net/http/httputil: fix joinURLPath unexpectedly appends a trailing slash

Fixes #50337

Author: potoo0

src/net/http/httputil/reverseproxy.go

@@ -207,6 +207,9 @@ type BufferPool interface {
 }
 
 func singleJoiningSlash(a, b string) string {
+	if b == "" {
+		return a
+	}
 	aslash := strings.HasSuffix(a, "/")
 	bslash := strings.HasPrefix(b, "/")
 	switch {
@@ -227,14 +230,16 @@ func joinURLPath(a, b *url.URL) (path, rawpath string) {
 	apath := a.EscapedPath()
 	bpath := b.EscapedPath()
 
-	aslash := strings.HasSuffix(apath, "/")
-	bslash := strings.HasPrefix(bpath, "/")
+	if bpath != "" {
+		aslash := strings.HasSuffix(apath, "/")
+		bslash := strings.HasPrefix(bpath, "/")
 
-	switch {
-	case aslash && bslash:
-		return a.Path + b.Path[1:], apath + bpath[1:]
-	case !aslash && !bslash:
-		return a.Path + "/" + b.Path, apath + "/" + bpath
+		switch {
+		case aslash && bslash:
+			return a.Path + b.Path[1:], apath + bpath[1:]
+		case !aslash && !bslash:
+			return a.Path + "/" + b.Path, apath + "/" + bpath
+		}
 	}
 	return a.Path + b.Path, apath + bpath
 }

src/net/http/httputil/reverseproxy_test.go

@@ -1662,6 +1662,29 @@ func TestJoinURLPath(t *testing.T) {
 	}
 }
 
+func TestJoinURLPath_trailingSlash(t *testing.T) {
+	tests := []struct {
+		a        *url.URL
+		b        *url.URL
+		wantPath string
+		wantRaw  string
+	}{
+		{&url.URL{Path: "/a/b"}, &url.URL{Path: ""}, "/a/b", ""},
+		{&url.URL{Path: "/a/b", RawPath: "/a%2Fb"}, &url.URL{Path: ""}, "/a/b", "/a%2Fb"},
+	}
+
+	for _, tt := range tests {
+		p, rp := joinURLPath(tt.a, tt.b)
+		if p != tt.wantPath || rp != tt.wantRaw {
+			t.Errorf("joinURLPath(URL(%q,%q),URL(%q,%q)) want (%q,%q) got (%q,%q)",
+				tt.a.Path, tt.a.RawPath,
+				tt.b.Path, tt.b.RawPath,
+				tt.wantPath, tt.wantRaw,
+				p, rp)
+		}
+	}
+}
+
 func TestReverseProxyRewriteReplacesOut(t *testing.T) {
 	const content = "response_content"
 	backend := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {