Do not restart enumerator if it prematurely terminates due to exception

Previously, if an exception occurred during enumeration, the next
call to Enumerator#next and similar methods would restart the
Enumerator from the beginning, which is unlikely to be the desired
behavior.

This changes the behavior so that after the enumerator has raised
an exception, future calls to Enumerator#next and similar methods
will raise StopIteration, similar to when calling Enumerator#next
after the enumerator has finished.

Fixes [Bug #16816]

enumerator.c

@@ -758,6 +758,37 @@ next_init(VALUE obj, struct enumerator *e)
     e->lookahead = Qundef;
 }
 
+struct get_next_values_args {
+    struct enumerator *e;
+    VALUE curr;
+};
+
+NORETURN(static void get_next_values_error(struct enumerator *, VALUE));
+
+static void
+get_next_values_error(struct enumerator *e, VALUE exc) {
+    e->fib = 0;
+    e->dst = Qnil;
+    e->lookahead = Qundef;
+    e->feedvalue = Qundef;
+    rb_exc_raise(exc);
+}
+
+static VALUE
+get_next_values_fiber_resume(VALUE vargs) {
+    struct get_next_values_args *args = (struct get_next_values_args*)vargs;
+    return rb_fiber_resume(args->e->fib, 1, &args->curr);
+}
+
+NORETURN(static VALUE get_next_values_fiber_resume_rescue(VALUE, VALUE));
+
+static VALUE
+get_next_values_fiber_resume_rescue(VALUE vargs, VALUE exc) {
+    struct enumerator *e = ((struct get_next_values_args*)vargs)->e;
+    e->stop_exc = rb_exc_new2(rb_eStopIteration, "iteration raised an error");
+    get_next_values_error(e, exc);
+}
+
 static VALUE
 get_next_values(VALUE obj, struct enumerator *e)
 {
@@ -772,13 +803,15 @@ get_next_values(VALUE obj, struct enumerator *e)
 	next_init(obj, e);
     }
 
-    vs = rb_fiber_resume(e->fib, 1, &curr);
+    struct get_next_values_args args;
+    args.e = e;
+    args.curr = curr;
+    vs = rb_rescue2(get_next_values_fiber_resume, (VALUE)&args,
+                    get_next_values_fiber_resume_rescue, (VALUE)&args,
+                    rb_eException, 0);
+
     if (e->stop_exc) {
-	e->fib = 0;
-	e->dst = Qnil;
-	e->lookahead = Qundef;
-	e->feedvalue = Qundef;
-	rb_exc_raise(e->stop_exc);
+        get_next_values_error(e, e->stop_exc);
     }
     return vs;
 }

test/ruby/test_enumerator.rb

@@ -38,6 +38,16 @@ def test_next
     assert_raise(StopIteration){e.next}
   end
 
+  def test_next_raise
+    f = Enumerator.new { |y|
+      3.times {|i| i == 2 ? raise(i.to_s) : (y.yield i) }
+    }
+
+    result = 4.times.map { f.next rescue $!.message}
+    expected = [0, 1, "2", "iteration raised an error"]
+    assert_equal(expected, result)
+  end
+
   def test_loop
     e = 3.times
     i = 0