Don't call accept_fork's error handler on cancellation

lib_eio/net.ml

@@ -182,6 +182,9 @@ let accept_fork ~sw (t : #listening_socket) ~on_error handle =
        Fiber.fork ~sw (fun () ->
            match child_started := true; handle (flow :> stream_socket) addr with
            | x -> Flow.close flow; x
+           | exception (Cancel.Cancelled _ as ex) ->
+             Flow.close flow;
+             raise ex
            | exception ex ->
              Flow.close flow;
              on_error (Exn.add_context ex "handling connection from %a" Sockaddr.pp addr)

lib_eio/net.mli

@@ -208,8 +208,12 @@ val accept_fork :
     @param on_error Called if [connection_handler] raises an exception.
                     This is typically a good place to log the error and continue.
                     If the exception is an {!Eio.Io} error then the caller's address is added to it.
+
                     If you don't want to handle connection errors,
-                    use [~on_error:raise] to cancel the caller's context. *)
+                    use [~on_error:raise] to cancel the caller's context.
+
+                    [on_error] is not called for {!Cancel.Cancelled} exceptions,
+                    which do not need to be reported. *)
 
 (** {2 Running Servers} *)
 

tests/network.md

@@ -403,6 +403,23 @@ If the fork itself fails, we still close the connection:
 Exception: Failure "Simulated error".
 ```
 
+`accept_fork` doesn't send cancellations to `on_error`:
+
+```ocaml
+# Eio_mock.Backend.run @@ fun () ->
+  let socket = Eio_mock.Net.listening_socket "tcp/80" in
+  let flow = Eio_mock.Flow.make "connection" in
+  let addr = `Tcp (Eio.Net.Ipaddr.V4.loopback, 1234) in
+  Eio_mock.Net.on_accept socket [`Return (flow, addr)];
+  Switch.run @@ fun sw ->
+  Eio.Net.accept_fork ~sw ~on_error:(traceln "BUG: %a" Fmt.exn) socket
+    (fun _flow _addr -> Fiber.await_cancel ());
+  Switch.fail sw (Failure "Simulated error");;
++tcp/80: accepted connection from tcp:127.0.0.1:1234
++connection: closed
+Exception: Failure "Simulated error".
+```
+
 ## Socketpair
 
 ```ocaml