CA-387699: Fix Protocol_async.with_lock bug spotted by Vincent

Monadic concurrency libraries can switch away to another 'promise'
whenever the bind operator is called.
In fact Async will always switch away, but Lwt would only switch away if the promise is blocked
(this is probably the origin of the bug).

Move the 't.m <- true' next to where we checked that it is false to ensure that we are
the only ones holding it.
(This is still vulnerable to race conditions with pure OCaml threads, but not with Async promises).

Another alternative would be to use Async.Throttle.Sequencer, but this change is a minimal one
that could be backported to Yangtze even.

Signed-off-by: Edwin Török <[email protected]>

ocaml/message-switch/async/protocol_async.ml

@@ -82,13 +82,13 @@ module M = struct
 
     let with_lock t f =
       let rec wait state =
-        if Bool.(t.m = state) then
+        if Bool.(t.m = state) then (
+          t.m <- true ;
           return ()
-        else
+        ) else
           Condition.wait t.c >>= fun () -> wait state
       in
       wait false >>= fun () ->
-      t.m <- true ;
       Monitor.protect f ~finally:(fun () ->
           t.m <- false ;
           Condition.broadcast t.c () ;