expfmt: precompute parsed goautoneg.Accept for well-known Format constants (#967)

Repeatedly parsing static Format constants with goautoneg.ParseAccept in
matchFormat on every call causes unnecessary heap allocations and latency
during content negotiation.

Precompute the parsed goautoneg.Accept structs for standard Format
constants at package initialization time, falling back to goautoneg.ParseAccept
for custom formats. Also add benchmarks with b.ReportAllocs() to track
negotiation performance.

Signed-off-by: David Ashpole <dashpole@google.com>

Author: dashpole

expfmt/encode.go

@@ -27,6 +27,24 @@ import (
 	"github.com/prometheus/common/model"
 )
 
+var formatToAccept = map[Format]goautoneg.Accept{}
+
+func init() {
+	for _, f := range []Format{
+		FmtText,
+		FmtProtoDelim,
+		FmtProtoText,
+		FmtProtoCompact,
+		FmtOpenMetrics_1_0_0,
+		fmtOpenMetrics_2_0_0,
+		FmtOpenMetrics_0_0_1,
+	} {
+		if parsed := goautoneg.ParseAccept(string(f)); len(parsed) > 0 {
+			formatToAccept[f] = parsed[0]
+		}
+	}
+}
+
 // Encoder types encode metric families into an underlying wire protocol.
 type Encoder interface {
 	Encode(*dto.MetricFamily) error
@@ -81,7 +99,7 @@ func NegotiateIncludingOpenMetrics(h http.Header) Format {
 // format if present in the accepted list, or the first accepted format (or FmtText
 // if accepted is empty).
 func NegotiateAccept(h http.Header, accepted ...Format) Format {
-	escapingScheme := Format(fmt.Sprintf("; escaping=%s", Format(model.NameEscapingScheme.String())))
+	escapingScheme := Format("; escaping=" + model.NameEscapingScheme.String())
 	for _, ac := range goautoneg.ParseAccept(h.Get(hdrAccept)) {
 		if escapeParam := ac.Params[model.EscapingKey]; escapeParam != "" {
 			switch Format(escapeParam) {
@@ -111,11 +129,14 @@ func NegotiateAccept(h http.Header, accepted ...Format) Format {
 
 // matchFormat checks if a parsed accept clause matches a given Format.
 func matchFormat(ac goautoneg.Accept, f Format) bool {
-	parsed := goautoneg.ParseAccept(string(f))
-	if len(parsed) == 0 {
-		return false
+	target, ok := formatToAccept[f]
+	if !ok {
+		parsed := goautoneg.ParseAccept(string(f))
+		if len(parsed) == 0 {
+			return false
+		}
+		target = parsed[0]
 	}
-	target := parsed[0]
 
 	if ac.Type != "*" && ac.Type != target.Type {
 		return false

expfmt/encode_test.go

@@ -583,3 +583,34 @@ func TestDottedEncode(t *testing.T) {
 		}
 	}
 }
+
+func BenchmarkNegotiate(b *testing.B) {
+	h := http.Header{}
+	h.Set(hdrAccept, "application/vnd.google.protobuf;proto=io.prometheus.client.MetricFamily;encoding=delimited;q=0.7,text/plain;version=0.0.4;q=0.3,application/json;q=0.1,*/*;q=0.01")
+	b.ReportAllocs()
+	b.ResetTimer()
+	for b.Loop() {
+		_ = Negotiate(h)
+	}
+}
+
+func BenchmarkNegotiateIncludingOpenMetrics(b *testing.B) {
+	h := http.Header{}
+	h.Set(hdrAccept, "application/openmetrics-text;version=1.0.0;q=0.8,application/openmetrics-text;version=0.0.1;q=0.5,text/plain;version=0.0.4;q=0.3,*/*;q=0.1")
+	b.ReportAllocs()
+	b.ResetTimer()
+	for b.Loop() {
+		_ = NegotiateIncludingOpenMetrics(h)
+	}
+}
+
+func BenchmarkNegotiateAccept(b *testing.B) {
+	h := http.Header{}
+	h.Set(hdrAccept, "application/openmetrics-text;version=1.0.0;q=0.8,text/plain;version=0.0.4;q=0.3,*/*;q=0.1")
+	accepted := []Format{FmtOpenMetrics_1_0_0, FmtOpenMetrics_0_0_1, FmtProtoDelim, FmtProtoText, FmtProtoCompact, FmtText}
+	b.ReportAllocs()
+	b.ResetTimer()
+	for b.Loop() {
+		_ = NegotiateAccept(h, accepted...)
+	}
+}