Don't call accept_fork's error handler on cancellation

This typically happens when the server is shutting down and wants to
stop all connection handlers. There is no need to have each connection
fiber log this; they just need to stop.

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