xml: Parser hook for non-UTF-8 charset converters

Adds an optional hook to Parser to let charset
converters step in when a processing directive
with a non-UTF-8 encoding is specified.

(Open to alternative proposals too...)

R=rsc
CC=golang-dev
https://golang.org/cl/4437061

src/pkg/xml/xml.go

@@ -163,6 +163,13 @@ type Parser struct {
 	//	"quot": `"`,
 	Entity map[string]string
 
+	// CharsetReader, if non-nil, defines a function to generate
+	// charset-conversion readers, converting from the provided
+	// non-UTF-8 charset into UTF-8. If CharsetReader is nil or
+	// returns an error, parsing stops with an error. One of the
+	// the CharsetReader's result values must be non-nil.
+	CharsetReader func(charset string, input io.Reader) (io.Reader, os.Error)
+
 	r         io.ByteReader
 	buf       bytes.Buffer
 	saved     *bytes.Buffer
@@ -186,17 +193,7 @@ func NewParser(r io.Reader) *Parser {
 		line:     1,
 		Strict:   true,
 	}
-
-	// Get efficient byte at a time reader.
-	// Assume that if reader has its own
-	// ReadByte, it's efficient enough.
-	// Otherwise, use bufio.
-	if rb, ok := r.(io.ByteReader); ok {
-		p.r = rb
-	} else {
-		p.r = bufio.NewReader(r)
-	}
-
+	p.switchToReader(r)
 	return p
 }
 
@@ -290,6 +287,18 @@ func (p *Parser) translate(n *Name, isElementName bool) {
 	}
 }
 
+func (p *Parser) switchToReader(r io.Reader) {
+	// Get efficient byte at a time reader.
+	// Assume that if reader has its own
+	// ReadByte, it's efficient enough.
+	// Otherwise, use bufio.
+	if rb, ok := r.(io.ByteReader); ok {
+		p.r = rb
+	} else {
+		p.r = bufio.NewReader(r)
+	}
+}
+
 // Parsing state - stack holds old name space translations
 // and the current set of open elements.  The translations to pop when
 // ending a given tag are *below* it on the stack, which is
@@ -487,6 +496,25 @@ func (p *Parser) RawToken() (Token, os.Error) {
 		}
 		data := p.buf.Bytes()
 		data = data[0 : len(data)-2] // chop ?>
+
+		if target == "xml" {
+			enc := procInstEncoding(string(data))
+			if enc != "" && enc != "utf-8" && enc != "UTF-8" {
+				if p.CharsetReader == nil {
+					p.err = fmt.Errorf("xml: encoding %q declared but Parser.CharsetReader is nil", enc)
+					return nil, p.err
+				}
+				newr, err := p.CharsetReader(enc, p.r.(io.Reader))
+				if err != nil {
+					p.err = fmt.Errorf("xml: opening charset %q: %v", enc, err)
+					return nil, p.err
+				}
+				if newr == nil {
+					panic("CharsetReader returned a nil Reader for charset " + enc)
+				}
+				p.switchToReader(newr)
+			}
+		}
 		return ProcInst{target, data}, nil
 
 	case '!':
@@ -1633,3 +1661,26 @@ func Escape(w io.Writer, s []byte) {
 	}
 	w.Write(s[last:])
 }
+
+// procInstEncoding parses the `encoding="..."` or `encoding='...'`
+// value out of the provided string, returning "" if not found.
+func procInstEncoding(s string) string {
+	// TODO: this parsing is somewhat lame and not exact.
+	// It works for all actual cases, though.
+	idx := strings.Index(s, "encoding=")
+	if idx == -1 {
+		return ""
+	}
+	v := s[idx+len("encoding="):]
+	if v == "" {
+		return ""
+	}
+	if v[0] != '\'' && v[0] != '"' {
+		return ""
+	}
+	idx = strings.IndexRune(v[1:], int(v[0]))
+	if idx == -1 {
+		return ""
+	}
+	return v[1 : idx+1]
+}

src/pkg/xml/xml_test.go

@@ -9,6 +9,7 @@ import (
 	"io"
 	"os"
 	"reflect"
+	"strings"
 	"testing"
 )
 
@@ -96,6 +97,19 @@ var cookedTokens = []Token{
 	Comment([]byte(" missing final newline ")),
 }
 
+const testInputAltEncoding = `
+<?xml version="1.0" encoding="x-testing-uppercase"?>
+<TAG>VALUE</TAG>`
+
+var rawTokensAltEncoding = []Token{
+	CharData([]byte("\n")),
+	ProcInst{"xml", []byte(`version="1.0" encoding="x-testing-uppercase"`)},
+	CharData([]byte("\n")),
+	StartElement{Name{"", "tag"}, nil},
+	CharData([]byte("value")),
+	EndElement{Name{"", "tag"}},
+}
+
 var xmlInput = []string{
 	// unexpected EOF cases
 	"<",
@@ -173,7 +187,64 @@ func StringReader(s string) io.Reader { return &stringReader{s, 0} }
 
 func TestRawToken(t *testing.T) {
 	p := NewParser(StringReader(testInput))
+	testRawToken(t, p, rawTokens)
+}
 
+type downCaser struct {
+	t *testing.T
+	r io.ByteReader
+}
+
+func (d *downCaser) ReadByte() (c byte, err os.Error) {
+	c, err = d.r.ReadByte()
+	if c >= 'A' && c <= 'Z' {
+		c += 'a' - 'A'
+	}
+	return
+}
+
+func (d *downCaser) Read(p []byte) (int, os.Error) {
+	d.t.Fatalf("unexpected Read call on downCaser reader")
+	return 0, os.EINVAL
+}
+
+func TestRawTokenAltEncoding(t *testing.T) {
+	sawEncoding := ""
+	p := NewParser(StringReader(testInputAltEncoding))
+	p.CharsetReader = func(charset string, input io.Reader) (io.Reader, os.Error) {
+		sawEncoding = charset
+		if charset != "x-testing-uppercase" {
+			t.Fatalf("unexpected charset %q", charset)
+		}
+		return &downCaser{t, input.(io.ByteReader)}, nil
+	}
+	testRawToken(t, p, rawTokensAltEncoding)
+}
+
+func TestRawTokenAltEncodingNoConverter(t *testing.T) {
+	p := NewParser(StringReader(testInputAltEncoding))
+	token, err := p.RawToken()
+	if token == nil {
+		t.Fatalf("expected a token on first RawToken call")
+	}
+	if err != nil {
+		t.Fatal(err)
+	}
+	token, err = p.RawToken()
+	if token != nil {
+		t.Errorf("expected a nil token; got %#v", token)
+	}
+	if err == nil {
+		t.Fatalf("expected an error on second RawToken call")
+	}
+	const encoding = "x-testing-uppercase"
+	if !strings.Contains(err.String(), encoding) {
+		t.Errorf("expected error to contain %q; got error: %v",
+			encoding, err)
+	}
+}
+
+func testRawToken(t *testing.T, p *Parser, rawTokens []Token) {
 	for i, want := range rawTokens {
 		have, err := p.RawToken()
 		if err != nil {
@@ -483,3 +554,26 @@ func TestDisallowedCharacters(t *testing.T) {
 		}
 	}
 }
+
+type procInstEncodingTest struct {
+	expect, got string
+}
+
+var procInstTests = []struct {
+	input, expect string
+}{
+	{`version="1.0" encoding="utf-8"`, "utf-8"},
+	{`version="1.0" encoding='utf-8'`, "utf-8"},
+	{`version="1.0" encoding='utf-8' `, "utf-8"},
+	{`version="1.0" encoding=utf-8`, ""},
+	{`encoding="FOO" `, "FOO"},
+}
+
+func TestProcInstEncoding(t *testing.T) {
+	for _, test := range procInstTests {
+		got := procInstEncoding(test.input)
+		if got != test.expect {
+			t.Errorf("procInstEncoding(%q) = %q; want %q", test.input, got, test.expect)
+		}
+	}
+}