net/http: add Pattern field in Request to return matched pattern info

api/next/66405.txt

@@ -0,0 +1 @@
+pkg net/http, type Request struct, Pattern string #66405

doc/next/6-stdlib/99-minor/net/http/66405.md

@@ -0,0 +1,3 @@
+For inbound requests, the new [Request.Pattern] field contains the [ServeMux]
+pattern (if any) that matched the request. This field is not set when
+`GODEBUG=httpmuxgo121=1` is set.

src/net/http/request.go

@@ -320,6 +320,10 @@ type Request struct {
 	// redirects.
 	Response *Response
 
+	// Pattern is the [ServeMux] pattern that matched the request.
+	// It is empty if the request was not matched against a pattern.
+	Pattern string
+
 	// ctx is either the client or server context. It should only
 	// be modified via copying the whole Request using Clone or WithContext.
 	// It is unexported to prevent people from using Context wrong

src/net/http/request_test.go

@@ -1527,7 +1527,7 @@ func TestPathValueNoMatch(t *testing.T) {
 	}
 }
 
-func TestPathValue(t *testing.T) {
+func TestPathValueAndPattern(t *testing.T) {
 	for _, test := range []struct {
 		pattern string
 		url     string
@@ -1568,6 +1568,9 @@ func TestPathValue(t *testing.T) {
 					t.Errorf("%q, %q: got %q, want %q", test.pattern, name, got, want)
 				}
 			}
+			if r.Pattern != test.pattern {
+				t.Errorf("pattern: got %s, want %s", r.Pattern, test.pattern)
+			}
 		})
 		server := httptest.NewServer(mux)
 		defer server.Close()

src/net/http/server.go

@@ -2682,7 +2682,7 @@ func (mux *ServeMux) ServeHTTP(w ResponseWriter, r *Request) {
 	if use121 {
 		h, _ = mux.mux121.findHandler(r)
 	} else {
-		h, _, r.pat, r.matches = mux.findHandler(r)
+		h, r.Pattern, r.pat, r.matches = mux.findHandler(r)
 	}
 	h.ServeHTTP(w, r)
 }