Scheme-GNUnet requires the new operations 'wait-until-port-readable-operation'
and 'wait-until-port-readable-operation' for communicating with services.
This patch has been previously submitted at <https://github.com/wingo/fibers/pull/50>,
on Sep 16, 2021.  As of Feb 3, 2022, upstream has not responded yet.

diff --git a/Makefile.am b/Makefile.am
index e2db57e..0134255 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -33,6 +33,7 @@ SOURCES = \
 	fibers/deque.scm \
 	fibers/epoll.scm \
 	fibers/interrupts.scm \
+	fibers/io-wakeup.scm \
 	fibers/nameset.scm \
 	fibers/operations.scm \
 	fibers/posix-clocks.scm \
@@ -67,6 +68,7 @@ TESTS = \
 	tests/conditions.scm \
 	tests/channels.scm \
 	tests/foreign.scm \
+	tests/io-wakeup.scm \
 	tests/parameters.scm \
 	tests/preemption.scm \
 	tests/speedup.scm
diff --git a/fibers.texi b/fibers.texi
index 52f7177..0990c8f 100644
--- a/fibers.texi
+++ b/fibers.texi
@@ -12,6 +12,7 @@ This manual is for Fibers (version @value{VERSION}, updated
 @value{UPDATED})
 
 Copyright 2016-2022 Andy Wingo
+Copyright 2021 Maxime Devos
 
 @quotation
 @c For more information, see COPYING.docs in the fibers
@@ -453,6 +454,7 @@ of operations for channels and timers, and an internals interface.
 * Channels::             Share memory by communicating.
 * Timers::               Operations on time.
 * Conditions::           Waiting for simple state changes.
+* Port Readiness::       Waiting until a port is ready for I/O.
 * REPL Commands::        Experimenting with Fibers at the console.
 * Schedulers and Tasks:: Fibers are built from lower-level primitives.
 @end menu
@@ -722,6 +724,28 @@ signalled.  Equivalent to @code{(perform-operation (wait-operation
 cvar))}.
 @end defun
 
+@node Port Readiness
+@section Port Readiness
+
+These two operations can be used on file ports to wait until
+they are readable or writable.  Spurious wake-ups are possible.
+This is complementary to Guile's suspendable ports.
+
+@example
+(use-modules (fibers io-wakeup))
+@end example
+
+@defun wait-until-port-readable-operation port
+Make an operation that will succeed with no values when the input
+port @var{port} becomes readable.  For passive sockets, this operation
+succeeds when a connection becomes available.
+@end defun
+
+@defun wait-until-port-writable-operation
+Make an operation that will succeed with no values when the output
+port @var{port} becomes writable.
+@end defun
+
 @node REPL Commands
 @section REPL Commands
 
diff --git a/fibers/io-wakeup.scm b/fibers/io-wakeup.scm
new file mode 100644
index 0000000..5df03f1
--- /dev/null
+++ b/fibers/io-wakeup.scm
@@ -0,0 +1,93 @@
+;; Fibers: cooperative, event-driven user-space threads.
+
+;;;; Copyright (C) 2016,2021 Free Software Foundation, Inc.
+;;;; Copyright (C) 2021 Maxime Devos
+;;;;
+;;;; This library is free software; you can redistribute it and/or
+;;;; modify it under the terms of the GNU Lesser General Public
+;;;; License as published by the Free Software Foundation; either
+;;;; version 3 of the License, or (at your option) any later version.
+;;;;
+;;;; This library is distributed in the hope that it will be useful,
+;;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+;;;; Lesser General Public License for more details.
+;;;;
+;;;; You should have received a copy of the GNU Lesser General Public
+;;;; License along with this library; if not, write to the Free Software
+;;;; Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+;;;;
+
+(define-module (fibers io-wakeup)
+  #:use-module (fibers scheduler)
+  #:use-module (fibers operations)
+  #:use-module (ice-9 atomic)
+  #:use-module (ice-9 match)
+  #:use-module (ice-9 threads)
+  #:use-module (ice-9 ports internal)
+  #:export (wait-until-port-readable-operation
+	    wait-until-port-writable-operation))
+
+(define *poll-sched* (make-atomic-box #f))
+
+(define (poll-sched)
+  (or (atomic-box-ref *poll-sched*)
+      (let ((sched (make-scheduler)))
+        (cond
+         ((atomic-box-compare-and-swap! *poll-sched* #f sched))
+         (else
+          ;; FIXME: Would be nice to clean up this thread at some point.
+          (call-with-new-thread
+           (lambda ()
+             (define (finished?) #f)
+             (run-scheduler sched finished?)))
+          sched)))))
+
+;; These procedure are subject to spurious wakeups.
+
+(define (readable? port)
+  "Test if PORT is writable."
+  (match (select (vector port) #() #() 0)
+    ((#() #() #()) #f)
+    ((#(_) #() #()) #t)))
+
+(define (writable? port)
+  "Test if PORT is writable."
+  (match (select #() (vector port) #() 0)
+    ((#() #() #()) #f)
+    ((#() #(_) #()) #t)))
+
+(define (make-wait-operation ready? schedule-when-ready port port-ready-fd this-procedure)
+  (make-base-operation #f
+                       (lambda _
+                         (and (ready? port) values))
+                       (lambda (flag sched resume)
+                         (define (commit)
+                           (match (atomic-box-compare-and-swap! flag 'W 'S)
+                             ('W (resume values))
+                             ('C (commit))
+                             ('S #f)))
+                         (if sched
+                             (schedule-when-ready
+                              sched (port-ready-fd port) commit)
+                             (schedule-task
+                              (poll-sched)
+                              (lambda ()
+                                (perform-operation (this-procedure port))
+                                (commit)))))))
+
+(define (wait-until-port-readable-operation port)
+  "Make an operation that will succeed when PORT is readable."
+  (unless (input-port? port)
+    (error "refusing to wait forever for input on non-input port"))
+  (make-wait-operation readable? schedule-task-when-fd-readable port
+                       port-read-wait-fd
+                       wait-until-port-readable-operation))
+
+(define (wait-until-port-writable-operation port)
+  "Make an operation that will succeed when PORT is writable."
+  (unless (output-port? port)
+    (error "refusing to wait forever for output on non-output port"))
+  (make-wait-operation writable? schedule-task-when-fd-writable port
+                       port-write-wait-fd
+                       wait-until-port-writable-operation))
diff --git a/tests/io-wakeup.scm b/tests/io-wakeup.scm
new file mode 100644
index 0000000..c14fa81
--- /dev/null
+++ b/tests/io-wakeup.scm
@@ -0,0 +1,167 @@
+;; Fibers: cooperative, event-driven user-space threads.
+
+;;;; Copyright (C) 2016 Free Software Foundation, Inc.
+;;;; Copyright (C) 2021 Maxime Devos
+;;;;
+;;;; This library is free software; you can redistribute it and/or
+;;;; modify it under the terms of the GNU Lesser General Public
+;;;; License as published by the Free Software Foundation; either
+;;;; version 3 of the License, or (at your option) any later version.
+;;;;
+;;;; This library is distributed in the hope that it will be useful,
+;;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+;;;; Lesser General Public License for more details.
+;;;;
+;;;; You should have received a copy of the GNU Lesser General Public
+;;;; License along with this library; if not, write to the Free Software
+;;;; Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+;;;;
+
+(define-module (tests io-wakeup)
+  #:use-module (rnrs bytevectors)
+  #:use-module (ice-9 control)
+  #:use-module (ice-9 suspendable-ports)
+  #:use-module (ice-9 binary-ports)
+  #:use-module (fibers)
+  #:use-module (fibers io-wakeup)
+  #:use-module (fibers operations)
+  #:use-module (fibers timers))
+
+(define failed? #f)
+
+(define-syntax-rule (assert-equal expected actual)
+  (let ((x expected))
+    (format #t "assert ~s equal to ~s: " 'actual x)
+    (force-output)
+    (let ((y actual))
+      (cond
+       ((equal? x y) (format #t "ok\n"))
+       (else
+        (format #t "no (got ~s)\n" y)
+        (set! failed? #t))))))
+
+(define-syntax-rule (assert-run-fibers-terminates exp)
+  (begin
+    (format #t "assert run-fibers on ~s terminates: " 'exp)
+    (force-output)
+    (let ((start (get-internal-real-time)))
+      (call-with-values (lambda () (run-fibers (lambda () exp)))
+        (lambda vals
+          (format #t "ok (~a s)\n" (/ (- (get-internal-real-time) start)
+                                      1.0 internal-time-units-per-second))
+          (apply values vals))))))
+
+(define-syntax-rule (assert-run-fibers-returns (expected ...) exp)
+  (begin
+    (call-with-values (lambda () (assert-run-fibers-terminates exp))
+      (lambda run-fiber-return-vals
+        (assert-equal '(expected ...) run-fiber-return-vals)))))
+
+
+;; Note that theoretically, on very slow systems, SECONDS might need
+;; to be increased.  However, readable/timeout? and writable/timeout?
+;; call this 5 times in a loop anyways, so the effective timeout is
+;; a fourth of a second, which should be plenty in practice.
+(define* (with-timeout op #:key (seconds 0.05) (wrap values))
+  (choice-operation op
+                    (wrap-operation (sleep-operation seconds) wrap)))
+
+(define* (readable/timeout? port #:key (allowed-spurious 5))
+  "Does waiting for readability time-out?
+Allow @var{allowed-spurious} spurious wakeups."
+  (or (perform-operation
+	(with-timeout
+	 (wrap-operation (wait-until-port-readable-operation port)
+			 (lambda () #f))
+	 #:wrap (lambda () #t)))
+      (and (> allowed-spurious 0)
+	   (readable/timeout? port #:allowed-spurious
+			      (- allowed-spurious 1)))))
+
+(define* (writable/timeout? port #:key (allowed-spurious 5))
+  "Does waiting for writability time-out?
+Allow @var{allowed-spurious} spurious wakeups."
+  (or (perform-operation
+       (with-timeout
+	(wrap-operation (wait-until-port-writable-operation port)
+			(lambda () #f))
+	#:wrap (lambda () #t)))
+      (and (> allowed-spurious 0)
+	   (writable/timeout? port #:allowed-spurious
+			      (- allowed-spurious 1)))))
+
+;; Tests:
+;;  * wait-until-port-readable-operaton / wait-until-port-writable-operation
+;;    blocks if the port isn't ready for input / output.
+;;
+;;    This is tested with a pipe (read & write)
+;;    and a listening socket (read, or accept in this case).
+;;
+;;    Due to the possibility of spurious wakeups,
+;;    a limited few spurious wakeups are tolerated.
+;;
+;;  * these operations succeed if the port is ready for input / output.
+;;
+;;    These are again tested with a pipe and a listening socket
+;;
+;; Blocking is detected with a small time-out.
+
+(define (make-listening-socket)
+  (let ((server (socket PF_INET SOCK_DGRAM 0)))
+    (bind server AF_INET INADDR_LOOPBACK 0)
+    server))
+
+(let ((s (make-listening-socket)))
+  (assert-run-fibers-returns (#t)
+			     (readable/timeout? s))
+  (assert-equal #t (readable/timeout? s))
+  (close s))
+
+(define (set-nonblocking! sock)
+  (let ((flags (fcntl sock F_GETFL)))
+    (fcntl sock F_SETFL (logior O_NONBLOCK flags))))
+
+(define-syntax-rule (with-pipes (A B) exp exp* ...)
+  (let* ((pipes (pipe))
+	 (A (car pipes))
+	 (B (cdr pipes)))
+    exp exp* ...
+    (close A)
+    (close B)))
+
+(with-pipes (A B)
+  (setvbuf A 'none)
+  (setvbuf B 'none)
+  (assert-run-fibers-returns (#t)
+			     (readable/timeout? A))
+  (assert-equal #t (readable/timeout? A))
+
+  ;; The buffer is empty, so writability is expected.
+  (assert-run-fibers-returns (#f)
+			     (writable/timeout? B))
+  (assert-equal #f (writable/timeout? B))
+
+  ;; Fill the buffer
+  (set-nonblocking! B)
+  (let ((bv (make-bytevector 1024)))
+    (let/ec k
+      (parameterize ((current-write-waiter k))
+	(let loop ()
+	  (put-bytevector B bv)
+	  (loop)))))
+
+  ;; As the buffer is full, writable/timeout? should return
+  ;; #t.
+  (assert-run-fibers-returns (#t)
+			     (writable/timeout? B))
+  ;; There's plenty to read now, so readable/timeout? should
+  ;; return #f.
+  (assert-run-fibers-returns (#f)
+			     (readable/timeout? A)))
+
+(exit (if failed? 1 0))
+
+;; Local Variables:
+;; eval: (put 'with-pipes 'scheme-indent-function 1)
+;; End: