exp/template/html: autoescape actions in HTML style attributes.

This does not wire up <style> elements as that is pending support
for raw text content in CL https://golang.org/cl/4964045/

This CL allows actions to appear in contexts like

selectors:        {{.Tag}}{{.Class}}{{.Id}}
property names:   border-{{.BidiLeadingEdge}}
property values:  color: {{.Color}}
strings:          font-family: "{{font-name}}"
URL strings:      background: "/foo?image={{.ImgQuery}}"
URL literals:     background: url("{{.Image}}")

but disallows actions inside CSS comments and disallows
embedding of JS in CSS entirely.

It is based on the CSS3 lexical grammar with affordances for
common browser extensions including line comments.

R=nigeltao
CC=golang-dev
https://golang.org/cl/4968058

src/pkg/exp/template/html/Makefile

@@ -7,7 +7,10 @@ include ../../../../Make.inc
 TARG=exp/template/html
 GOFILES=\
 	context.go\
+	css.go\
 	escape.go\
+	html.go\
 	js.go\
+	url.go\
 
 include ../../../../Make.pkg

src/pkg/exp/template/html/context.go

@@ -63,23 +63,47 @@ const (
 	stateJSBlockCmt
 	// stateJSLineCmt occurs inside a JavaScript // line comment.
 	stateJSLineCmt
+	// stateCSS occurs inside a <style> element or style attribute.
+	stateCSS
+	// stateCSSDqStr occurs inside a CSS double quoted string.
+	stateCSSDqStr
+	// stateCSSSqStr occurs inside a CSS single quoted string.
+	stateCSSSqStr
+	// stateCSSDqURL occurs inside a CSS double quoted url("...").
+	stateCSSDqURL
+	// stateCSSSqURL occurs inside a CSS single quoted url('...').
+	stateCSSSqURL
+	// stateCSSURL occurs inside a CSS unquoted url(...).
+	stateCSSURL
+	// stateCSSBlockCmt occurs inside a CSS /* block comment */.
+	stateCSSBlockCmt
+	// stateCSSLineCmt occurs inside a CSS // line comment.
+	stateCSSLineCmt
 	// stateError is an infectious error state outside any valid
 	// HTML/CSS/JS construct.
 	stateError
 )
 
 var stateNames = [...]string{
-	stateText:       "stateText",
-	stateTag:        "stateTag",
-	stateAttr:       "stateAttr",
-	stateURL:        "stateURL",
-	stateJS:         "stateJS",
-	stateJSDqStr:    "stateJSDqStr",
-	stateJSSqStr:    "stateJSSqStr",
-	stateJSRegexp:   "stateJSRegexp",
-	stateJSBlockCmt: "stateJSBlockCmt",
-	stateJSLineCmt:  "stateJSLineCmt",
-	stateError:      "stateError",
+	stateText:        "stateText",
+	stateTag:         "stateTag",
+	stateAttr:        "stateAttr",
+	stateURL:         "stateURL",
+	stateJS:          "stateJS",
+	stateJSDqStr:     "stateJSDqStr",
+	stateJSSqStr:     "stateJSSqStr",
+	stateJSRegexp:    "stateJSRegexp",
+	stateJSBlockCmt:  "stateJSBlockCmt",
+	stateJSLineCmt:   "stateJSLineCmt",
+	stateCSS:         "stateCSS",
+	stateCSSDqStr:    "stateCSSDqStr",
+	stateCSSSqStr:    "stateCSSSqStr",
+	stateCSSDqURL:    "stateCSSDqURL",
+	stateCSSSqURL:    "stateCSSSqURL",
+	stateCSSURL:      "stateCSSURL",
+	stateCSSBlockCmt: "stateCSSBlockCmt",
+	stateCSSLineCmt:  "stateCSSLineCmt",
+	stateError:       "stateError",
 }
 
 func (s state) String() string {
@@ -132,8 +156,8 @@ const (
 	// urlPartQueryOrFrag occurs in the query portion between the ^s in
 	// "http://auth/path?^k=v#frag^".
 	urlPartQueryOrFrag
-	// urlPartUnknown occurs due to joining of contexts both before and after
-	// the query separator.
+	// urlPartUnknown occurs due to joining of contexts both before and
+	// after the query separator.
 	urlPartUnknown
 )
 

src/pkg/exp/template/html/css.go

@@ -0,0 +1,255 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package html
+
+import (
+	"bytes"
+	"fmt"
+	"unicode"
+	"utf8"
+)
+
+// endsWithCSSKeyword returns whether b ends with an ident that
+// case-insensitively matches the lower-case kw.
+func endsWithCSSKeyword(b []byte, kw string) bool {
+	i := len(b) - len(kw)
+	if i < 0 {
+		// Too short.
+		return false
+	}
+	if i != 0 {
+		r, _ := utf8.DecodeLastRune(b[:i])
+		if isCSSNmchar(r) {
+			// Too long.
+			return false
+		}
+	}
+	// Many CSS keywords, such as "!important" can have characters encoded,
+	// but the URI production does not allow that according to
+	// http://www.w3.org/TR/css3-syntax/#TOK-URI
+	// This does not attempt to recognize encoded keywords. For example,
+	// given "\75\72\6c" and "url" this return false.
+	return string(bytes.ToLower(b[i:])) == kw
+}
+
+// isCSSNmchar returns whether rune is allowed anywhere in a CSS identifier.
+func isCSSNmchar(rune int) bool {
+	// Based on the CSS3 nmchar production but ignores multi-rune escape
+	// sequences.
+	// http://www.w3.org/TR/css3-syntax/#SUBTOK-nmchar
+	return 'a' <= rune && rune <= 'z' ||
+		'A' <= rune && rune <= 'Z' ||
+		'0' <= rune && rune <= '9' ||
+		'-' == rune ||
+		'_' == rune ||
+		// Non-ASCII cases below.
+		0x80 <= rune && rune <= 0xd7ff ||
+		0xe000 <= rune && rune <= 0xfffd ||
+		0x10000 <= rune && rune <= 0x10ffff
+}
+
+// decodeCSS decodes CSS3 escapes given a sequence of stringchars.
+// If there is no change, it returns the input, otherwise it returns a slice
+// backed by a new array.
+// http://www.w3.org/TR/css3-syntax/#SUBTOK-stringchar defines stringchar.
+func decodeCSS(s []byte) []byte {
+	i := bytes.IndexByte(s, '\\')
+	if i == -1 {
+		return s
+	}
+	// The UTF-8 sequence for a codepoint is never longer than 1 + the
+	// number hex digits need to represent that codepoint, so len(s) is an
+	// upper bound on the output length.
+	b := make([]byte, 0, len(s))
+	for len(s) != 0 {
+		i := bytes.IndexByte(s, '\\')
+		if i == -1 {
+			i = len(s)
+		}
+		b, s = append(b, s[:i]...), s[i:]
+		if len(s) < 2 {
+			break
+		}
+		// http://www.w3.org/TR/css3-syntax/#SUBTOK-escape
+		// escape ::= unicode | '\' [#x20-#x7E#x80-#xD7FF#xE000-#xFFFD#x10000-#x10FFFF]
+		if isHex(s[1]) {
+			// http://www.w3.org/TR/css3-syntax/#SUBTOK-unicode
+			//   unicode ::= '\' [0-9a-fA-F]{1,6} wc?
+			j := 2
+			for j < len(s) && j < 7 && isHex(s[j]) {
+				j++
+			}
+			rune := hexDecode(s[1:j])
+			if rune > unicode.MaxRune {
+				rune, j = rune/16, j-1
+			}
+			n := utf8.EncodeRune(b[len(b):cap(b)], rune)
+			// The optional space at the end allows a hex
+			// sequence to be followed by a literal hex.
+			// string(decodeCSS([]byte(`\A B`))) == "\nB"
+			b, s = b[:len(b)+n], skipCSSSpace(s[j:])
+		} else {
+			// `\\` decodes to `\` and `\"` to `"`.
+			_, n := utf8.DecodeRune(s[1:])
+			b, s = append(b, s[1:1+n]...), s[1+n:]
+		}
+	}
+	return b
+}
+
+// isHex returns whether the given character is a hex digit.
+func isHex(c byte) bool {
+	return '0' <= c && c <= '9' || 'a' <= c && c <= 'f' || 'A' <= c && c <= 'F'
+}
+
+// hexDecode decodes a short hex digit sequence: "10" -> 16.
+func hexDecode(s []byte) int {
+	n := 0
+	for _, c := range s {
+		n <<= 4
+		switch {
+		case '0' <= c && c <= '9':
+			n |= int(c - '0')
+		case 'a' <= c && c <= 'f':
+			n |= int(c-'a') + 10
+		case 'A' <= c && c <= 'F':
+			n |= int(c-'A') + 10
+		default:
+			panic(fmt.Sprintf("Bad hex digit in %q", s))
+		}
+	}
+	return n
+}
+
+// skipCSSSpace returns a suffix of c, skipping over a single space.
+func skipCSSSpace(c []byte) []byte {
+	if len(c) == 0 {
+		return c
+	}
+	// wc ::= #x9 | #xA | #xC | #xD | #x20
+	switch c[0] {
+	case '\t', '\n', '\f', ' ':
+		return c[1:]
+	case '\r':
+		// This differs from CSS3's wc production because it contains a
+		// probable spec error whereby wc contains all the single byte
+		// sequences in nl (newline) but not CRLF.
+		if len(c) >= 2 && c[1] == '\n' {
+			return c[2:]
+		}
+		return c[1:]
+	}
+	return c
+}
+
+// cssEscaper escapes HTML and CSS special characters using \<hex>+ escapes.
+func cssEscaper(args ...interface{}) string {
+	s := stringify(args...)
+	var b bytes.Buffer
+	written := 0
+	for i, r := range s {
+		var repl string
+		switch r {
+		case 0:
+			repl = `\0`
+		case '\t':
+			repl = `\9`
+		case '\n':
+			repl = `\a`
+		case '\f':
+			repl = `\c`
+		case '\r':
+			repl = `\d`
+		// Encode HTML specials as hex so the output can be embedded
+		// in HTML attributes without further encoding.
+		case '"':
+			repl = `\22`
+		case '&':
+			repl = `\26`
+		case '\'':
+			repl = `\27`
+		case '(':
+			repl = `\28`
+		case ')':
+			repl = `\29`
+		case '+':
+			repl = `\2b`
+		case '/':
+			repl = `\2f`
+		case ':':
+			repl = `\3a`
+		case ';':
+			repl = `\3b`
+		case '<':
+			repl = `\3c`
+		case '>':
+			repl = `\3e`
+		case '\\':
+			repl = `\\`
+		case '{':
+			repl = `\7b`
+		case '}':
+			repl = `\7d`
+		default:
+			continue
+		}
+		b.WriteString(s[written:i])
+		b.WriteString(repl)
+		written = i + utf8.RuneLen(r)
+		if repl != `\\` && (written == len(s) || isHex(s[written])) {
+			b.WriteByte(' ')
+		}
+	}
+	if written == 0 {
+		return s
+	}
+	b.WriteString(s[written:])
+	return b.String()
+}
+
+var expressionBytes = []byte("expression")
+var mozBindingBytes = []byte("mozbinding")
+
+// cssValueFilter allows innocuous CSS values in the output including CSS
+// quantities (10px or 25%), ID or class literals (#foo, .bar), keyword values
+// (inherit, blue), and colors (#888).
+// It filters out unsafe values, such as those that affect token boundaries,
+// and anything that might execute scripts.
+func cssValueFilter(args ...interface{}) string {
+	s, id := decodeCSS([]byte(stringify(args...))), make([]byte, 0, 64)
+
+	// CSS3 error handling is specified as honoring string boundaries per
+	// http://www.w3.org/TR/css3-syntax/#error-handling :
+	//     Malformed declarations. User agents must handle unexpected
+	//     tokens encountered while parsing a declaration by reading until
+	//     the end of the declaration, while observing the rules for
+	//     matching pairs of (), [], {}, "", and '', and correctly handling
+	//     escapes. For example, a malformed declaration may be missing a
+	//     property, colon (:) or value.
+	// So we need to make sure that values do not have mismatched bracket
+	// or quote characters to prevent the browser from restarting parsing
+	// inside a string that might embed JavaScript source.
+	for i, c := range s {
+		switch c {
+		case 0, '"', '\'', '(', ')', '/', ';', '@', '[', '\\', ']', '`', '{', '}':
+			return filterFailsafe
+		case '-':
+			// Disallow <!-- or -->.
+			// -- should not appear in valid identifiers.
+			if i != 0 && '-' == s[i-1] {
+				return filterFailsafe
+			}
+		default:
+			if c < 0x80 && isCSSNmchar(int(c)) {
+				id = append(id, c)
+			}
+		}
+	}
+	id = bytes.ToLower(id)
+	if bytes.Index(id, expressionBytes) != -1 || bytes.Index(id, mozBindingBytes) != -1 {
+		return filterFailsafe
+	}
+	return string(s)
+}