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

Fixes #66405

Change-Id: Icd80944b6ca081aa7addd4fb85d2b3c29b6c9542
GitHub-Last-Rev: c6e32742c4b733230c82627571b423de45997c24
GitHub-Pull-Request: golang/go#66618
Reviewed-on: https://go-review.googlesource.com/c/go/+/574997
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

3 files changed, 9 insertions, 2 deletions

diff --git a/src/net/http/request.go b/src/net/http/request.go
index bdd18adf3f..f208b95c46 100644
--- a/src/net/http/request.go
+++ b/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
diff --git a/src/net/http/request_test.go b/src/net/http/request_test.go
index a7deba46e3..9b6eb6e1a8 100644
--- a/src/net/http/request_test.go
+++ b/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
@@ -1576,6 +1576,9 @@ 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()
diff --git a/src/net/http/server.go b/src/net/http/server.go
index b76c869567..9786a68129 100644
--- a/src/net/http/server.go
+++ b/src/net/http/server.go
@@ -2703,7 +2703,7 @@ 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)
 }