http plugin: honor cache when upstream sends no-store/max-age=0 (#31028)

plugin/http_test.go

@@ -12,14 +12,19 @@ import (
 )
 
 type httpHandler struct {
-	val string
-	req *http.Request
-	cnt int
+	val          string
+	req          *http.Request
+	cnt          int
+	cacheBusting bool
 }
 
 func (h *httpHandler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
 	h.req = req
 	h.val = lo.RandomString(16, lo.LettersCharset)
+	if h.cacheBusting {
+		w.Header().Set("Cache-Control", "no-store, no-cache, max-age=0, must-revalidate")
+		w.Header().Set("Pragma", "no-cache")
+	}
 	_, _ = w.Write([]byte(h.val))
 	h.cnt++
 }
@@ -72,6 +77,30 @@ func (suite *httpTestSuite) TestCacheGet() {
 	}
 }
 
+func (suite *httpTestSuite) TestCacheGetNoStore() {
+	// upstream sends cache-busting headers, cache must still take effect (#31025)
+	suite.h.cacheBusting = true
+	defer func() { suite.h.cacheBusting = false }()
+
+	uri := suite.srv.URL + "/foo/bar?baz=2"
+	p := NewHTTP(util.NewLogger("foo"), http.MethodGet, uri, false, time.Minute)
+
+	g, err := p.StringGetter()
+	suite.Require().NoError(err)
+
+	suite.h.cnt = 0
+	res, err := g()
+	suite.Require().NoError(err)
+	first := suite.h.cnt
+
+	for range 3 {
+		val, err := g()
+		suite.Require().NoError(err)
+		suite.Require().Equal(res, val)
+		suite.Require().Equal(first, suite.h.cnt)
+	}
+}
+
 func (suite *httpTestSuite) TestSetQuery() {
 	uri := suite.srv.URL + "/foo/bar?baz={{.baz}}"
 	p := NewHTTP(util.NewLogger("foo"), http.MethodGet, uri, false, 0)