Adds support for interacting with devices on macOS

Block devices currently did not work on macOS,
due to a bug that prevents poll(2) from working with
block devices. This fixes that by creating a new thread
that monitors block devices using select(2). The thread is
created the first time a user computation attempts to read
or write to a block devices. The implementation of the new
select manager is in select.ml

Additionally, the select manager also needed an eventfd system.
The eventfd system from sched.ml was refactoring into a
re-usable model that encapsulates some of the internal state
in posix_eventfd.ml.

Author: aaronjeline

lib_eio_posix/.claude/settings.local.json

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+{
+  "permissions": {
+    "allow": [
+      "Bash(timeout 3 sh:*)",
+      "Bash(timeout 5 sh:*)",
+      "Bash(dune build:*)"
+    ]
+  }
+}

lib_eio_posix/posix_eventfd.ml

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+(*
+ * Copyright (C) 2025 Aaron Eline
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ *)
+module Rcfd = Eio_unix.Private.Rcfd
+
+module Writer = struct
+  type t = Rcfd.t
+  let wake_buffer = Bytes.of_string "!"
+  let wakeup t = 
+    Rcfd.use t
+      ~if_closed:ignore
+      (fun fd -> 
+        try 
+          ignore (Unix.single_write fd wake_buffer 0 1)
+        with
+         (* If the pipe is full then a wake up is pending anyway. *)
+          | Unix.Unix_error ((Unix.EACCES | EWOULDBLOCK), _, _) -> ()
+         (* We're shutting down; the event has already been processed. *)
+          | Unix.Unix_error (Unix.EPIPE, _, _) -> ()
+      )
+end 
+
+module Owner = struct 
+  type t = {
+    read : Unix.file_descr;
+    write : Rcfd.t;
+  }
+
+  let is_reader fd t = fd = t.read
+
+  let reader_fd t = t.read
+
+  let create_writer t = t.write
+
+  let clear t = 
+    let buf = Bytes.create 8 in
+    let got = Unix.read t.read buf 0 (Bytes.length buf) in
+    assert (got > 0)
+
+  let create () = 
+    let read, write = Unix.pipe ~cloexec:true () in
+    Unix.set_nonblock read;
+    Unix.set_nonblock write;
+    let write = Rcfd.make write in
+    {
+      read; write
+    }
+
+  let cleanup t = 
+    Unix.close t.read;
+    let was_open = Rcfd.close t.write in
+    assert was_open
+
+end

lib_eio_posix/posix_eventfd.mli

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+(*
+ * Copyright (C) 2025 Aaron Eline
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ *)
+
+(**
+    This module implements a thread safe re-usable abstraction for POSIX compliant eventfds.
+    Eventfds are backed by a pipe.
+*)
+
+(**
+ The [Writer] module is the writing end of the eventfd. 
+ Having a [Writer.t] allows you to signal to the holder of the owner to wake up
+ It is safe to share between threads
+*)
+module Writer :
+  sig type t
+
+  (** Signal to the owning thread to wake up *)
+  val wakeup : t -> unit
+end
+
+(**
+    The [Owner] module is the reading end of the event fd.
+    It is not safe to share between threads.
+    Anyone who holds a [Writer.t] can send wake up signals to an [Owner.t]
+*)
+module Owner :
+  sig
+    type t
+
+    (** Check if a given fd is the read end of the event fd *)
+    val is_reader : Unix.file_descr -> t -> bool
+
+    (** @return the reading fd of the eventfd. This is not safe to share between threads. *)
+    val reader_fd : t -> Unix.file_descr
+
+    (** Get a view into the write end of the event fd.
+        @return the writing end. Safe to share between threads. *)
+    val create_writer : t -> Writer.t
+
+    (** Clear pending events from the event fd *)
+    val clear : t -> unit
+
+    (** @return a new eventfd owner *)
+    val create : unit -> t
+
+    (** Close the file descriptors associated with the eventfd *)
+    val cleanup : t -> unit
+  end