Simplify default argument specification. (#6507)

ext/openssl/extconf.rb

@@ -27,7 +27,6 @@ if with_config("debug") or enable_config("debug")
 end
 
 have_func("rb_io_maybe_wait") # Ruby 3.1
-have_func("rb_io_timeout") # Ruby 3.2
 
 Logging::message "=== Checking for system dependent stuff... ===\n"
 have_library("nsl", "t_open")

ext/openssl/ossl_ssl.c

@@ -1641,21 +1641,15 @@ no_exception_p(VALUE opts)
     return 0;
 }
 
-inline static
-VALUE io_timeout()
-{
-#ifdef HAVE_RB_IO_TIMEOUT
-    return Qundef;
-#else
-    return Qnil;
+#ifndef RB_IO_TIMEOUT_DEFAULT
+#define RB_IO_TIMEOUT_DEFAULT Qnil
 #endif
-}
 
 static void
 io_wait_writable(rb_io_t *fptr)
 {
 #ifdef HAVE_RB_IO_MAYBE_WAIT
-    rb_io_maybe_wait_writable(errno, fptr->self, io_timeout());
+    rb_io_maybe_wait_writable(errno, fptr->self, RB_IO_TIMEOUT_DEFAULT);
 #else
     rb_io_wait_writable(fptr->fd);
 #endif
@@ -1665,7 +1659,7 @@ static void
 io_wait_readable(rb_io_t *fptr)
 {
 #ifdef HAVE_RB_IO_MAYBE_WAIT
-    rb_io_maybe_wait_readable(errno, fptr->self, io_timeout());
+    rb_io_maybe_wait_readable(errno, fptr->self, RB_IO_TIMEOUT_DEFAULT);
 #else
     rb_io_wait_readable(fptr->fd);
 #endif

include/ruby/io.h

@@ -58,6 +58,9 @@
 // IO#wait, IO#wait_readable, IO#wait_writable, IO#wait_priority are defined by this implementation.
 #define RUBY_IO_WAIT_METHODS
 
+// Used as the default timeout argument to `rb_io_wait` to use the `IO#timeout` value.
+#define RUBY_IO_TIMEOUT_DEFAULT Qundef
+
 RBIMPL_SYMBOL_EXPORT_BEGIN()
 
 struct stat;
@@ -884,6 +887,8 @@ VALUE rb_io_set_timeout(VALUE io, VALUE timeout);
  * @param[in]  io                   An IO object to wait.
  * @param[in]  events               See above.
  * @param[in]  timeout              Time, or numeric seconds since UNIX epoch.
+ *                                  If Qnil, wait forever. If Qundef, use the
+ *                                  default timeout.
  * @exception  rb_eIOError          `io` is not open.
  * @exception  rb_eRangeError       `timeout` is out of range.
  * @exception  rb_eSystemCallError  `select(2)` failed for some reason.