fix: StringIO#write transcodes strings with a different encoding

CHANGELOG.md

@@ -24,6 +24,7 @@ Bug fixes:
 * Fix constants lookup when `BasicObject#instance_eval` method is called with a String (#2810, @andrykonchin).
 * Don't trigger the `method_added` event when changing a method's visibility or calling `module_function` (@paracycle, @nirvdrum).
 * Fix `rb_time_timespec_new` function to not call `Time.at` method directly (@andrykonchin).
+* Fix `StringIO#write` to transcode strings with encodings that don't match the `StringIO`'s `external_encoding`. (#2839, @flavorjones)
 
 Compatibility:
 

lib/truffle/stringio.rb

@@ -280,6 +280,13 @@ def write(str)
     str = String(str)
     return 0 if str.empty?
 
+    if external_encoding &&
+       external_encoding != str.encoding &&
+       external_encoding != Encoding::BINARY &&
+       str.encoding != Encoding::BINARY
+      str = str.encode(external_encoding)
+    end
+
     d = @__data__
     TruffleRuby.synchronized(d) do
       pos = d.pos

spec/ruby/library/stringio/write_spec.rb

@@ -17,3 +17,23 @@
 describe "StringIO#write when in append mode" do
   it_behaves_like :stringio_write_append, :write
 end
+
+describe "StringIO#write transcoding" do
+  describe "when UTF-16 encoding is set" do
+    it "accepts a UTF-8-encoded string and transcodes it" do
+      io = StringIO.new.set_encoding(Encoding::UTF_16)
+      utf8_str = "hello"
+
+      io.write(utf8_str)
+
+      result = io.string
+      expected = [
+        254, 255, # BOM
+        0, 104, 0, 101, 0, 108, 0, 108, 0, 111, # double-width "hello"
+      ]
+
+      io.external_encoding.should == Encoding::UTF_16
+      result.bytes.should == expected
+    end
+  end
+end