Handle invalid negative args to n_times

Addresses https://github.com/ocsigen/lwt/pull/1032/files/efca0a8174ee9f888c6bbb4beca0258ac639f58c#r1806822912

src/retry/lwt_retry.ml

@@ -58,9 +58,10 @@ let with_sleep ?(duration=default_sleep_duration) (attempts : _ attempt Lwt_stre
   end
 
 let n_times n attempts =
+  if n < 0 then invalid_arg "Lwt_retry.n_times: n must be non-negative";
   (* The first attempt is a try, and re-tries start counting from n + 1 *)
   let retries = n + 1 in
   let+ attempts = Lwt_stream.nget retries attempts in
   match List.rev attempts with
   | last :: _ -> last
-  | _ -> failwith "Lwt_retry.n_times impossible"
+  | _ -> failwith "Lwt_retry.n_times: impossible"

test/retry/main.ml

@@ -132,6 +132,16 @@ let suite = suite "lwt_retry" [
         let+ success = Retry.(operation |> on_error |> n_times 5) in
         success = Ok ());
 
+    test_direct "n_times on negative raises Invalid_argument"  (fun () ->
+        let invalid_negative_retries = -5 in
+        let operation () = Lwt.return_error (`Retry ()) in
+        let attempts = Retry.(operation |> on_error) in
+        try
+          let _ = Retry.(attempts |> n_times invalid_negative_retries) in
+          false (* We failed to raise the invalid argument exception *)
+        with
+          Invalid_argument _ -> true);
+
     (* test that the sleeps actually throttle computations as desired *)
     test "with_sleep really does sleep"  (fun ()  ->
       let duration _ = 0.1 in