Raise ArgumentError if sprintf format string ends with %

* Add tests and specs. See ruby/spec#401.
  Patch by Yuta Iwama and Shintaro Morikawa.
  [ruby-core:80153] [Bug #13315] [Fix GH-1560]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58890 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

spec/rubyspec/core/string/modulo_spec.rb

@@ -14,9 +14,18 @@ describe "String#%" do
     ("%d%% %s" % [10, "of chickens!"]).should == "10% of chickens!"
   end
 
-  it "formats single % character at the end as literal %" do
-    ("%" % []).should == "%"
-    ("foo%" % []).should == "foo%"
+  ruby_version_is ""..."2.5" do
+    it "formats single % character at the end as literal %" do
+      ("%" % []).should == "%"
+      ("foo%" % []).should == "foo%"
+    end
+  end
+
+  ruby_version_is "2.5" do
+    it "raises an error if single % appears at the end" do
+      lambda { ("%" % []) }.should raise_error(ArgumentError)
+      lambda { ("foo%" % [])}.should raise_error(ArgumentError)
+    end
   end
 
   it "formats single % character before a newline as literal %" do

sprintf.c

@@ -520,7 +520,10 @@ rb_str_format(int argc, const VALUE *argv, VALUE fmt)
 	VALUE sym = Qnil;
 
 	for (t = p; t < end && *t != '%'; t++) ;
-	if (t + 1 == end) ++t;
+	if (t + 1 == end) {
+	    if (*t == '%') rb_raise(rb_eArgError, "incomplete format specifier");
+	    ++t;
+	}
 	PUSH(p, t - p);
 	if (coderange != ENC_CODERANGE_BROKEN && scanned < blen) {
 	    scanned += rb_str_coderange_scan_restartable(buf+scanned, buf+blen, enc, &coderange);

test/ruby/test_sprintf.rb

@@ -424,6 +424,16 @@ class TestSprintf < Test::Unit::TestCase
     assert_equal("%" * BSIZ, sprintf("%%" * BSIZ))
   end
 
+  def test_percent_sign_at_end
+    assert_raise_with_message(ArgumentError, "incomplete format specifier") do
+      sprintf("%")
+    end
+
+    assert_raise_with_message(ArgumentError, "incomplete format specifier") do
+      sprintf("abc%")
+    end
+  end
+
   def test_rb_sprintf
     assert_match(/^#<TestSprintf::T012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789:0x[0-9a-f]+>$/,
                  T012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789.new.inspect)