Enable protobuf negotiation only when histograms are enabled

Signed-off-by: Ganesh Vernekar <ganeshvern@gmail.com>

cmd/prometheus/main.go

@@ -198,6 +198,7 @@ func (c *flagConfig) setFeatureListOptions(logger log.Logger) error {
 				level.Info(logger).Log("msg", "No default port will be appended to scrape targets' addresses.")
 			case "native-histograms":
 				c.tsdb.EnableNativeHistograms = true
+				c.scrape.EnableProtobufNegotiation = true
 				level.Info(logger).Log("msg", "Experimental native histogram support enabled.")
 			case "":
 				continue

scrape/manager.go

@@ -132,6 +132,9 @@ type Options struct {
 	// Option to enable the experimental in-memory metadata storage and append
 	// metadata to the WAL.
 	EnableMetadataStorage bool
+	// Option to enable protobuf negotiation with the client. Note that the client can already
+	// send protobuf without needing to enable this.
+	EnableProtobufNegotiation bool
 	// Option to increase the interval used by scrape manager to throttle target groups updates.
 	DiscoveryReloadInterval model.Duration
 

scrape/scrape.go

@@ -243,6 +243,8 @@ type scrapePool struct {
 	newLoop func(scrapeLoopOptions) loop
 
 	noDefaultPort bool
+
+	enableProtobufNegotiation bool
 }
 
 type labelLimits struct {
@@ -293,6 +295,7 @@ func newScrapePool(cfg *config.ScrapeConfig, app storage.Appendable, jitterSeed
 		logger:                    logger,
 		httpOpts:                  options.HTTPClientOptions,
 		noDefaultPort:             options.NoDefaultPort,
+		enableProtobufNegotiation: options.EnableProtobufNegotiation,
 	}
 	sp.newLoop = func(opts scrapeLoopOptions) loop {
 		// Update the targets retrieval function for metadata to a new scrape cache.
@@ -433,8 +436,12 @@ func (sp *scrapePool) reload(cfg *config.ScrapeConfig) error {
 
 		t := sp.activeTargets[fp]
 		interval, timeout, err := t.intervalAndTimeout(interval, timeout)
+		acceptHeader := scrapeAcceptHeader
+		if sp.enableProtobufNegotiation {
+			acceptHeader = scrapeAcceptHeaderWithProtobuf
+		}
 		var (
-			s       = &targetScraper{Target: t, client: sp.client, timeout: timeout, bodySizeLimit: bodySizeLimit}
+			s       = &targetScraper{Target: t, client: sp.client, timeout: timeout, bodySizeLimit: bodySizeLimit, acceptHeader: acceptHeader}
 			newLoop = sp.newLoop(scrapeLoopOptions{
 				target:          t,
 				scraper:         s,
@@ -537,8 +544,11 @@ func (sp *scrapePool) sync(targets []*Target) {
 			// for every target.
 			var err error
 			interval, timeout, err = t.intervalAndTimeout(interval, timeout)
-
+			acceptHeader := scrapeAcceptHeader
-			s := &targetScraper{Target: t, client: sp.client, timeout: timeout, bodySizeLimit: bodySizeLimit}
+			if sp.enableProtobufNegotiation {
+				acceptHeader = scrapeAcceptHeaderWithProtobuf
+			}
+			s := &targetScraper{Target: t, client: sp.client, timeout: timeout, bodySizeLimit: bodySizeLimit, acceptHeader: acceptHeader}
 			l := sp.newLoop(scrapeLoopOptions{
 				target:          t,
 				scraper:         s,
@@ -757,11 +767,15 @@ type targetScraper struct {
 	buf   *bufio.Reader
 
 	bodySizeLimit int64
+	acceptHeader  string
 }
 
 var errBodySizeLimit = errors.New("body size limit exceeded")
 
-const acceptHeader = `application/vnd.google.protobuf; proto=io.prometheus.client.MetricFamily; encoding=delimited,application/openmetrics-text;version=1.0.0;q=0.75,text/plain;version=0.0.4;q=0.5,*/*;q=0.1`
+const (
+	scrapeAcceptHeader             = `encoding=delimited,application/openmetrics-text;version=1.0.0;q=0.75,text/plain;version=0.0.4;q=0.5,*/*;q=0.1`
+	scrapeAcceptHeaderWithProtobuf = `application/vnd.google.protobuf; proto=io.prometheus.client.MetricFamily; encoding=delimited,application/openmetrics-text;version=1.0.0;q=0.75,text/plain;version=0.0.4;q=0.5,*/*;q=0.1`
+)
 
 var UserAgent = fmt.Sprintf("Prometheus/%s", version.Version)
 
@@ -771,7 +785,7 @@ func (s *targetScraper) scrape(ctx context.Context, w io.Writer) (string, error)
 		if err != nil {
 			return "", err
 		}
-		req.Header.Add("Accept", acceptHeader)
+		req.Header.Add("Accept", s.acceptHeader)
 		req.Header.Add("Accept-Encoding", "gzip")
 		req.Header.Set("User-Agent", UserAgent)
 		req.Header.Set("X-Prometheus-Scrape-Timeout-Seconds", strconv.FormatFloat(s.timeout.Seconds(), 'f', -1, 64))

scrape/scrape_test.go

@@ -2147,12 +2147,16 @@ func TestTargetScraperScrapeOK(t *testing.T) {
 		expectedTimeout = "1.5"
 	)
 
+	var protobufParsing bool
+
 	server := httptest.NewServer(
 		http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			if protobufParsing {
 				accept := r.Header.Get("Accept")
 				if !strings.HasPrefix(accept, "application/vnd.google.protobuf;") {
 					t.Errorf("Expected Accept header to prefer application/vnd.google.protobuf, got %q", accept)
 				}
+			}
 
 			timeout := r.Header.Get("X-Prometheus-Scrape-Timeout-Seconds")
 			if timeout != expectedTimeout {
@@ -2170,6 +2174,7 @@ func TestTargetScraperScrapeOK(t *testing.T) {
 		panic(err)
 	}
 
+	runTest := func(acceptHeader string) {
 		ts := &targetScraper{
 			Target: &Target{
 				labels: labels.FromStrings(
@@ -2179,6 +2184,7 @@ func TestTargetScraperScrapeOK(t *testing.T) {
 			},
 			client:       http.DefaultClient,
 			timeout:      configTimeout,
+			acceptHeader: acceptHeader,
 		}
 		var buf bytes.Buffer
 
@@ -2186,6 +2192,11 @@ func TestTargetScraperScrapeOK(t *testing.T) {
 		require.NoError(t, err)
 		require.Equal(t, "text/plain; version=0.0.4", contentType)
 		require.Equal(t, "metric_a 1\nmetric_b 2\n", buf.String())
+	}
+
+	runTest(scrapeAcceptHeader)
+	protobufParsing = true
+	runTest(scrapeAcceptHeaderWithProtobuf)
 }
 
 func TestTargetScrapeScrapeCancel(t *testing.T) {
@@ -2211,6 +2222,7 @@ func TestTargetScrapeScrapeCancel(t *testing.T) {
 			),
 		},
 		client:       http.DefaultClient,
+		acceptHeader: scrapeAcceptHeader,
 	}
 	ctx, cancel := context.WithCancel(context.Background())
 
@@ -2264,6 +2276,7 @@ func TestTargetScrapeScrapeNotFound(t *testing.T) {
 			),
 		},
 		client:       http.DefaultClient,
+		acceptHeader: scrapeAcceptHeader,
 	}
 
 	_, err = ts.scrape(context.Background(), io.Discard)
@@ -2305,6 +2318,7 @@ func TestTargetScraperBodySizeLimit(t *testing.T) {
 		},
 		client:        http.DefaultClient,
 		bodySizeLimit: bodySizeLimit,
+		acceptHeader:  scrapeAcceptHeader,
 	}
 	var buf bytes.Buffer