Support for non-blocking I/O

* doc/ref/api-io.texi (I/O Extensions): Document read_wait_fd /
  write_wait_fd members.
  (Non-Blocking I/O): New section.
* libguile/fports.c (fport_read, fport_write): Return -1 if the
  operation would block.
  (fport_wait_fd, scm_make_fptob): Add read/write wait-fd
  implementation.
* libguile/ports-internal.h (scm_t_port_type): Add read_wait_fd /
  write_wait_fd.
* libguile/ports.c (default_read_wait_fd, default_write_wait_fd): New
  functions.
  (scm_make_port_type): Initialize default read/write wait fd impls.
  (trampoline_to_c_read, trampoline_to_scm_read)
  (trampoline_to_c_write, trampoline_to_scm_write): To Scheme, a return
  of #f indicates EWOULDBLOCk.
  (scm_set_port_read_wait_fd, scm_set_port_write_wait_fd): New
  functions.
  (port_read_wait_fd, port_write_wait_fd, scm_port_read_wait_fd)
  (scm_port_write_wait_fd, port_poll, scm_port_poll): New functions.
  (scm_i_read_bytes, scm_i_write_bytes): Poll if the read or write would
  block.
* libguile/ports.h (scm_set_port_read_wait_fd)
  (scm_set_port_write_wait_fd): Add declarations.
* module/ice-9/ports.scm: Shunt port-poll and port-{read,write}-wait-fd
  to the internals module.
* module/ice-9/sports.scm (current-write-waiter):
  (current-read-waiter): Implement.
* test-suite/tests/ports.test: Adapt non-blocking test to new behavior.
* NEWS: Add entry.

module/ice-9/ports.scm

@@ -179,7 +179,10 @@ interpret its input and output."
             specialize-port-encoding!
             port-random-access?
             port-decode-char
-            port-read-buffering))
+            port-read-buffering
+            port-poll
+            port-read-wait-fd
+            port-write-wait-fd))
 
 (define-syntax-rule (port-buffer-bytevector buf) (vector-ref buf 0))
 (define-syntax-rule (port-buffer-cur buf) (vector-ref buf 1))
@@ -209,7 +212,10 @@ interpret its input and output."
                        specialize-port-encoding!
                        port-decode-char
                        port-random-access?
-                       port-read-buffering)
+                       port-read-buffering
+                       port-poll
+                       port-read-wait-fd
+                       port-write-wait-fd)
 
 ;; And we're back.
 (define-module (ice-9 ports))

module/ice-9/sports.scm

@@ -54,7 +54,9 @@
   #:replace (peek-char
              read-char)
   #:export (lookahead-u8
-            get-u8))
+            get-u8
+            current-read-waiter
+            current-write-waiter))
 
 (define (write-bytes port src start count)
   (let ((written ((port-write port) port src start count)))
@@ -77,11 +79,25 @@
       (set-port-buffer-end! buf 0)
       (write-bytes port (port-buffer-bytevector buf) cur (- end cur)))))
 
+(define (default-read-waiter port) (port-poll port "r"))
+(define (default-write-waiter port) (port-poll port "w"))
+
+(define current-read-waiter  (make-parameter default-read-waiter))
+(define current-write-waiter (make-parameter default-write-waiter))
+
+(define (wait-for-readable port) ((current-read-waiter) port))
+(define (wait-for-writable port) ((current-write-waiter) port))
+
 (define (read-bytes port dst start count)
-  (let ((read ((port-read port) port dst start count)))
-    (unless (<= 0 read count)
-      (error "bad return from port read function" read))
-    read))
+  (cond
+   (((port-read port) port dst start count)
+    => (lambda (read)
+         (unless (<= 0 read count)
+           (error "bad return from port read function" read))
+         read))
+   (else
+    (wait-for-readable port)
+    (read-bytes port dst start count))))
 
 (define utf8-bom #vu8(#xEF #xBB #xBF))
 (define utf16be-bom #vu8(#xFE #xFF))