html/template: support parsing complex JS template literals

This change undoes the restrictions added in CL 482079, which added a
blanket ban on using actions within JS template literal strings, and
adds logic to support actions while properly applies contextual escaping
based on the correct context within the literal.

Since template literals can contain both normal strings, and nested JS
contexts, logic is required to properly track those context switches
during parsing.

ErrJsTmplLit is deprecated, and the GODEBUG flag jstmpllitinterp no
longer does anything.

Fixes #61619

Change-Id: I0338cc6f663723267b8f7aaacc55aa28f60906f2
Reviewed-on: https://go-review.googlesource.com/c/go/+/507995
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Roland Shoemaker <roland@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>

api/next/61619.txt

@@ -0,0 +1 @@
+pkg html/template, const ErrJSTemplate //deprecated #61619

src/html/template/context.go

@@ -21,6 +21,8 @@ type context struct {
 	delim           delim
 	urlPart         urlPart
 	jsCtx           jsCtx
+	jsTmplExprDepth int
+	jsBraceDepth    int
 	attr            attr
 	element         element
 	n               parse.Node // for range break/continue
@@ -120,8 +122,8 @@ const (
 	stateJSDqStr
 	// stateJSSqStr occurs inside a JavaScript single quoted string.
 	stateJSSqStr
-	// stateJSBqStr occurs inside a JavaScript back quoted string.
-	stateJSBqStr
+	// stateJSTmplLit occurs inside a JavaScript back quoted string.
+	stateJSTmplLit
 	// stateJSRegexp occurs inside a JavaScript regexp literal.
 	stateJSRegexp
 	// stateJSBlockCmt occurs inside a JavaScript /* block comment */.
@@ -182,7 +184,7 @@ func isInScriptLiteral(s state) bool {
 	// stateJSHTMLOpenCmt, stateJSHTMLCloseCmt) because their content is already
 	// omitted from the output.
 	switch s {
-	case stateJSDqStr, stateJSSqStr, stateJSBqStr, stateJSRegexp:
+	case stateJSDqStr, stateJSSqStr, stateJSTmplLit, stateJSRegexp:
 		return true
 	}
 	return false

src/html/template/error.go

@@ -221,6 +221,10 @@ const (
 	// Discussion:
 	//   Package html/template does not support actions inside of JS template
 	//   literals.
+	//
+	// Deprecated: ErrJSTemplate is no longer returned when an action is present
+	// in a JS template literal. Actions inside of JS template literals are now
+	// escaped as expected.
 	ErrJSTemplate
 )
 

src/html/template/escape.go

@@ -70,6 +70,7 @@ var funcMap = template.FuncMap{
 	"_html_template_htmlescaper":      htmlEscaper,
 	"_html_template_jsregexpescaper":  jsRegexpEscaper,
 	"_html_template_jsstrescaper":     jsStrEscaper,
+	"_html_template_jstmpllitescaper": jsTmplLitEscaper,
 	"_html_template_jsvalescaper":     jsValEscaper,
 	"_html_template_nospaceescaper":   htmlNospaceEscaper,
 	"_html_template_rcdataescaper":    rcdataEscaper,
@@ -227,16 +228,8 @@ func (e *escaper) escapeAction(c context, n *parse.ActionNode) context {
 		c.jsCtx = jsCtxDivOp
 	case stateJSDqStr, stateJSSqStr:
 		s = append(s, "_html_template_jsstrescaper")
-	case stateJSBqStr:
-		if debugAllowActionJSTmpl.Value() == "1" {
-			debugAllowActionJSTmpl.IncNonDefault()
-			s = append(s, "_html_template_jsstrescaper")
-		} else {
-			return context{
-				state: stateError,
-				err:   errorf(ErrJSTemplate, n, n.Line, "%s appears in a JS template literal", n),
-			}
-		}
+	case stateJSTmplLit:
+		s = append(s, "_html_template_jstmpllitescaper")
 	case stateJSRegexp:
 		s = append(s, "_html_template_jsregexpescaper")
 	case stateCSS:
@@ -395,6 +388,9 @@ var redundantFuncs = map[string]map[string]bool{
 	"_html_template_jsstrescaper": {
 		"_html_template_attrescaper": true,
 	},
+	"_html_template_jstmpllitescaper": {
+		"_html_template_attrescaper": true,
+	},
 	"_html_template_urlescaper": {
 		"_html_template_urlnormalizer": true,
 	},

src/html/template/escape_test.go

@@ -31,7 +31,7 @@ func (x *goodMarshaler) MarshalJSON() ([]byte, error) {
 func TestEscape(t *testing.T) {
 	data := struct {
 		F, T       bool
-		C, G, H string
+		C, G, H, I string
 		A, E       []string
 		B, M       json.Marshaler
 		N          int
@@ -52,6 +52,7 @@ func TestEscape(t *testing.T) {
 		U: nil,
 		Z: nil,
 		W: HTML(`&iexcl;<b class="foo">Hello</b>, <textarea>O'World</textarea>!`),
+		I: "${ asd `` }",
 	}
 	pdata := &data
 
@@ -718,6 +719,21 @@ func TestEscape(t *testing.T) {
 			"<p name=\"{{.U}}\">",
 			"<p name=\"\">",
 		},
+		{
+			"JS template lit special characters",
+			"<script>var a = `{{.I}}`</script>",
+			"<script>var a = `\\u0024\\u007b asd \\u0060\\u0060 \\u007d`</script>",
+		},
+		{
+			"JS template lit special characters, nested lit",
+			"<script>var a = `${ `{{.I}}` }`</script>",
+			"<script>var a = `${ `\\u0024\\u007b asd \\u0060\\u0060 \\u007d` }`</script>",
+		},
+		{
+			"JS template lit, nested JS",
+			"<script>var a = `${ var a = \"{{\"a \\\" d\"}}\" }`</script>",
+			"<script>var a = `${ var a = \"a \\u0022 d\" }`</script>",
+		},
 	}
 
 	for _, test := range tests {
@@ -976,6 +992,31 @@ func TestErrors(t *testing.T) {
 			"<script>var a = `${a+b}`</script>`",
 			"",
 		},
+		{
+			"<script>var tmpl = `asd`;</script>",
+			``,
+		},
+		{
+			"<script>var tmpl = `${1}`;</script>",
+			``,
+		},
+		{
+			"<script>var tmpl = `${return ``}`;</script>",
+			``,
+		},
+		{
+			"<script>var tmpl = `${return {{.}} }`;</script>",
+			``,
+		},
+		{
+			"<script>var tmpl = `${ let a = {1:1} {{.}} }`;</script>",
+			``,
+		},
+		{
+			"<script>var tmpl = `asd ${return \"{\"}`;</script>",
+			``,
+		},
+
 		// Error cases.
 		{
 			"{{if .Cond}}<a{{end}}",
@@ -1122,10 +1163,26 @@ func TestErrors(t *testing.T) {
 			// html is allowed since it is the last command in the pipeline, but urlquery is not.
 			`predefined escaper "urlquery" disallowed in template`,
 		},
-		{
-			"<script>var tmpl = `asd {{.}}`;</script>",
-			`{{.}} appears in a JS template literal`,
-		},
+		// {
+		// 	"<script>var tmpl = `asd {{.}}`;</script>",
+		// 	`{{.}} appears in a JS template literal`,
+		// },
+		// {
+		// 	"<script>var v = `${function(){return `{{.V}}+1`}()}`;</script>",
+		// 	`{{.V}} appears in a JS template literal`,
+		// },
+		// {
+		// 	"<script>var a = `asd ${function(){b = {1:2}; return`{{.}}`}}`</script>",
+		// 	`{{.}} appears in a JS template literal`,
+		// },
+		// {
+		// 	"<script>var tmpl = `${return `{{.}}`}`;</script>",
+		// 	`{{.}} appears in a JS template literal`,
+		// },
+		// {
+		// 	"<script>var tmpl = `${return {`{{.}}`}`;</script>",
+		// 	`{{.}} appears in a JS template literal`,
+		// },
 	}
 	for _, test := range tests {
 		buf := new(bytes.Buffer)
@@ -1349,7 +1406,7 @@ func TestEscapeText(t *testing.T) {
 		},
 		{
 			"<a onclick=\"`foo",
-			context{state: stateJSBqStr, delim: delimDoubleQuote, attr: attrScript},
+			context{state: stateJSTmplLit, delim: delimDoubleQuote, attr: attrScript},
 		},
 		{
 			`<A ONCLICK="'`,
@@ -1691,6 +1748,58 @@ func TestEscapeText(t *testing.T) {
 			`<svg:a svg:onclick="x()">`,
 			context{},
 		},
+		{
+			"<script>var a = `",
+			context{state: stateJSTmplLit, element: elementScript},
+		},
+		{
+			"<script>var a = `${",
+			context{state: stateJS, element: elementScript},
+		},
+		{
+			"<script>var a = `${}",
+			context{state: stateJSTmplLit, element: elementScript},
+		},
+		{
+			"<script>var a = `${`",
+			context{state: stateJSTmplLit, element: elementScript},
+		},
+		{
+			"<script>var a = `${var a = \"",
+			context{state: stateJSDqStr, element: elementScript},
+		},
+		{
+			"<script>var a = `${var a = \"`",
+			context{state: stateJSDqStr, element: elementScript},
+		},
+		{
+			"<script>var a = `${var a = \"}",
+			context{state: stateJSDqStr, element: elementScript},
+		},
+		{
+			"<script>var a = `${``",
+			context{state: stateJS, element: elementScript},
+		},
+		{
+			"<script>var a = `${`}",
+			context{state: stateJSTmplLit, element: elementScript},
+		},
+		{
+			"<script>`${ {} } asd`</script><script>`${ {} }",
+			context{state: stateJSTmplLit, element: elementScript},
+		},
+		{
+			"<script>var foo = `${ (_ => { return \"x\" })() + \"${",
+			context{state: stateJSDqStr, element: elementScript},
+		},
+		{
+			"<script>var a = `${ {</script><script>var b = `${ x }",
+			context{state: stateJSTmplLit, element: elementScript, jsCtx: jsCtxDivOp},
+		},
+		{
+			"<script>var foo = `x` + \"${",
+			context{state: stateJSDqStr, element: elementScript},
+		},
 	}
 
 	for _, test := range tests {

src/html/template/js.go

@@ -238,6 +238,11 @@ func jsStrEscaper(args ...any) string {
 	return replace(s, jsStrReplacementTable)
 }
 
+func jsTmplLitEscaper(args ...any) string {
+	s, _ := stringify(args...)
+	return replace(s, jsBqStrReplacementTable)
+}
+
 // jsRegexpEscaper behaves like jsStrEscaper but escapes regular expression
 // specials so the result is treated literally when included in a regular
 // expression literal. /foo{{.X}}bar/ matches the string "foo" followed by
@@ -324,6 +329,31 @@ var jsStrReplacementTable = []string{
 	'\\': `\\`,
 }
 
+// jsBqStrReplacementTable is like jsStrReplacementTable except it also contains
+// the special characters for JS template literals: $, {, and }.
+var jsBqStrReplacementTable = []string{
+	0:    `\u0000`,
+	'\t': `\t`,
+	'\n': `\n`,
+	'\v': `\u000b`, // "\v" == "v" on IE 6.
+	'\f': `\f`,
+	'\r': `\r`,
+	// Encode HTML specials as hex so the output can be embedded
+	// in HTML attributes without further encoding.
+	'"':  `\u0022`,
+	'`':  `\u0060`,
+	'&':  `\u0026`,
+	'\'': `\u0027`,
+	'+':  `\u002b`,
+	'/':  `\/`,
+	'<':  `\u003c`,
+	'>':  `\u003e`,
+	'\\': `\\`,
+	'$':  `\u0024`,
+	'{':  `\u007b`,
+	'}':  `\u007d`,
+}
+
 // jsStrNormReplacementTable is like jsStrReplacementTable but does not
 // overencode existing escapes since this table has no entry for `\`.
 var jsStrNormReplacementTable = []string{

src/html/template/state_string.go

@@ -21,7 +21,7 @@ func _() {
 	_ = x[stateJS-10]
 	_ = x[stateJSDqStr-11]
 	_ = x[stateJSSqStr-12]
-	_ = x[stateJSBqStr-13]
+	_ = x[stateJSTmplLit-13]
 	_ = x[stateJSRegexp-14]
 	_ = x[stateJSBlockCmt-15]
 	_ = x[stateJSLineCmt-16]
@@ -39,9 +39,9 @@ func _() {
 	_ = x[stateDead-28]
 }
 
-const _state_name = "stateTextstateTagstateAttrNamestateAfterNamestateBeforeValuestateHTMLCmtstateRCDATAstateAttrstateURLstateSrcsetstateJSstateJSDqStrstateJSSqStrstateJSBqStrstateJSRegexpstateJSBlockCmtstateJSLineCmtstateJSHTMLOpenCmtstateJSHTMLCloseCmtstateCSSstateCSSDqStrstateCSSSqStrstateCSSDqURLstateCSSSqURLstateCSSURLstateCSSBlockCmtstateCSSLineCmtstateErrorstateDead"
+const _state_name = "stateTextstateTagstateAttrNamestateAfterNamestateBeforeValuestateHTMLCmtstateRCDATAstateAttrstateURLstateSrcsetstateJSstateJSDqStrstateJSSqStrstateJSTmplLitstateJSRegexpstateJSBlockCmtstateJSLineCmtstateJSHTMLOpenCmtstateJSHTMLCloseCmtstateCSSstateCSSDqStrstateCSSSqStrstateCSSDqURLstateCSSSqURLstateCSSURLstateCSSBlockCmtstateCSSLineCmtstateErrorstateDead"
 
-var _state_index = [...]uint16{0, 9, 17, 30, 44, 60, 72, 83, 92, 100, 111, 118, 130, 142, 154, 167, 182, 196, 214, 233, 241, 254, 267, 280, 293, 304, 320, 335, 345, 354}
+var _state_index = [...]uint16{0, 9, 17, 30, 44, 60, 72, 83, 92, 100, 111, 118, 130, 142, 156, 169, 184, 198, 216, 235, 243, 256, 269, 282, 295, 306, 322, 337, 347, 356}
 
 func (i state) String() string {
 	if i >= state(len(_state_index)-1) {

src/html/template/transition.go

@@ -27,8 +27,8 @@ var transitionFunc = [...]func(context, []byte) (context, int){
 	stateJS:             tJS,
 	stateJSDqStr:        tJSDelimited,
 	stateJSSqStr:        tJSDelimited,
-	stateJSBqStr:        tJSDelimited,
 	stateJSRegexp:       tJSDelimited,
+	stateJSTmplLit:      tJSTmpl,
 	stateJSBlockCmt:     tBlockCmt,
 	stateJSLineCmt:      tLineCmt,
 	stateJSHTMLOpenCmt:  tLineCmt,
@@ -270,7 +270,7 @@ func tURL(c context, s []byte) (context, int) {
 
 // tJS is the context transition function for the JS state.
 func tJS(c context, s []byte) (context, int) {
-	i := bytes.IndexAny(s, "\"`'/<-#")
+	i := bytes.IndexAny(s, "\"`'/{}<-#")
 	if i == -1 {
 		// Entire input is non string, comment, regexp tokens.
 		c.jsCtx = nextJSCtx(s, c.jsCtx)
@@ -283,7 +283,7 @@ func tJS(c context, s []byte) (context, int) {
 	case '\'':
 		c.state, c.jsCtx = stateJSSqStr, jsCtxRegexp
 	case '`':
-		c.state, c.jsCtx = stateJSBqStr, jsCtxRegexp
+		c.state, c.jsCtx = stateJSTmplLit, jsCtxRegexp
 	case '/':
 		switch {
 		case i+1 < len(s) && s[i+1] == '/':
@@ -320,12 +320,67 @@ func tJS(c context, s []byte) (context, int) {
 		if i+1 < len(s) && s[i+1] == '!' {
 			c.state, i = stateJSLineCmt, i+1
 		}
+	case '{':
+		c.jsBraceDepth++
+	case '}':
+		if c.jsTmplExprDepth == 0 {
+			return c, i + 1
+		}
+		for j := 0; j <= i; j++ {
+			switch s[j] {
+			case '\\':
+				j++
+			case '{':
+				c.jsBraceDepth++
+			case '}':
+				c.jsBraceDepth--
+			}
+		}
+		if c.jsBraceDepth >= 0 {
+			return c, i + 1
+		}
+		c.jsTmplExprDepth--
+		c.jsBraceDepth = 0
+		c.state = stateJSTmplLit
 	default:
 		panic("unreachable")
 	}
 	return c, i + 1
 }
 
+func tJSTmpl(c context, s []byte) (context, int) {
+	var k int
+	for {
+		i := k + bytes.IndexAny(s[k:], "`\\$")
+		if i < k {
+			break
+		}
+		switch s[i] {
+		case '\\':
+			i++
+			if i == len(s) {
+				return context{
+					state: stateError,
+					err:   errorf(ErrPartialEscape, nil, 0, "unfinished escape sequence in JS string: %q", s),
+				}, len(s)
+			}
+		case '$':
+			if len(s) >= i+2 && s[i+1] == '{' {
+				c.jsTmplExprDepth++
+				c.state = stateJS
+				return c, i + 2
+			}
+		case '`':
+			// end
+			c.state = stateJS
+			return c, i + 1
+		}
+		k = i + 1
+	}
+
+	return c, len(s)
+}
+
 // tJSDelimited is the context transition function for the JS string and regexp
 // states.
 func tJSDelimited(c context, s []byte) (context, int) {
@@ -333,8 +388,6 @@ func tJSDelimited(c context, s []byte) (context, int) {
 	switch c.state {
 	case stateJSSqStr:
 		specials = `\'`
-	case stateJSBqStr:
-		specials = "`\\"
 	case stateJSRegexp:
 		specials = `\/[]`
 	}