GODRIVER-2526 Optimize description.selectByKind. (#1059)

* GODRIVER-2526 Optimize description.selectByKind by eliminating struct appending.

Co-authored-by: Benjamin Rewis <[email protected]>

Author: qingyang-hu

mongo/description/selector_test.go

@@ -230,6 +230,35 @@ func TestSelector_Sharded(t *testing.T) {
 	require.Equal([]Server{s}, result)
 }
 
+func BenchmarkSelector_Sharded(b *testing.B) {
+	subject := readpref.Primary()
+
+	s := Server{
+		Addr:              address.Address("localhost:27017"),
+		HeartbeatInterval: time.Duration(10) * time.Second,
+		LastWriteTime:     time.Date(2017, 2, 11, 14, 0, 0, 0, time.UTC),
+		LastUpdateTime:    time.Date(2017, 2, 11, 14, 0, 2, 0, time.UTC),
+		Kind:              Mongos,
+		WireVersion:       &VersionRange{Min: 0, Max: 5},
+	}
+	servers := make([]Server, 100)
+	for i := 0; i < len(servers); i++ {
+		servers[i] = s
+	}
+	servers[0].Kind = LoadBalancer
+	c := Topology{
+		Kind:    Sharded,
+		Servers: servers,
+	}
+
+	b.ResetTimer()
+	b.RunParallel(func(p *testing.PB) {
+		for p.Next() {
+			_, _ = ReadPrefSelector(subject).SelectServer(c, c.Servers)
+		}
+	})
+}
+
 func TestSelector_Single(t *testing.T) {
 	t.Parallel()
 

mongo/description/server_selector.go

@@ -296,13 +296,18 @@ func selectByTagSet(candidates []Server, tagSets []tag.Set) []Server {
 }
 
 func selectByKind(candidates []Server, kind ServerKind) []Server {
-	var result []Server
-	for _, s := range candidates {
+	// Record the indices of viable candidates first and then append those to the returned slice
+	// to avoid appending costly Server structs directly as an optimization.
+	viableIndexes := make([]int, 0, len(candidates))
+	for i, s := range candidates {
 		if s.Kind == kind {
-			result = append(result, s)
+			viableIndexes = append(viableIndexes, i)
 		}
 	}
-
+	result := make([]Server, len(viableIndexes))
+	for i, idx := range viableIndexes {
+		result[i] = candidates[idx]
+	}
 	return result
 }