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

SetPattern will parse string with parsePattern function. This allows third party frameworks set pattern info in requests.
Pattern will return matched pattern string if it is set.

Fixes golang#66405

Author: sillydong

src/net/http/request.go

@@ -1561,3 +1561,39 @@ func (r *Request) requiresHTTP1() bool {
 	return hasToken(r.Header.Get("Connection"), "upgrade") &&
 		ascii.EqualFold(r.Header.Get("Upgrade"), "websocket")
 }
+
+// SetPattern parses a string into a Pattern and set in Request.
+// The string's syntax is
+//
+//	[METHOD] [HOST]/[PATH]
+//
+// where:
+//   - METHOD is an HTTP method
+//   - HOST is a hostname
+//   - PATH consists of slash-separated segments, where each segment is either
+//     a literal or a wildcard of the form "{name}", "{name...}", or "{$}".
+//
+// METHOD, HOST and PATH are all optional; that is, the string can be "/".
+// If METHOD is present, it must be followed by at least one space or tab.
+// Wildcard names must be valid Go identifiers.
+// The "{$}" and "{name...}" wildcard must occur at the end of PATH.
+// PATH may end with a '/'.
+// Wildcard names in a path must be distinct.
+func (r *Request) SetPattern(s string) error {
+	pat, err := parsePattern(s)
+	if err != nil {
+		return err
+	}
+	r.pat = pat
+	return nil
+}
+
+// Pattern returns the pattern that matched this request, if the request
+// resulted from matching a pattern registered on a ServeMux.
+// Otherwise, it returns the empty string.
+func (r *Request) Pattern() string {
+	if r.pat == nil {
+		return ""
+	}
+	return r.pat.str
+}

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()
@@ -1652,3 +1655,25 @@ func TestStatus(t *testing.T) {
 		}
 	}
 }
+
+func TestSetPattern(t *testing.T) {
+	patterns := []string{
+		"GET /",
+		"HEAD /test",
+		"/",
+		"/foo/bar",
+		"GET /foo/{b...}",
+		"GET /{f}",
+	}
+
+	req := new(Request)
+	for _, pattern := range patterns {
+		err := req.SetPattern(pattern)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if req.Pattern() != pattern {
+			t.Errorf("pattern doesn't match, got %s, want %s", req.Pattern(), pattern)
+		}
+	}
+}