Fix StringIO#initialize and preserve initial string's encoding when mode is w so the initial string is truncated

* close #3599

Author: andrykonchin

CHANGELOG.md

@@ -33,6 +33,7 @@ Compatibility:
 * Fix `Enumerable#reduce` to handle non-Symbol method name parameter (#2931, @andrykonchin).
 * Fix `RangeError` message to match CRuby for `Integer#chr` called with invalid codepoint argument (#2795, @andrykonchin).
 * Joni has been updated from 2.1.44 to 2.2.1 (@andrykonchin).
+* Fix `StringIO#initialize` and preserve initial string's encoding when mode is `w` so the initial string is truncated (#3599, @andrykonchin).
 
 Performance:
 

lib/truffle/stringio.rb

@@ -691,7 +691,7 @@ def yaml_initialize(type, val)
 
     d = @__data__ # no sync, only called from initialize
     raise Errno::EACCES, 'Permission denied' if @writable && d.string.frozen?
-    d.string.replace('') if truncate
+    d.string.replace(''.force_encoding(d.string.encoding)) if truncate
   end
 
   private def mode_from_integer(mode)
@@ -708,7 +708,7 @@ def yaml_initialize(type, val)
     end
 
     @append = true if (mode & IO::APPEND) != 0
-    d.string.replace('') if (mode & IO::TRUNC) != 0
+    d.string.replace(''.force_encoding(d.string.encoding)) if (mode & IO::TRUNC) != 0
   end
 
   private def getline(arg_error, sep, limit, chomp = false)

spec/ruby/library/stringio/initialize_spec.rb

@@ -130,6 +130,26 @@
     -> { @io.send(:initialize, str, "w") }.should raise_error(Errno::EACCES)
     -> { @io.send(:initialize, str, "a") }.should raise_error(Errno::EACCES)
   end
+
+  it "truncates all the content if passed w mode" do
+    io = StringIO.allocate
+    source = +"example".encode(Encoding::ISO_8859_1);
+
+    io.send(:initialize, source, "w")
+
+    io.string.should.empty?
+    io.string.encoding.should == Encoding::ISO_8859_1
+  end
+
+  it "truncates all the content if passed IO::TRUNC mode" do
+    io = StringIO.allocate
+    source = +"example".encode(Encoding::ISO_8859_1);
+
+    io.send(:initialize, source, IO::TRUNC)
+
+    io.string.should.empty?
+    io.string.encoding.should == Encoding::ISO_8859_1
+  end
 end
 
 describe "StringIO#initialize when passed [Object]" do