Use streaming encoding of metrics

Signed-off-by: beorn7 <beorn@soundcloud.com>

prometheus/promhttp/http.go

@@ -32,7 +32,6 @@
 package promhttp
 
 import (
-	"bytes"
 	"compress/gzip"
 	"fmt"
 	"io"
@@ -59,12 +58,6 @@ var gzipPool = sync.Pool{
 	},
 }
 
-var bufPool = sync.Pool{
-	New: func() interface{} {
-		return &bytes.Buffer{}
-	},
-}
-
 // Handler returns an http.Handler for the prometheus.DefaultGatherer, using
 // default HandlerOpts, i.e. it reports the first error as an HTTP error, it has
 // no error logging, and it applies compression if requested by the client.
@@ -97,13 +90,13 @@ func HandlerFor(reg prometheus.Gatherer, opts HandlerOpts) http.Handler {
 		inFlightSem = make(chan struct{}, opts.MaxRequestsInFlight)
 	}
 
-	h := http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
+	h := http.HandlerFunc(func(rsp http.ResponseWriter, req *http.Request) {
 		if inFlightSem != nil {
 			select {
 			case inFlightSem <- struct{}{}: // All good, carry on.
 				defer func() { <-inFlightSem }()
 			default:
-				http.Error(w, fmt.Sprintf(
+				http.Error(rsp, fmt.Sprintf(
 					"Limit of concurrent requests reached (%d), try again later.", opts.MaxRequestsInFlight,
 				), http.StatusServiceUnavailable)
 				return
@@ -119,49 +112,21 @@ func HandlerFor(reg prometheus.Gatherer, opts HandlerOpts) http.Handler {
 				panic(err)
 			case ContinueOnError:
 				if len(mfs) == 0 {
-					http.Error(w, "No metrics gathered, last error:\n\n"+err.Error(), http.StatusInternalServerError)
+					// Still report the error if no metrics have been gathered.
+					httpError(rsp, err)
 					return
 				}
 			case HTTPErrorOnError:
-				http.Error(w, "An error has occurred during metrics gathering:\n\n"+err.Error(), http.StatusInternalServerError)
+				httpError(rsp, err)
 				return
 			}
 		}
 
 		contentType := expfmt.Negotiate(req.Header)
-		buf := bufPool.Get().(*bytes.Buffer)
+		header := rsp.Header()
-		buf.Reset()
-		enc := expfmt.NewEncoder(buf, contentType)
-
-		defer bufPool.Put(buf)
-
-		var lastErr error
-		for _, mf := range mfs {
-			if err := enc.Encode(mf); err != nil {
-				lastErr = err
-				if opts.ErrorLog != nil {
-					opts.ErrorLog.Println("error encoding metric family:", err)
-				}
-				switch opts.ErrorHandling {
-				case PanicOnError:
-					panic(err)
-				case ContinueOnError:
-					// Handled later.
-				case HTTPErrorOnError:
-					http.Error(w, "An error has occurred during metrics encoding:\n\n"+err.Error(), http.StatusInternalServerError)
-					return
-				}
-			}
-		}
-
-		if lastErr != nil && buf.Len() == 0 {
-			http.Error(w, "No metrics encoded, last error:\n\n"+lastErr.Error(), http.StatusInternalServerError)
-			return
-		}
-		header := w.Header()
 		header.Set(contentTypeHeader, string(contentType))
-		header.Set(contentLengthHeader, fmt.Sprint(buf.Len()))
 
+		w := io.Writer(rsp)
 		if !opts.DisableCompression && gzipAccepted(req.Header) {
 			header.Set(contentEncodingHeader, "gzip")
 			gz := gzipPool.Get().(*gzip.Writer)
@@ -170,12 +135,33 @@ func HandlerFor(reg prometheus.Gatherer, opts HandlerOpts) http.Handler {
 			gz.Reset(w)
 			defer gz.Close()
 
-			zipWriter := gzipResponseWriter{gz, w}
+			w = gz
-			writeResult(zipWriter, buf, opts)
+		}
-			return
+
+		enc := expfmt.NewEncoder(w, contentType)
+
+		var lastErr error
+		for _, mf := range mfs {
+			if err := enc.Encode(mf); err != nil {
+				lastErr = err
+				if opts.ErrorLog != nil {
+					opts.ErrorLog.Println("error encoding and sending metric family:", err)
+				}
+				switch opts.ErrorHandling {
+				case PanicOnError:
+					panic(err)
+				case ContinueOnError:
+					// Handled later.
+				case HTTPErrorOnError:
+					httpError(rsp, err)
+					return
+				}
+			}
+		}
+
+		if lastErr != nil {
+			httpError(rsp, lastErr)
 		}
-		writeResult(w, buf, opts)
-		// TODO(beorn7): Consider streaming serving of metrics.
 	})
 
 	if opts.Timeout <= 0 {
@@ -296,29 +282,8 @@ type HandlerOpts struct {
 	Timeout time.Duration
 }
 
-// gzipResponseWriter in charge of wrapping io.Writer and http.ReponseWriter
+// gzipAccepted returns whether the client will accept gzip-encoded content.
-// together, allowing to get a single struct which implements both interface.
-type gzipResponseWriter struct {
-	io.Writer
-	http.ResponseWriter
-}
-
-func (w gzipResponseWriter) Write(b []byte) (int, error) {
-	return w.Writer.Write(b)
-}
-
-// writeResult to buf using http.ResponseWriter.
-// If ErrorLog is enabled, err is logged in.
-func writeResult(w http.ResponseWriter, buf *bytes.Buffer, opts HandlerOpts) {
-	if _, err := w.Write(buf.Bytes()); err != nil && opts.ErrorLog != nil {
-		opts.ErrorLog.Println("error while sending encoded metrics:", err)
-	}
-}
-
-// gzipHandler return a http.HandlerFunc in charge of compressing the content
-// of the given http.HandlerFunc
 func gzipAccepted(header http.Header) bool {
-
 	a := header.Get(acceptEncodingHeader)
 	parts := strings.Split(a, ",")
 	for _, part := range parts {
@@ -329,3 +294,18 @@ func gzipAccepted(header http.Header) bool {
 	}
 	return false
 }
+
+// httpError removes any content-encoding header and then calls http.Error with
+// the provided error and http.StatusInternalServerErrer. Error contents is
+// supposed to be uncompressed plain text. However, same as with a plain
+// http.Error, any header settings will be void if the header has already been
+// sent. The error message will still be written to the writer, but it will
+// probably be of limited use.
+func httpError(rsp http.ResponseWriter, err error) {
+	rsp.Header().Del(contentEncodingHeader)
+	http.Error(
+		rsp,
+		"An error has occurred while serving metrics:\n\n"+err.Error(),
+		http.StatusInternalServerError,
+	)
+}

prometheus/promhttp/http_test.go

@@ -103,7 +103,7 @@ func TestHandlerErrorHandling(t *testing.T) {
 	})
 	wantMsg := `error gathering metrics: error collecting metric Desc{fqName: "invalid_metric", help: "not helpful", constLabels: {}, variableLabels: []}: collect error
 `
-	wantErrorBody := `An error has occurred during metrics gathering:
+	wantErrorBody := `An error has occurred while serving metrics:
 
 error collecting metric Desc{fqName: "invalid_metric", help: "not helpful", constLabels: {}, variableLabels: []}: collect error
 `

prometheus/registry_test.go

@@ -250,7 +250,7 @@ metric: <
 		},
 	}
 
-	expectedMetricFamilyInvalidLabelValueAsText := []byte(`An error has occurred during metrics gathering:
+	expectedMetricFamilyInvalidLabelValueAsText := []byte(`An error has occurred while serving metrics:
 
 collected metric "name" { label:<name:"constname" value:"\377" > label:<name:"labelname" value:"different_val" > counter:<value:42 > } has a label named "constname" whose value is not utf8: "\xff"
 `)
@@ -299,15 +299,15 @@ complex_bucket 1
 			},
 		},
 	}
-	bucketCollisionMsg := []byte(`An error has occurred during metrics gathering:
+	bucketCollisionMsg := []byte(`An error has occurred while serving metrics:
 
 collected metric named "complex_bucket" collides with previously collected histogram named "complex"
 `)
-	summaryCountCollisionMsg := []byte(`An error has occurred during metrics gathering:
+	summaryCountCollisionMsg := []byte(`An error has occurred while serving metrics:
 
 collected metric named "complex_count" collides with previously collected summary named "complex"
 `)
-	histogramCountCollisionMsg := []byte(`An error has occurred during metrics gathering:
+	histogramCountCollisionMsg := []byte(`An error has occurred while serving metrics:
 
 collected metric named "complex_count" collides with previously collected histogram named "complex"
 `)
@@ -333,7 +333,7 @@ collected metric named "complex_count" collides with previously collected histog
 			},
 		},
 	}
-	duplicateLabelMsg := []byte(`An error has occurred during metrics gathering:
+	duplicateLabelMsg := []byte(`An error has occurred while serving metrics:
 
 collected metric "broken_metric" { label:<name:"foo" value:"bar" > label:<name:"foo" value:"baz" > counter:<value:2.7 > } has two or more labels with the same name: foo
 `)