logtail, net/portmapper, wgengine/magicsock: use fmt.Appendf

Fixes #5206

Change-Id: I490bb92e774ce7c044040537e2cd864fcf1dbe5a
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

logtail/buffer.go

@@ -5,7 +5,6 @@
 package logtail
 
 import (
-	"bytes"
 	"errors"
 	"fmt"
 	"sync"
@@ -51,9 +50,7 @@ func (m *memBuffer) TryReadLine() ([]byte, error) {
 	case ent := <-m.pending:
 		if ent.dropCount > 0 {
 			m.next = ent.msg
-			buf := new(bytes.Buffer)
-			fmt.Fprintf(buf, "----------- %d logs dropped ----------", ent.dropCount)
-			return buf.Bytes(), nil
+			return fmt.Appendf(nil, "----------- %d logs dropped ----------", ent.dropCount), nil
 		}
 		return ent.msg, nil
 	default:

logtail/logtail.go

@@ -270,7 +270,7 @@ func (l *Logger) drainPending(scratch []byte) (res []byte) {
 		if err == io.EOF {
 			break
 		} else if err != nil {
-			b = []byte(fmt.Sprintf("reading ringbuffer: %v", err))
+			b = fmt.Appendf(nil, "reading ringbuffer: %v", err)
 			batchDone = true
 		} else if b == nil {
 			if entries > 0 {