net/url: add PathEscape, PathUnescape

Fixes #13737.

Change-Id: Ib655dbf06f44709f687f8a2410c80f31e4075f13
Reviewed-on: https://go-review.googlesource.com/31322
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/url/url.go

@@ -74,6 +74,7 @@ type encoding int
 
 const (
 	encodePath encoding = 1 + iota
+	encodePathSegment
 	encodeHost
 	encodeZone
 	encodeUserPassword
@@ -132,9 +133,14 @@ func shouldEscape(c byte, mode encoding) bool {
 			// The RFC allows : @ & = + $ but saves / ; , for assigning
 			// meaning to individual path segments. This package
 			// only manipulates the path as a whole, so we allow those
-			// last two as well. That leaves only ? to escape.
+			// last three as well. That leaves only ? to escape.
 			return c == '?'
 
+		case encodePathSegment: // §3.3
+			// The RFC allows : @ & = + $ but saves / ; , for assigning
+			// meaning to individual path segments.
+			return c == '/' || c == ';' || c == ',' || c == '?'
+
 		case encodeUserPassword: // §3.2.1
 			// The RFC allows ';', ':', '&', '=', '+', '$', and ',' in
 			// userinfo, so we must escape only '@', '/', and '?'.
@@ -164,6 +170,15 @@ func QueryUnescape(s string) (string, error) {
 	return unescape(s, encodeQueryComponent)
 }
 
+// PathUnescape does the inverse transformation of PathEscape, converting
+// %AB into the byte 0xAB. It returns an error if any % is not followed by
+// two hexadecimal digits.
+//
+// PathUnescape is identical to QueryUnescape except that it does not unescape '+' to ' ' (space).
+func PathUnescape(s string) (string, error) {
+	return unescape(s, encodePathSegment)
+}
+
 // unescape unescapes a string; the mode specifies
 // which section of the URL string is being unescaped.
 func unescape(s string, mode encoding) (string, error) {
@@ -250,6 +265,12 @@ func QueryEscape(s string) string {
 	return escape(s, encodeQueryComponent)
 }
 
+// PathEscape escapes the string so it can be safely placed
+// inside a URL path segment.
+func PathEscape(s string) string {
+	return escape(s, encodePathSegment)
+}
+
 func escape(s string, mode encoding) string {
 	spaceCount, hexCount := 0, 0
 	for i := 0; i < len(s); i++ {

src/net/url/url_test.go

@@ -800,6 +800,16 @@ var unescapeTests = []EscapeTest{
 		"",
 		EscapeError("%zz"),
 	},
+	{
+		"a+b",
+		"a b",
+		nil,
+	},
+	{
+		"a%20b",
+		"a b",
+		nil,
+	},
 }
 
 func TestUnescape(t *testing.T) {
@@ -808,10 +818,33 @@ func TestUnescape(t *testing.T) {
 		if actual != tt.out || (err != nil) != (tt.err != nil) {
 			t.Errorf("QueryUnescape(%q) = %q, %s; want %q, %s", tt.in, actual, err, tt.out, tt.err)
 		}
+
+		in := tt.in
+		out := tt.out
+		if strings.Contains(tt.in, "+") {
+			in = strings.Replace(tt.in, "+", "%20", -1)
+			actual, err := PathUnescape(in)
+			if actual != tt.out || (err != nil) != (tt.err != nil) {
+				t.Errorf("PathUnescape(%q) = %q, %s; want %q, %s", in, actual, err, tt.out, tt.err)
+			}
+			if tt.err == nil {
+				s, err := QueryUnescape(strings.Replace(tt.in, "+", "XXX", -1))
+				if err != nil {
+					continue
+				}
+				in = tt.in
+				out = strings.Replace(s, "XXX", "+", -1)
+			}
+		}
+
+		actual, err = PathUnescape(in)
+		if actual != out || (err != nil) != (tt.err != nil) {
+			t.Errorf("PathUnescape(%q) = %q, %s; want %q, %s", in, actual, err, out, tt.err)
+		}
 	}
 }
 
-var escapeTests = []EscapeTest{
+var queryEscapeTests = []EscapeTest{
 	{
 		"",
 		"",
@@ -839,8 +872,8 @@ var escapeTests = []EscapeTest{
 	},
 }
 
-func TestEscape(t *testing.T) {
-	for _, tt := range escapeTests {
+func TestQueryEscape(t *testing.T) {
+	for _, tt := range queryEscapeTests {
 		actual := QueryEscape(tt.in)
 		if tt.out != actual {
 			t.Errorf("QueryEscape(%q) = %q, want %q", tt.in, actual, tt.out)
@@ -854,6 +887,54 @@ func TestEscape(t *testing.T) {
 	}
 }
 
+var pathEscapeTests = []EscapeTest{
+	{
+		"",
+		"",
+		nil,
+	},
+	{
+		"abc",
+		"abc",
+		nil,
+	},
+	{
+		"abc+def",
+		"abc+def",
+		nil,
+	},
+	{
+		"one two",
+		"one%20two",
+		nil,
+	},
+	{
+		"10%",
+		"10%25",
+		nil,
+	},
+	{
+		" ?&=#+%!<>#\"{}|\\^[]`☺\t:/@$'()*,;",
+		"%20%3F&=%23+%25%21%3C%3E%23%22%7B%7D%7C%5C%5E%5B%5D%60%E2%98%BA%09:%2F@$%27%28%29%2A%2C%3B",
+		nil,
+	},
+}
+
+func TestPathEscape(t *testing.T) {
+	for _, tt := range pathEscapeTests {
+		actual := PathEscape(tt.in)
+		if tt.out != actual {
+			t.Errorf("PathEscape(%q) = %q, want %q", tt.in, actual, tt.out)
+		}
+
+		// for bonus points, verify that escape:unescape is an identity.
+		roundtrip, err := PathUnescape(actual)
+		if roundtrip != tt.in || err != nil {
+			t.Errorf("PathUnescape(%q) = %q, %s; want %q, %s", actual, roundtrip, err, tt.in, "[no error]")
+		}
+	}
+}
+
 //var userinfoTests = []UserinfoTest{
 //	{"user", "password", "user:password"},
 //	{"foo:bar", "~!@#$%^&*()_+{}|[]\\-=`:;'\"<>?,./",