net/http: introduce ReadCookies() API for Request

This commit implements the new API proposed with
golang#61472

This change set implements the ReadCookies function
on the net/http Request so that it's conveniently possible
to retrieve all cookies from the Cookie header that match
a given name.

It does so by just wrapping the already existing readCookies
function.

Closes golang#61472

Author: timofurrer

src/net/http/request.go

@@ -416,6 +416,12 @@ func (r *Request) Cookies() []*Cookie {
 	return readCookies(r.Header, "")
 }
 
+// ReadCookies parses and returns the named HTTP cookies sent with the request
+// or an empty slice if none matched.
+func (r *Request) ReadCookies(name string) []*Cookie {
+	return readCookies(r.Header, name)
+}
+
 // ErrNoCookie is returned by Request's Cookie method when a cookie is not found.
 var ErrNoCookie = errors.New("http: named cookie not present")
 

src/net/http/request_test.go

@@ -1201,6 +1201,53 @@ func TestRequestCookie(t *testing.T) {
 	}
 }
 
+func TestRequestFilterCookiesByName(t *testing.T) {
+	for _, tt := range []struct {
+		requestCookies  []*Cookie
+		filterForCookie string
+		expectedCookies int
+	}{
+		{
+			requestCookies: []*Cookie{
+				{Name: "foo", Value: "foo-1"},
+				{Name: "bar", Value: "bar"},
+			},
+			filterForCookie: "foo",
+			expectedCookies: 1,
+		},
+		{
+			requestCookies: []*Cookie{
+				{Name: "foo", Value: "foo-1"},
+				{Name: "foo", Value: "foo-2"},
+				{Name: "bar", Value: "bar"},
+			},
+			filterForCookie: "foo",
+			expectedCookies: 2,
+		},
+		{
+			requestCookies: []*Cookie{
+				{Name: "bar", Value: "bar"},
+			},
+			filterForCookie: "foo",
+			expectedCookies: 0,
+		},
+	} {
+		req, err := NewRequest("GET", "http://example.com/", nil)
+		if err != nil {
+			t.Fatal(err)
+		}
+		for _, c := range tt.requestCookies {
+			req.AddCookie(c)
+		}
+
+		cs := req.ReadCookies(tt.filterForCookie)
+
+		if len(cs) != tt.expectedCookies {
+			t.Errorf("got %d cookies, want %d", len(cs), tt.expectedCookies)
+		}
+	}
+}
+
 const (
 	fileaContents = "This is a test file."
 	filebContents = "Another test file."