Add a benchmark demonstrating lock contention when Marshaling/Unmarshaling maps.

This benchmark is derived from a Google service which experienced poor performance when handling protos with maps.

The current implementation of map decoding uses reflection. In particular reflect.New, reflect.NewAt, and reflect.(*Value).Addr all call reflect.(*rvalue).ptrTo.

reflect.(*rvalue).ptrTo uses a cache protected by a mutex. Grabbing this lock is what causes the problem.

reflect.(*rvalue).ptrTo also implements a fast path (which avoids critical sections) for certain types known to the compiler. Hopefully we can extend the compiler to generate descriptors for more types (https://golang.org/issue/17931) so that we can hit the fast path for all types needed for proto decoding.
PiperOrigin-RevId: 139337589

proto/map_test.go

@@ -0,0 +1,46 @@
+package proto_test
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/golang/protobuf/proto"
+	ppb "github.com/golang/protobuf/proto/proto3_proto"
+)
+
+func marshalled() []byte {
+	m := &ppb.IntMaps{}
+	for i := 0; i < 1000; i++ {
+		m.Maps = append(m.Maps, &ppb.IntMap{
+			Rtt: map[int32]int32{1: 2},
+		})
+	}
+	b, err := proto.Marshal(m)
+	if err != nil {
+		panic(fmt.Sprintf("Can't marshal %+v: %v", m, err))
+	}
+	return b
+}
+
+func BenchmarkConcurrentMapUnmarshal(b *testing.B) {
+	in := marshalled()
+	b.RunParallel(func(pb *testing.PB) {
+		for pb.Next() {
+			var out ppb.IntMaps
+			if err := proto.Unmarshal(in, &out); err != nil {
+				b.Errorf("Can't unmarshal ppb.IntMaps: %v", err)
+			}
+		}
+	})
+}
+
+func BenchmarkSequentialMapUnmarshal(b *testing.B) {
+	in := marshalled()
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		var out ppb.IntMaps
+		if err := proto.Unmarshal(in, &out); err != nil {
+			b.Errorf("Can't unmarshal ppb.IntMaps: %v", err)
+		}
+	}
+}

proto/proto3_proto/proto3.proto

@@ -76,3 +76,12 @@ message Nested {
 message MessageWithMap {
   map<bool, bytes> byte_mapping = 1;
 }
+
+
+message IntMap {
+  map<int32, int32> rtt = 1;
+}
+
+message IntMaps {
+  repeated IntMap maps = 1;
+}