Fix position updates in StringIO to always use byte lengths.

Author: aardvark179

CHANGELOG.md

@@ -6,6 +6,7 @@ New features:
 
 Bug fixes:
 
+* Fix `StringIO` to set position correctly after reading multi-byte characters (#2207, @aardvark179).
 
 Compatibility:
 

lib/truffle/stringio.rb

@@ -211,7 +211,7 @@ def each_byte
     d = @__data__
     string = d.string
 
-    while d.pos < string.length
+    while d.pos < string.bytesize
       check_readable
       byte = string.getbyte d.pos
       d.pos += 1
@@ -468,7 +468,7 @@ def read(length = nil, buffer = nil)
       buffer.replace str if buffer
     end
 
-    d.pos += str.length
+    d.pos += str.bytesize
     str
   end