rdynamic reason the case
below already records, so every x86 dev loop is a two-process one and this is
the answer it gets.

test/test_dev.ml

@@ -2839,6 +2839,20 @@ let () =
               ignore (ask "(:op \"describe\")");
               contains_sub (Buffer.contents seen) "42"))
       then fail "--two-process: the reload was never installed";
+      (* And the one verb this shape cannot have. Running [main] again means
+         waking a thread that parked inside this process, and here the program
+         is a child: when it finishes it is gone, and there is nothing to wake.
+         Refused by naming what this daemon is rather than with the message a
+         merged one gives, because "the program is already running" would send
+         somebody back to try again after it had exited — and [--x86] arrives
+         here too, since it refuses the merged daemon for the -rdynamic reason
+         given below. *)
+      let r = ask "(:op \"rerun\")" in
+      let why = Option.value ~default:(status r) (Wire.string_field r "message") in
+      if status r <> "error" then
+        fail "--two-process answered a rerun it cannot perform"
+      else if not (contains_sub why "two-process") then
+        fail "--two-process refuses a rerun as: %s" why;
       ignore (ask "(:op \"close\")");
       Unix.close tc
     end;