net/url: preserve leading slashes when resolving path

When doing resolvePath, if there are multiple leading slashes in the
target, preserve them. This prevents an issue where the Go http.Client
cleans up multiple leading slashes in the Location header in a
redirect, resulting in a redirection to the incorrect target.

Fixes #21158.

Change-Id: I6a21ea61ca3bc7033f3c8a6ccc21ecaa3e996fa8
Reviewed-on: https://go-review.googlesource.com/51050Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>

src/net/url/url.go

@@ -911,7 +911,7 @@ func resolvePath(base, ref string) string {
 		// Add final slash to the joined path.
 		dst = append(dst, "")
 	}
-	return "/" + strings.TrimLeft(strings.Join(dst, "/"), "/")
+	return "/" + strings.TrimPrefix(strings.Join(dst, "/"), "/")
 }
 
 // IsAbs reports whether the URL is absolute.

src/net/url/url_test.go

@@ -1032,6 +1032,10 @@ var resolveReferenceTests = []struct {
 	{"http://foo.com/bar?a=b", "/baz?", "http://foo.com/baz?"},
 	{"http://foo.com/bar?a=b", "/baz?c=d", "http://foo.com/baz?c=d"},
 
+	// Multiple slashes
+	{"http://foo.com/bar", "http://foo.com//baz", "http://foo.com//baz"},
+	{"http://foo.com/bar", "http://foo.com///baz/quux", "http://foo.com///baz/quux"},
+
 	// Scheme-relative
 	{"https://foo.com/bar?a=b", "//bar.com/quux", "https://bar.com/quux"},