eio: fix semaphore acquisition, graceful stop, and time source

- Acquire semaphore BEFORE spawning handler fiber: replace
  Eio.Net.accept_fork with manual accept + Semaphore.acquire + Fiber.fork
  so we bound the number of in-flight fibers rather than spawning
  unlimited fibers that all block on the semaphore.

- Graceful stop: remove Eio.Switch.fail sw Exit from stop(), just set
  running to false so existing handlers can complete naturally instead
  of being cancelled immediately.

- Replace Unix.gettimeofday with Eio.Time.now clock to use the Eio
  clock abstraction instead of direct Unix calls.

src/eio/tiny_httpd_eio.ml

@@ -129,7 +129,8 @@ let io_backend ?addr ?port ?unix_sock ?max_connections ?max_buf_pool_size
   let module M = struct
     let init_addr () = addr
     let init_port () = port
-    let get_time_s () = Unix.gettimeofday ()
+    let clock = Eio.Stdenv.clock stdenv
+    let get_time_s () = Eio.Time.now clock
     let max_connections = get_max_connection_ ?max_connections ()
 
     let pool_size =
@@ -172,8 +173,7 @@ let io_backend ?addr ?port ?unix_sock ?max_connections ?max_buf_pool_size
                 running = (fun () -> Atomic.get running);
                 stop =
                   (fun () ->
-                    Atomic.set running false;
+                    Atomic.set running false);
-                    Eio.Switch.fail sw Exit);
                 endpoint = (fun () -> actual_addr, actual_port);
                 active_connections = (fun () -> Atomic.get active_conns);
               }
@@ -182,28 +182,33 @@ let io_backend ?addr ?port ?unix_sock ?max_connections ?max_buf_pool_size
             after_init tcp_server;
 
             while Atomic.get running do
-              Eio.Net.accept_fork ~sw
+              match Eio.Net.accept ~sw sock with
-                ~on_error:(fun exn ->
+              | exception (Eio.Cancel.Cancelled _ | Eio.Io _) when not (Atomic.get running) ->
-                  Log.error (fun k ->
+                (* Socket closed or switch cancelled during shutdown; exit loop *)
-                      k "error in client handler: %s" (Printexc.to_string exn)))
+                ()
-                sock
+              | conn, client_addr ->
-                (fun flow client_addr ->
+                (* Acquire semaphore BEFORE spawning a fiber so we
-                  Eio.Semaphore.acquire sem;
+                   bound the number of in-flight fibers. *)
-                  Eio_unix.Fd.use_exn "setsockopt"
+                Eio.Semaphore.acquire sem;
-                    (Eio_unix.Net.fd flow) (fun fd ->
+                Eio.Fiber.fork ~sw (fun () ->
-                      Unix.setsockopt fd Unix.TCP_NODELAY true);
-                  Atomic.incr active_conns;
                   let@ () =
                     Fun.protect ~finally:(fun () ->
                         Log.debug (fun k ->
                             k "Tiny_httpd_eio: client handler returned");
                         Atomic.decr active_conns;
-                        Eio.Semaphore.release sem)
+                        Eio.Semaphore.release sem;
+                        Eio.Flow.close conn)
                   in
+                  (try
+                    Eio_unix.Fd.use_exn "setsockopt"
+                      (Eio_unix.Net.fd conn) (fun fd ->
+                        Unix.setsockopt fd Unix.TCP_NODELAY true)
+                  with Unix.Unix_error _ -> ());
+                  Atomic.incr active_conns;
                   let ic_closed = ref false in
                   let oc_closed = ref false in
-                  let ic = ic_of_flow ~closed:ic_closed ~buf_pool:cstruct_pool flow in
+                  let ic = ic_of_flow ~closed:ic_closed ~buf_pool:cstruct_pool conn in
-                  let oc = oc_of_flow ~closed:oc_closed ~buf_pool:cstruct_pool flow in
+                  let oc = oc_of_flow ~closed:oc_closed ~buf_pool:cstruct_pool conn in
 
                   Log.debug (fun k ->
                       k "handling client on %a…" Eio.Net.Sockaddr.pp client_addr);