File.dirname: add a spec for Shift JIS handling

While trying to speedup various `File.*` methods, I realized they
were way slower and complicated than they should for no apparent
reason. However after asking Nobu he explained that Shift JIS encoded
text can contain `0x5C` (ASCII backslash) as the second byte of a
two byte character sequence.

Since on Windows `0x5C` is `File::ALT_SEPARATOR`, this can easily
break naive path related algorithms searching for directory separators.

Author: byroot

core/file/dirname_spec.rb

@@ -78,6 +78,37 @@ def object.to_int; 2; end
     File.dirname("foo/../").should == "foo"
   end
 
+  it "rejects strings encoded with non ASCII-compatible or dummy encodings" do
+    valid, invalid = Encoding.list.partition { |e| e.ascii_compatible? && !e.dummy? }
+
+    valid.reject do |enc|
+      path = "/foo/bar".encode(enc)
+      expected = "/foo".encode(enc)
+
+      File.dirname(path) == expected
+    end.should == []
+
+    invalid.reject do |enc|
+      path = "/foo/bar".encode(enc)
+    rescue Encoding::ConverterNotFoundError
+      true
+    else
+      begin
+        File.dirname(path)
+        false
+      rescue Encoding::CompatibilityError
+        true
+      end
+    end.should == []
+  end
+
+  it "handles Shift JIS 0x5C (\\) as second byte of a multi-byte sequence" do
+    # dir/fileソname.txt
+    path = "dir/file\x83\x5cname.txt".b.force_encoding(Encoding::SHIFT_JIS)
+    path.valid_encoding?.should be_true
+    File.dirname(path).should == "dir"
+  end
+
   platform_is_not :windows do
     it "returns all the components of filename except the last one (edge cases on non-windows)" do
       File.dirname('/////').should == '/'