Re-use same buffer when receiving messages in TCP collector (#399)

Not a huge improvement all things considered, but an improvement
nonetheless and it makes the TCP collector consistent with the UDP
collector.

Signed-off-by: Antonin Bas <[email protected]>

pkg/collector/tcp.go

@@ -87,6 +87,7 @@ func (cp *CollectingProcess) handleTCPClient(conn net.Conn) {
 	go func() {
 		defer cp.wg.Done()
 		defer close(doneCh)
+		var b bytes.Buffer
 		for {
 			length, err := getMessageLength(reader)
 			if errors.Is(err, io.EOF) {
@@ -97,13 +98,21 @@ func (cp *CollectingProcess) handleTCPClient(conn net.Conn) {
 				klog.ErrorS(err, "Error when retrieving message length")
 				return
 			}
-			buff := make([]byte, length)
+			// Make sure we have enough capacity for the message.
+			b.Grow(length)
+			// The buff slice is guaranteed to have a capacity >= length, and will have
+			// a length of 0.
+			buff := b.AvailableBuffer()
+			// Increase the length of buff to fit the message. Note that slices can be
+			// resliced up to their capacity.
+			buff = buff[:length]
 			_, err = io.ReadFull(reader, buff)
 			if err != nil {
 				klog.ErrorS(err, "Error when reading the message")
 				return
 			}
-			message, err := cp.decodePacket(session, bytes.NewBuffer(buff), address)
+			b.Write(buff)
+			message, err := cp.decodePacket(session, &b, address)
 			if err != nil {
 				// This can be an invalid template record, or invalid data record.
 				// We close the connection, which is the best way to let the client
@@ -113,6 +122,7 @@ func (cp *CollectingProcess) handleTCPClient(conn net.Conn) {
 			}
 			klog.V(4).InfoS("Processed message from exporter",
 				"observationDomainID", message.GetObsDomainID(), "setType", message.GetSet().GetSetType(), "numRecords", message.GetSet().GetNumberOfRecords())
+			b.Reset()
 		}
 	}()
 	select {