Reuse the byte buffer from GRPC response in the frontend. (cortexproj…

…ect#4377)

* Reuse the byte buffer from GRPC response in the frontend.

This PR allow to reuse the GRPC byte buffer casted as a `io.Reader` in the http rountripper of the frontend.
This way we don't have to copy the content from the reader into another buffer when reading the http response.

I've been testing this with in Loki and this shows good memory saving (around 1GB).

Signed-off-by: Cyril Tovena <cyril.tovena@gmail.com>

* Use safe cast

Signed-off-by: Cyril Tovena <cyril.tovena@gmail.com>

* Review feedback.

Signed-off-by: Cyril Tovena <cyril.tovena@gmail.com>
Signed-off-by: Alvin Lin <alvinlin@amazon.com>

pkg/frontend/transport/roundtripper.go

@@ -3,6 +3,7 @@ package transport
 import (
 	"bytes"
 	"context"
+	"io"
 	"io/ioutil"
 	"net/http"
 
@@ -24,6 +25,15 @@ type grpcRoundTripperAdapter struct {
 	roundTripper GrpcRoundTripper
 }
 
+type buffer struct {
+	buff []byte
+	io.ReadCloser
+}
+
+func (b *buffer) Bytes() []byte {
+	return b.buff
+}
+
 func (a *grpcRoundTripperAdapter) RoundTrip(r *http.Request) (*http.Response, error) {
 	req, err := server.HTTPRequest(r)
 	if err != nil {
@@ -37,7 +47,7 @@ func (a *grpcRoundTripperAdapter) RoundTrip(r *http.Request) (*http.Response, er
 
 	httpResp := &http.Response{
 		StatusCode:    int(resp.Code),
-		Body:          ioutil.NopCloser(bytes.NewReader(resp.Body)),
+		Body:          &buffer{buff: resp.Body, ReadCloser: ioutil.NopCloser(bytes.NewReader(resp.Body))},
 		Header:        http.Header{},
 		ContentLength: int64(len(resp.Body)),
 	}

pkg/querier/queryrange/query_range.go

@@ -266,20 +266,15 @@ func (prometheusCodec) DecodeResponse(ctx context.Context, r *http.Response, _ R
 	log, ctx := spanlogger.New(ctx, "ParseQueryRangeResponse") //nolint:ineffassign,staticcheck
 	defer log.Finish()
 
-	// Preallocate the buffer with the exact size so we don't waste allocations
-	// while progressively growing an initial small buffer. The buffer capacity
-	// is increased by MinRead to avoid extra allocations due to how ReadFrom()
-	// internally works.
-	buf := bytes.NewBuffer(make([]byte, 0, r.ContentLength+bytes.MinRead))
-	if _, err := buf.ReadFrom(r.Body); err != nil {
+	buf, err := bodyBuffer(r)
+	if err != nil {
 		log.Error(err)
-		return nil, httpgrpc.Errorf(http.StatusInternalServerError, "error decoding response: %v", err)
+		return nil, err
 	}
-
-	log.LogFields(otlog.Int("bytes", buf.Len()))
+	log.LogFields(otlog.Int("bytes", len(buf)))
 
 	var resp PrometheusResponse
-	if err := json.Unmarshal(buf.Bytes(), &resp); err != nil {
+	if err := json.Unmarshal(buf, &resp); err != nil {
 		return nil, httpgrpc.Errorf(http.StatusInternalServerError, "error decoding response: %v", err)
 	}
 
@@ -289,6 +284,29 @@ func (prometheusCodec) DecodeResponse(ctx context.Context, r *http.Response, _ R
 	return &resp, nil
 }
 
+// Buffer can be used to read a response body.
+// This allows to avoid reading the body multiple times from the `http.Response.Body`.
+type Buffer interface {
+	Bytes() []byte
+}
+
+func bodyBuffer(res *http.Response) ([]byte, error) {
+	// Attempt to cast the response body to a Buffer and use it if possible.
+	// This is because the frontend may have already read the body and buffered it.
+	if buffer, ok := res.Body.(Buffer); ok {
+		return buffer.Bytes(), nil
+	}
+	// Preallocate the buffer with the exact size so we don't waste allocations
+	// while progressively growing an initial small buffer. The buffer capacity
+	// is increased by MinRead to avoid extra allocations due to how ReadFrom()
+	// internally works.
+	buf := bytes.NewBuffer(make([]byte, 0, res.ContentLength+bytes.MinRead))
+	if _, err := buf.ReadFrom(res.Body); err != nil {
+		return nil, httpgrpc.Errorf(http.StatusInternalServerError, "error decoding response: %v", err)
+	}
+	return buf.Bytes(), nil
+}
+
 func (prometheusCodec) EncodeResponse(ctx context.Context, res Response) (*http.Response, error) {
 	sp, _ := opentracing.StartSpanFromContext(ctx, "APIResponse.ToHTTPResponse")
 	defer sp.Finish()
@@ -392,7 +410,6 @@ func matrixMerge(resps []*PrometheusResponse) []SampleStream {
 // bigger than the given minTs. Empty slice is returned if minTs is bigger than all the
 // timestamps in samples.
 func sliceSamples(samples []cortexpb.Sample, minTs int64) []cortexpb.Sample {
-
 	if len(samples) <= 0 || minTs < samples[0].TimestampMs {
 		return samples
 	}