topdown: http.send to cache responses based on status code

Currently http.send caches all responses. Now if the response
had a status code of `500`(Internal Server Error ) for example,
it's possible OPA will return a reponse from the cache for the
next query. This can have unintened consequences as OPA will
keep serving the cached response till it's fresh while at the
same time it's possible the server has a proper response available.
To avoid such as scenario this change updates the caching behavior
to take into account the status code of the HTTP response before
inserting a value into the cache. The list of status codes that
can be cached is per https://www.rfc-editor.org/rfc/rfc7231#section-6.1.

Fixes: #5617

Signed-off-by: Ashutosh Narkar <anarkar4387@gmail.com>

docs/content/policy-reference.md

@@ -902,7 +902,8 @@ instead of halting evaluation, if `http.send` encounters an error, it can return
 set to `0` and `error` describing the actual error. This can be activated by setting the `raise_error` field
 in the `request` object to `false`.
 
-If the `cache` field in the `request` object is `true`, `http.send` will return a cached response after it checks its freshness and validity.
+If the `cache` field in the `request` object is `true`, `http.send` will return a cached response after it checks its
+freshness and validity.
 
 `http.send` uses the `Cache-Control` and `Expires` response headers to check the freshness of the cached response.
 Specifically if the [max-age](https://tools.ietf.org/html/rfc7234#section-5.2.2.8) `Cache-Control` directive is set, `http.send`
@@ -919,6 +920,10 @@ conjunction with the `force_cache_duration_seconds` field. If `force_cache` is `
 
 Also, if `force_cache` is `true`, it overrides the `cache` field.
 
+`http.send` only caches responses with the following HTTP status codes: `200`, `203`, `204`, `206`, `300`, `301`,
+`404`, `405`, `410`, `414`, and `501`. This is behavior is as per https://www.rfc-editor.org/rfc/rfc7231#section-6.1 and
+is enforced when caching responses within a single query or across queries via the `cache` and `force_cache` request fields.
+
 {{< info >}}
 `http.send` uses the `Date` response header to calculate the current age of the response by comparing it with the current time.
 This value is used to determine the freshness of the cached response. As per https://tools.ietf.org/html/rfc7231#section-7.1.1.2,

topdown/http.go

@@ -69,8 +69,24 @@ var allowedKeyNames = [...]string{
 	"caching_mode",
 }
 
+// ref: https://www.rfc-editor.org/rfc/rfc7231#section-6.1
+var cacheableHTTPStatusCodes = [...]int{
+	http.StatusOK,
+	http.StatusNonAuthoritativeInfo,
+	http.StatusNoContent,
+	http.StatusPartialContent,
+	http.StatusMultipleChoices,
+	http.StatusMovedPermanently,
+	http.StatusNotFound,
+	http.StatusMethodNotAllowed,
+	http.StatusGone,
+	http.StatusRequestURITooLong,
+	http.StatusNotImplemented,
+}
+
 var (
 	allowedKeys                 = ast.NewSet()
+	cacheableCodes              = ast.NewSet()
 	requiredKeys                = ast.NewSet(ast.StringTerm("method"), ast.StringTerm("url"))
 	httpSendLatencyMetricKey    = "rego_builtin_" + strings.ReplaceAll(ast.HTTPSend.Name, ".", "_")
 	httpSendInterQueryCacheHits = httpSendLatencyMetricKey + "_interquery_cache_hits"
@@ -162,6 +178,7 @@ func getHTTPResponse(bctx BuiltinContext, req ast.Object) (*ast.Term, error) {
 
 func init() {
 	createAllowedKeys()
+	createCacheableHTTPStatusCodes()
 	initDefaults()
 	RegisterBuiltinFunc(ast.HTTPSend.Name, builtinHTTPSend)
 }
@@ -779,7 +796,7 @@ func (c *interQueryCache) checkHTTPSendInterQueryCache() (ast.Value, error) {
 
 // insertIntoHTTPSendInterQueryCache inserts given key and value in the inter-query cache
 func insertIntoHTTPSendInterQueryCache(bctx BuiltinContext, key ast.Value, resp *http.Response, respBody []byte, cacheParams *forceCacheParams) error {
-	if resp == nil || (!forceCaching(cacheParams) && !canStore(resp.Header)) {
+	if resp == nil || (!forceCaching(cacheParams) && !canStore(resp.Header)) || !cacheableCodes.Contains(ast.IntNumberTerm(resp.StatusCode)) {
 		return nil
 	}
 
@@ -817,6 +834,12 @@ func createAllowedKeys() {
 	}
 }
 
+func createCacheableHTTPStatusCodes() {
+	for _, element := range cacheableHTTPStatusCodes {
+		cacheableCodes.Add(ast.IntNumberTerm(element))
+	}
+}
+
 func parseTimeout(timeoutVal ast.Value) (time.Duration, error) {
 	var timeout time.Duration
 	switch t := timeoutVal.(type) {
@@ -1321,7 +1344,10 @@ func (c *intraQueryCache) InsertIntoCache(value *http.Response) (ast.Value, erro
 		return nil, handleHTTPSendErr(c.bctx, err)
 	}
 
-	insertIntoHTTPSendCache(c.bctx, c.key, result)
+	if cacheableCodes.Contains(ast.IntNumberTerm(value.StatusCode)) {
+		insertIntoHTTPSendCache(c.bctx, c.key, result)
+	}
+
 	return result, nil
 }
 

topdown/http_test.go

@@ -2626,6 +2626,108 @@ func TestCertSelectionLogic(t *testing.T) {
 	}
 }
 
+func TestHTTPSendCacheDefaultStatusCodesIntraQueryCache(t *testing.T) {
+
+	// run test server
+	var requests []*http.Request
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		requests = append(requests, r)
+		if len(requests)%2 == 0 {
+			headers := w.Header()
+			headers["Cache-Control"] = []string{"max-age=290304000, public"}
+			w.WriteHeader(http.StatusOK)
+		} else {
+			w.WriteHeader(http.StatusInternalServerError)
+		}
+	}))
+
+	defer ts.Close()
+
+	t.Run("non-cacheable status code: intra-query cache", func(t *testing.T) {
+		base := fmt.Sprintf(`http.send({"method": "get", "url": %q, "cache": true})`, ts.URL)
+		query := fmt.Sprintf("%v;%v;%v", base, base, base)
+
+		q := NewQuery(ast.MustParseBody(query))
+
+		// Execute three http.send calls within a query.
+		// Since the server returns a http.StatusInternalServerError on the first request, this should NOT be cached as
+		// http.StatusInternalServerError is not a cacheable status code. The second request should result in OPA reaching
+		// out to the server again and getting a http.StatusOK response status code.
+		// The third request should now be served from the cache.
+
+		_, err := q.Run(context.Background())
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		expectedReqCount := 2
+		if len(requests) != expectedReqCount {
+			t.Fatalf("Expected to get %d requests, got %d", expectedReqCount, len(requests))
+		}
+	})
+}
+
+func TestHTTPSendCacheDefaultStatusCodesInterQueryCache(t *testing.T) {
+
+	// run test server
+	var requests []*http.Request
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		requests = append(requests, r)
+		if len(requests)%2 == 0 {
+			headers := w.Header()
+			headers["Cache-Control"] = []string{"max-age=290304000, public"}
+			w.WriteHeader(http.StatusOK)
+		} else {
+			w.WriteHeader(http.StatusInternalServerError)
+		}
+	}))
+
+	defer ts.Close()
+
+	t.Run("non-cacheable status code: inter-query cache", func(t *testing.T) {
+
+		// add an inter-query cache
+		config, _ := iCache.ParseCachingConfig(nil)
+		interQueryCache := iCache.NewInterQueryCache(config)
+
+		m := metrics.New()
+
+		q := NewQuery(ast.MustParseBody(fmt.Sprintf(`http.send({"method": "get", "url": %q, "cache": true})`, ts.URL))).
+			WithMetrics(m).WithInterQueryBuiltinCache(interQueryCache)
+
+		// Execute three queries.
+		// Since the server returns a http.StatusInternalServerError on the first request, this should NOT be cached as
+		// http.StatusInternalServerError is not a cacheable status code. The second request should result in OPA reaching
+		// out to the server again and getting a http.StatusOK response status code.
+		// The third request should now be served from the cache.
+
+		_, err := q.Run(context.Background())
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		_, err = q.Run(context.Background())
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		_, err = q.Run(context.Background())
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		expectedReqCount := 2
+		if len(requests) != expectedReqCount {
+			t.Fatalf("Expected to get %d requests, got %d", expectedReqCount, len(requests))
+		}
+
+		// verify http.send inter-query cache hit metric is incremented due to the third request.
+		if exp, act := uint64(1), m.Counter(httpSendInterQueryCacheHits).Value(); exp != act {
+			t.Fatalf("expected %d cache hits, got %d", exp, act)
+		}
+	})
+}
+
 func TestHTTPSendMetrics(t *testing.T) {
 
 	// run test server