net/url: warn that JoinPath arguments should be escaped

Fixes #75799

Change-Id: I483f7b1129799d8dd7f359a04e6ebc1b6a5d0b08
Reviewed-on: https://go-review.googlesource.com/c/go/+/719000
Reviewed-by: Mark Freeman <markfreeman@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/net/url/url.go

@@ -1205,7 +1205,13 @@ func (u *URL) UnmarshalBinary(text []byte) error {
 // JoinPath returns a new [URL] with the provided path elements joined to
 // any existing path and the resulting path cleaned of any ./ or ../ elements.
 // Any sequences of multiple / characters will be reduced to a single /.
+// Path elements must already be in escaped form, as produced by [PathEscape].
 func (u *URL) JoinPath(elem ...string) *URL {
+	url, _ := u.joinPath(elem...)
+	return url
+}
+
+func (u *URL) joinPath(elem ...string) (*URL, error) {
 	elem = append([]string{u.EscapedPath()}, elem...)
 	var p string
 	if !strings.HasPrefix(elem[0], "/") {
@@ -1222,8 +1228,8 @@ func (u *URL) JoinPath(elem ...string) *URL {
 		p += "/"
 	}
 	url := *u
-	url.setPath(p)
-	return &url
+	err := url.setPath(p)
+	return &url, err
 }
 
 // validUserinfo reports whether s is a valid userinfo string per RFC 3986
@@ -1281,11 +1287,15 @@ func stringContainsCTLByte(s string) bool {
 
 // JoinPath returns a [URL] string with the provided path elements joined to
 // the existing path of base and the resulting path cleaned of any ./ or ../ elements.
+// Path elements must already be in escaped form, as produced by [PathEscape].
 func JoinPath(base string, elem ...string) (result string, err error) {
 	url, err := Parse(base)
 	if err != nil {
 		return
 	}
-	result = url.JoinPath(elem...).String()
-	return
+	res, err := url.joinPath(elem...)
+	if err != nil {
+		return "", err
+	}
+	return res.String(), nil
 }

src/net/url/url_test.go

@@ -2159,11 +2159,6 @@ func TestJoinPath(t *testing.T) {
 			elem: []string{"../go"},
 			out:  "https://go.googlesource.com/go",
 		},
-		{
-			base: "https://go.googlesource.com",
-			elem: []string{"../go"},
-			out:  "https://go.googlesource.com/go",
-		},
 		{
 			base: "https://go.googlesource.com",
 			elem: []string{"../go", "../../go", "../../../go"},
@@ -2228,6 +2223,10 @@ func TestJoinPath(t *testing.T) {
 			elem: []string{"/go"},
 			out:  "https://go.googlesource.com/a/b/go",
 		},
+		{
+			base: "https://go.googlesource.com/",
+			elem: []string{"100%"},
+		},
 		{
 			base: "/",
 			elem: nil,
@@ -2269,17 +2268,25 @@ func TestJoinPath(t *testing.T) {
 		if tt.out == "" {
 			wantErr = "non-nil error"
 		}
-		if out, err := JoinPath(tt.base, tt.elem...); out != tt.out || (err == nil) != (tt.out != "") {
+		out, err := JoinPath(tt.base, tt.elem...)
+		if out != tt.out || (err == nil) != (tt.out != "") {
 			t.Errorf("JoinPath(%q, %q) = %q, %v, want %q, %v", tt.base, tt.elem, out, err, tt.out, wantErr)
 		}
-		var out string
+
 		u, err := Parse(tt.base)
-		if err == nil {
-			u = u.JoinPath(tt.elem...)
-			out = u.String()
+		if err != nil {
+			if tt.out != "" {
+				t.Errorf("Parse(%q) = %v", tt.base, err)
+			}
+			continue
 		}
-		if out != tt.out || (err == nil) != (tt.out != "") {
-			t.Errorf("Parse(%q).JoinPath(%q) = %q, %v, want %q, %v", tt.base, tt.elem, out, err, tt.out, wantErr)
+		if tt.out == "" {
+			// URL.JoinPath doesn't return an error, so leave it unchanged
+			tt.out = tt.base
+		}
+		out = u.JoinPath(tt.elem...).String()
+		if out != tt.out {
+			t.Errorf("Parse(%q).JoinPath(%q) = %q, want %q", tt.base, tt.elem, out, tt.out)
 		}
 	}
 }