Fix InfluxDB output UDP line splitting (#5439)

(cherry picked from commit 5dfa3fa)

plugins/outputs/influxdb/udp.go

@@ -1,6 +1,8 @@
 package influxdb
 
 import (
+	"bufio"
+	"bytes"
 	"context"
 	"fmt"
 	"log"
@@ -45,9 +47,9 @@ func NewUDPClient(config *UDPConfig) (*udpClient, error) {
 	serializer := config.Serializer
 	if serializer == nil {
 		s := influx.NewSerializer()
-		s.SetMaxLineBytes(config.MaxPayloadSize)
 		serializer = s
 	}
+	serializer.SetMaxLineBytes(size)
 
 	dialer := config.Dialer
 	if dialer == nil {
@@ -96,7 +98,11 @@ func (c *udpClient) Write(ctx context.Context, metrics []telegraf.Metric) error
 			continue
 		}
 
-		_, err = c.conn.Write(octets)
+		scanner := bufio.NewScanner(bytes.NewReader(octets))
+		scanner.Split(scanLines)
+		for scanner.Scan() {
+			_, err = c.conn.Write(scanner.Bytes())
+		}
 		if err != nil {
 			c.conn.Close()
 			c.conn = nil
@@ -118,3 +124,15 @@ type netDialer struct {
 func (d *netDialer) DialContext(ctx context.Context, network, address string) (Conn, error) {
 	return d.Dialer.DialContext(ctx, network, address)
 }
+
+func scanLines(data []byte, atEOF bool) (advance int, token []byte, err error) {
+	if atEOF && len(data) == 0 {
+		return 0, nil, nil
+	}
+	if i := bytes.IndexByte(data, '\n'); i >= 0 {
+		// We have a full newline-terminated line.
+		return i + 1, data[0 : i+1], nil
+
+	}
+	return 0, nil, nil
+}

plugins/serializers/influx/influx.go

@@ -240,6 +240,7 @@ func (s *Serializer) writeMetric(w io.Writer, m telegraf.Metric) error {
 				return err
 			}
 
+			pairsLen = 0
 			firstField = true
 			bytesNeeded = len(s.header) + len(s.pair) + len(s.footer)
 

plugins/serializers/influx/influx_test.go

@@ -275,6 +275,24 @@ var tests = []struct {
 		),
 		output: []byte("cpu abc=123i 1519194109000000042\ncpu def=456i 1519194109000000042\n"),
 	},
+	{
+		name:     "split_fields_overflow",
+		maxBytes: 43,
+		input: MustMetric(
+			metric.New(
+				"cpu",
+				map[string]string{},
+				map[string]interface{}{
+					"abc": 123,
+					"def": 456,
+					"ghi": 789,
+					"jkl": 123,
+				},
+				time.Unix(1519194109, 42),
+			),
+		),
+		output: []byte("cpu abc=123i,def=456i 1519194109000000042\ncpu ghi=789i,jkl=123i 1519194109000000042\n"),
+	},
 	{
 		name: "name newline",
 		input: MustMetric(