net/url: preserve a trailing slash in JoinPath

Fixes #52074

Change-Id: I30897f32e70a6ca0c4e11aaf07088c27336efaba
Reviewed-on: https://go-review.googlesource.com/c/go/+/397256
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matt Layher <mdlayher@gmail.com>
Trust: Matt Layher <mdlayher@gmail.com>

src/net/url/url.go

@@ -1189,11 +1189,18 @@ 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 /.
 func (u *URL) JoinPath(elem ...string) *URL {
 	url := *u
 	if len(elem) > 0 {
 		elem = append([]string{u.Path}, elem...)
-		url.setPath(path.Join(elem...))
+		p := path.Join(elem...)
+		// path.Join will remove any trailing slashes.
+		// Preserve at least one.
+		if strings.HasSuffix(elem[len(elem)-1], "/") && !strings.HasSuffix(p, "/") {
+			p += "/"
+		}
+		url.setPath(p)
 	}
 	return &url
 }

src/net/url/url_test.go

@@ -2099,6 +2099,31 @@ func TestJoinPath(t *testing.T) {
 			base: "http://[fe80::1%en0]:8080/",
 			elem: []string{"/go"},
 		},
+		{
+			base: "https://go.googlesource.com",
+			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: nil,
+			out:  "https://go.googlesource.com",
+		},
+		{
+			base: "https://go.googlesource.com/",
+			elem: nil,
+			out:  "https://go.googlesource.com/",
+		},
+		{
+			base: "/",
+			elem: nil,
+			out:  "/",
+		},
 	}
 	for _, tt := range tests {
 		wantErr := "nil"