bugfix: check that timer servicing worked

src/lowering/desugar.ml

@@ -367,8 +367,10 @@ and call_system_func_opt name es obj_typ =
            let timer =
              blockE
                [ expD T.(callE (varE (var id.it note)) [Any]
-                   (varE (var "@set_global_timer" (Func (Local, Returns, [], [Prim Nat64], []))))) ]
-               (unitE ()) in
+                   (varE (var "@set_global_timer" Mo_frontend.Typing.global_timer_set_type))) ]
+               T.(callE
+                    (varE (var "@check_timer_send" (Func (Local, Returns, [], [], [])))) []
+                    (unitE())) in
            { timer with at }
         | "heartbeat" ->
           blockE
@@ -606,7 +608,9 @@ and build_actor at ts self_id es obj_typ =
           | None when !Mo_config.Flags.global_timer ->
             blockE
               [ expD T.(callE (varE (var "@timer_helper" Mo_frontend.Typing.heartbeat_type)) [unit] (unitE())) ]
-              (unitE ())
+              T.(callE
+                   (varE (var "@check_timer_send" (Func (Local, Returns, [], [], [])))) []
+                   (unitE()))
           | None -> tupE []);
        inspect =
          (match call_system_func_opt "inspect" es obj_typ with

src/mo_frontend/typing.ml

@@ -389,9 +389,11 @@ let infer_mut mut : T.typ -> T.typ =
 let heartbeat_type =
   T.(Func (Local, Returns, [scope_bind], [], [Async (Fut, Var (default_scope_var, 0), unit)]))
 
+let global_timer_set_type = T.(Func (Local, Returns, [], [Prim Nat64], []))
+
 let timer_type =
   T.(Func (Local, Returns, [scope_bind],
-    [Func (Local, Returns, [], [Prim Nat64], [])],
+    [global_timer_set_type],
     [Async (Fut, Var (default_scope_var, 0), unit)]))
 
 let system_funcs tfs =

src/mo_frontend/typing.mli

@@ -13,3 +13,4 @@ val check_actors : ?viper_mode:bool -> ?check_actors:bool -> scope -> Syntax.pro
 val check_stab_sig : scope -> Syntax.stab_sig -> (field list) Diag.result
 
 val heartbeat_type : typ
+val global_timer_set_type : typ

src/prelude/internals.mo

@@ -675,5 +675,13 @@ func @cancelTimer(id : Nat) {
   }
 };
 
-
 func @set_global_timer(time : Nat64) = ignore (prim "global_timer_set" : Nat64 -> Nat64) time;
+
+// Function called by backend to check if self-send worked.
+// DO NOT RENAME without modifying compilation.
+func @check_timer_send() {
+  if (not @call_succeeded()) {
+    // i.e. self-call queue full: expire soon and retry
+    ignore (prim "global_timer_set" : Nat64 -> Nat64) 1;
+  }
+};