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 +280,8 @@ def write(str)
     str = String(str)
     return 0 if str.empty?
 
+    str = Truffle::IOOperations.write_transcoding(str, external_encoding)
+
     d = @__data__
     TruffleRuby.synchronized(d) do
       pos = d.pos

spec/ruby/core/io/shared/write.rb

@@ -97,3 +97,55 @@
     end
   end
 end
+
+describe :io_write_transcode, shared: true do
+  before :each do
+    @transcode_filename = tmp("io_write_transcode")
+  end
+
+  after :each do
+    rm_r @transcode_filename
+  end
+
+  describe "transcoding when UTF-16 encoding is set" do
+    it "accepts a UTF-8-encoded string and transcodes it" do
+      utf8_str = "hello"
+
+      File.open(@transcode_filename, "w", external_encoding: Encoding::UTF_16BE) do |file|
+        file.external_encoding.should == Encoding::UTF_16BE
+        file.send(@method, utf8_str)
+      end
+
+      result = File.binread(@transcode_filename)
+      expected = [0, 104, 0, 101, 0, 108, 0, 108, 0, 111] # double-width "hello"
+
+      result.bytes.should == expected
+    end
+  end
+end
+
+describe :io_write_no_transcode, shared: true do
+  before :each do
+    @transcode_filename = tmp("io_write_no_transcode")
+  end
+
+  after :each do
+    rm_r @transcode_filename
+  end
+
+  describe "transcoding when UTF-16 encoding is set" do
+    it "accepts a UTF-8-encoded string and transcodes it" do
+      utf8_str = "hello"
+
+      File.open(@transcode_filename, "w", external_encoding: Encoding::UTF_16BE) do |file|
+        file.external_encoding.should == Encoding::UTF_16BE
+        file.send(@method, utf8_str)
+      end
+
+      result = File.binread(@transcode_filename)
+      expected = [104, 101, 108, 108, 111] # not transcoded to UTF-16BE
+
+      result.bytes.should == expected
+    end
+  end
+end

spec/ruby/core/io/syswrite_spec.rb

@@ -78,4 +78,5 @@
 
 describe "IO#syswrite" do
   it_behaves_like :io_write, :syswrite
+  it_behaves_like :io_write_no_transcode, :syswrite
 end

spec/ruby/core/io/write_nonblock_spec.rb

@@ -50,6 +50,7 @@
 
   describe "IO#write_nonblock" do
     it_behaves_like :io_write, :write_nonblock
+    it_behaves_like :io_write_no_transcode, :write_nonblock
   end
 end
 

spec/ruby/core/io/write_spec.rb

@@ -220,6 +220,7 @@
 
 describe "IO#write" do
   it_behaves_like :io_write, :write
+  it_behaves_like :io_write_transcode, :write
 
   it "accepts multiple arguments" do
     IO.pipe do |r, w|

spec/ruby/library/stringio/shared/write.rb

@@ -88,6 +88,25 @@
     @io.write "fghi"
     @io.string.should == "12fghi"
   end
+
+  describe "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_16BE)
+        utf8_str = "hello"
+
+        io.send(@method, utf8_str)
+
+        result = io.string
+        expected = [
+          0, 104, 0, 101, 0, 108, 0, 108, 0, 111, # double-width "hello"
+        ]
+
+        io.external_encoding.should == Encoding::UTF_16BE
+        result.bytes.should == expected
+      end
+    end
+  end
 end
 
 describe :stringio_write_not_writable, shared: true do

src/main/ruby/truffleruby/core/io.rb

@@ -2324,11 +2324,7 @@ def write(*objects)
 
       ensure_open_and_writable
 
-      if external_encoding && external_encoding != string.encoding && external_encoding != Encoding::BINARY
-        unless string.ascii_only? && external_encoding.ascii_compatible?
-          string = string.encode(external_encoding)
-        end
-      end
+      string = Truffle::IOOperations.write_transcoding(string, external_encoding)
 
       count = Truffle::POSIX.write_string self, string, true
       bytes_written += count

src/main/ruby/truffleruby/core/truffle/io_operations.rb

@@ -72,6 +72,15 @@ def self.puts(io, *args)
       nil
     end
 
+    def self.write_transcoding(string, external_encoding)
+      if external_encoding && external_encoding != string.encoding && external_encoding != Encoding::BINARY &&
+          !(string.ascii_only? && external_encoding.ascii_compatible?)
+        string.encode(external_encoding)
+      else
+        string
+      end
+    end
+
     def self.dup2_with_cloexec(old_fd, new_fd)
       if new_fd < 3
         # STDIO should not be made close-on-exec. `dup2` clears the close-on-exec bit for the destination FD.