Update rb_str_modify and rb_str_modify_expand to raise a FrozenError for frozen strings

Author: bjfish

CHANGELOG.md

@@ -9,6 +9,7 @@ Bug fixes:
 Compatibility:
 
 * Implement `Process::Status.wait` (#2378).
+* Update `rb_str_modify` and `rb_str_modify_expand` to raise a `FrozenError` when given a frozen string (#2392).
 
 Performance:
 

lib/cext/ABI_check.txt

@@ -1 +1 @@
-3
+4

spec/ruby/optional/capi/string_spec.rb

@@ -665,6 +665,12 @@ def inspect
     end
   end
 
+  describe "rb_str_modify" do
+    it "raises an error if the string is frozen" do
+      -> { @s.rb_str_modify("frozen".freeze) }.should raise_error(FrozenError)
+    end
+  end
+
   describe "rb_str_modify_expand" do
     it "grows the capacity to bytesize + expand, not changing the bytesize" do
       str = @s.rb_str_buf_new(256, "abcd")
@@ -685,6 +691,10 @@ def inspect
       @s.RSTRING_LEN(str).should == 3
       @s.rb_str_capacity(str).should == 1027
     end
+
+    it "raises an error if the string is frozen" do
+      -> { @s.rb_str_modify_expand("frozen".freeze, 10) }.should raise_error(FrozenError)
+    end
   end
 
   describe "rb_str_resize" do

src/main/c/cext/string.c

@@ -174,6 +174,7 @@ VALUE rb_str_split(VALUE string, const char *split) {
 }
 
 void rb_str_modify(VALUE string) {
+  rb_check_frozen(string);
   ENC_CODERANGE_CLEAR(string);
 }
 
@@ -337,6 +338,7 @@ void rb_str_modify_expand(VALUE str, long expand) {
     rb_raise(rb_eArgError, "string size too big");
   }
 
+  rb_check_frozen(str);
   if (expand > 0) {
     // rb_str_modify_expand() resizes the native buffer but does not change
     // RSTRING_LEN() (and therefore String#bytesize).