From f5052d45be1b564a683c347dd72fd2f7b8638fd8 Mon Sep 17 00:00:00 2001
From: nobu <nobu@b2dd03c8-39d4-4d8f-98ff-823fe69b080e>
Date: Sun, 18 Jun 2017 04:38:01 +0000
Subject: [PATCH] string.c: check just before modification

* string.c (rb_str_chomp_bang): check if modifiable after checking
  an argument and just before modification, as it can get frozen
  during the argument conversion to String.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59112 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
---
 string.c                 | 3 ++-
 test/ruby/test_string.rb | 8 ++++++++
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/string.c b/string.c
index cd633e1226..052e0197a1 100644
--- a/string.c
+++ b/string.c
@@ -8255,6 +8255,7 @@ rb_str_chomp_string(VALUE str, VALUE rs)
     long olen = RSTRING_LEN(str);
     long len = chompped_length(str, rs);
     if (len >= olen) return Qnil;
+    str_modify_keep_cr(str);
     STR_SET_LEN(str, len);
     TERM_FILL(&RSTRING_PTR(str)[len], TERM_LEN(str));
     if (ENC_CODERANGE(str) != ENC_CODERANGE_7BIT) {
@@ -8275,7 +8276,7 @@ static VALUE
 rb_str_chomp_bang(int argc, VALUE *argv, VALUE str)
 {
     VALUE rs;
-    str_modify_keep_cr(str);
+    str_modifiable(str);
     if (RSTRING_LEN(str) == 0) return Qnil;
     rs = chomp_rs(argc, argv);
     if (NIL_P(rs)) return Qnil;
diff --git a/test/ruby/test_string.rb b/test/ruby/test_string.rb
index dc76b53810..0fd0538d00 100644
--- a/test/ruby/test_string.rb
+++ b/test/ruby/test_string.rb
@@ -444,6 +444,14 @@ CODE
 
     s = S("").freeze
     assert_raise_with_message(RuntimeError, /frozen/) {s.chomp!}
+
+    s = S("ax")
+    o = Struct.new(:s).new(s)
+    def o.to_str
+      s.freeze
+      "x"
+    end
+    assert_raise_with_message(RuntimeError, /frozen/) {s.chomp!(o)}
   ensure
     $/ = save
   end