Restore old behavior for \" within interpolations

Before the parser rewrite, HCL would silently convert `\"` within
interpolation braces to `"`.

After the conversion, this became a syntax error.

We've found several instances of Terraform configs in the wild using
this syntax. It results in a hard "syntax error" message during config
parsing. While avoiding the extra escape on double quotes within
interpolations is definitely preferred, the UX of the syntax error feels
harsh enough to be worth inserting this backwards compatibility for now,
leaving us the option of deprecating it with a warning down the line.

decoder_test.go

@@ -50,6 +50,13 @@ func TestDecode_interface(t *testing.T) {
 				"foo": "bar\"baz\\n",
 			},
 		},
+		{
+			"interpolate_escape.hcl",
+			false,
+			map[string]interface{}{
+				"foo": "${file(\"bing/bong.txt\")}",
+			},
+		},
 		{
 			"float.hcl",
 			false,

hcl/parser/test-fixtures/complex.hcl

@@ -3,6 +3,8 @@ variable "foo" {
 	description = "bar"
 }
 
+variable "groups" { }
+
 provider "aws" {
 	access_key = "foo"
 	secret_key = "bar"
@@ -19,8 +21,9 @@ resource "aws_security_group" "firewall" {
 resource aws_instance "web" {
 	ami = "${var.foo}"
 	security_groups = [
-		"foo", 
+		"foo",
 		"${aws_security_group.firewall.foo}",
+		"${element(split(\",\", var.groups)}",
 	]
 	network_interface = {
 		device_index = 0

hcl/scanner/scanner_test.go

@@ -79,6 +79,7 @@ var tokenLists = map[string][]tokenPair{
 		{token.STRING, `"a"`},
 		{token.STRING, `"本"`},
 		{token.STRING, `"${file("foo")}"`},
+		{token.STRING, `"${file(\"foo\")}"`},
 		{token.STRING, `"\a"`},
 		{token.STRING, `"\b"`},
 		{token.STRING, `"\f"`},

hcl/strconv/quote.go

@@ -49,7 +49,7 @@ func Unquote(s string) (t string, err error) {
 	for len(s) > 0 {
 		// If we're starting a '${}' then let it through un-unquoted.
 		// Specifically: we don't unquote any characters within the `${}`
-		// section.
+		// section, except for escaped quotes, which we handle specifically.
 		if s[0] == '$' && len(s) > 1 && s[1] == '{' {
 			buf = append(buf, '$', '{')
 			s = s[2:]
@@ -64,6 +64,14 @@ func Unquote(s string) (t string, err error) {
 
 				s = s[size:]
 
+				// We special case escaped double quotes in interpolations, converting
+				// them to straight double quotes.
+				if r == '\\' {
+					if q, _ := utf8.DecodeRuneInString(s); q == '"' {
+						continue
+					}
+				}
+
 				n := utf8.EncodeRune(runeTmp[:], r)
 				buf = append(buf, runeTmp[:n]...)
 

hcl/strconv/quote_test.go

@@ -1,8 +1,6 @@
 package strconv
 
-import (
-	"testing"
-)
+import "testing"
 
 type quoteTest struct {
 	in    string
@@ -38,6 +36,7 @@ var unquotetests = []unQuoteTest{
 	{`"\a\b\f\n\r\t\v\\\""`, "\a\b\f\n\r\t\v\\\""},
 	{`"'"`, "'"},
 	{`"${file("foo")}"`, `${file("foo")}`},
+	{`"${file(\"foo\")}"`, `${file("foo")}`},
 }
 
 var misquoted = []string{
@@ -72,7 +71,7 @@ var misquoted = []string{
 
 func TestUnquote(t *testing.T) {
 	for _, tt := range unquotetests {
-		if out, err := Unquote(tt.in); err != nil && out != tt.out {
+		if out, err := Unquote(tt.in); err != nil || out != tt.out {
 			t.Errorf("Unquote(%#q) = %q, %v want %q, nil", tt.in, out, err, tt.out)
 		}
 	}

hcl/token/token_test.go

@@ -50,6 +50,7 @@ func TestTokenValue(t *testing.T) {
 		{Token{Type: IDENT, Text: `foo`}, "foo"},
 		{Token{Type: STRING, Text: `"foo"`}, "foo"},
 		{Token{Type: STRING, Text: `"foo\nbar"`}, "foo\nbar"},
+		{Token{Type: STRING, Text: `"${file(\"foo\")}"`}, `${file("foo")}`},
 		{Token{Type: HEREDOC, Text: "<<EOF\nfoo\nbar\nEOF"}, "foo\nbar"},
 	}
 

json/scanner/scanner_test.go

@@ -33,6 +33,7 @@ var tokenLists = map[string][]tokenPair{
 		{token.STRING, `"a"`},
 		{token.STRING, `"本"`},
 		{token.STRING, `"${file("foo")}"`},
+		{token.STRING, `"${file(\"foo\")}"`},
 		{token.STRING, `"\a"`},
 		{token.STRING, `"\b"`},
 		{token.STRING, `"\f"`},

test-fixtures/interpolate_escape.hcl

@@ -0,0 +1 @@
+foo="${file(\"bing/bong.txt\")}"