rb_wait_for_single_fd: do not OOM or segfault with invalid FD on select()

Instead, match the poll() implementation used on Linux for now;
as the Linux poll(2) manpage describes using negative FD to
easily ignore an FD in a larger FD set while (sleeping the given
timeout).  I'm not entirely sure if matching poll() behavior
is a good idea for a single FD, but it's better than segfaulting
or NoMemoryError.

* thread.c (init_set_fd): ignore negative FD
* test/-ext-/wait_for_single_fd/test_wait_for_single_fd.rb
  (test_wait_for_invalid_fd): check values which may trigger
  segfaults or OOM

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58925 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/-ext-/wait_for_single_fd/test_wait_for_single_fd.rb

@@ -22,6 +22,13 @@ class TestWaitForSingleFD < Test::Unit::TestCase
   end
 
   def test_wait_for_invalid_fd
+    # Negative FDs should not cause NoMemoryError or segfault when
+    # using select().  For now, match the poll() implementation
+    # used on Linux, which sleeps the given amount of time given
+    # when fd is negative (as documented in the Linux poll(2) manpage)
+    assert_equal 0, IO.wait_for_single_fd(-999, RB_WAITFD_IN, 0)
+    assert_equal 0, IO.wait_for_single_fd(-1, RB_WAITFD_OUT, 0)
+
     # FreeBSD 8.2 or prior sticks this
     # http://bugs.ruby-lang.org/issues/5524
     skip if /freebsd[1-8]/ =~ RUBY_PLATFORM

thread.c

@@ -3969,6 +3969,9 @@ rb_wait_for_single_fd(int fd, int events, struct timeval *tv)
 static rb_fdset_t *
 init_set_fd(int fd, rb_fdset_t *fds)
 {
+    if (fd < 0) {
+	return 0;
+    }
     rb_fd_init(fds);
     rb_fd_set(fd, fds);