* string.c (rb_str_set_len): should fail to modify shared string.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@28865 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/-ext-/string/test_set_len.rb

@@ -0,0 +1,25 @@
+require 'test/unit'
+require "-test-/string/string"
+
+class Test_StrSetLen < Test::Unit::TestCase
+  def setup
+    @s0 = [*"a".."z"].join("").freeze
+    @s1 = Bug::String.new(@s0)
+  end
+
+  def teardown
+    orig = [*"a".."z"].join("")
+    assert_equal(orig, @s0)
+  end
+
+  def test_non_shared
+    @s1.modify!
+    assert_equal("abc", @s1.set_len(3))
+  end
+
+  def test_shared
+    assert_raise(RuntimeError) {
+      assert_equal("abc", @s1.set_len(3))
+    }
+  end
+end