Use socket pairs for remaining scheduler control fds on Windows (fixes #5245)

src/core/base/harbor/harbor.ml

@@ -1240,7 +1240,11 @@ module Make (T : Transport_t) : T with type socket = T.socket = struct
         (fun cur bind_addr -> if bind_addr <> "" then bind_addr :: cur else cur)
         [] conf_harbor_bind_addrs#get
     in
-    let in_s, out_s = Unix.pipe ~cloexec:true () in
+    (* A socket pair rather than a pipe: on Windows, a single non-socket fd in
+       the scheduler forces [Unix.select] into its worker-thread emulation for
+       every call, which leaks native memory (see #5245). With sockets only,
+       select uses the plain winsock fast path. *)
+    let in_s, out_s = Unix_utils.socketpair ~cloexec:true () in
     let events = `Read in_s :: List.map (open_socket port) bind_addrs in
     Task.add Tutils.scheduler
       {

src/modules/duppy/duppy.ml

@@ -777,7 +777,11 @@ module Monad = struct
       module Control = Control
 
       let tmp = Bytes.create 1024
-      let x, y = Unix.pipe ()
+
+      (* A socket pair rather than a pipe: on Windows, a single non-socket fd
+         in the scheduler forces [Unix.select] into its worker-thread emulation
+         for every call, which leaks native memory. See [create] above. *)
+      let x, y = Unix_utils.socketpair ()
       let stop = ref false
       let wake_up () = ignore (Unix_utils.write y (Bytes.of_string " ") 0 1)
       let ctl_m = Mutex_o.create ()