logtail: avoid bytes.Buffer allocation

Re-use a pre-allocated bytes.Buffer struct and
shallow the copy the result of bytes.NewBuffer into it
to avoid allocating the struct.

Note that we're only reusing the bytes.Buffer struct itself
and not the underling []byte temporarily stored within it.

Updates #cleanup
Updates tailscale/corp#18514
Updates golang/go#67004

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

logtail/logtail.go

@@ -213,6 +213,7 @@ type Logger struct {
 	procSequence uint64
 	flushTimer   tstime.TimerController // used when flushDelay is >0
 	writeBuf     [bufferSize]byte       // owned by Write for reuse
+	bytesBuf     bytes.Buffer           // owned by appendTextOrJSONLocked for reuse
 	jsonDec      jsontext.Decoder       // owned by appendTextOrJSONLocked for reuse
 
 	shutdownStartMu sync.Mutex    // guards the closing of shutdownStart
@@ -724,9 +725,16 @@ func (l *Logger) appendTextOrJSONLocked(dst, src []byte, level int) []byte {
 	// whether it contains the reserved "logtail" name at the top-level.
 	var logtailKeyOffset, logtailValOffset, logtailValLength int
 	validJSON := func() bool {
-		// TODO(dsnet): Avoid allocation of bytes.Buffer struct.
+		// The jsontext.NewDecoder API operates on an io.Reader, for which
+		// bytes.Buffer provides a means to convert a []byte into an io.Reader.
+		// However, bytes.NewBuffer normally allocates unless
+		// we immediately shallow copy it into a pre-allocated Buffer struct.
+		// See https://go.dev/issue/67004.
+		l.bytesBuf = *bytes.NewBuffer(src)
+		defer func() { l.bytesBuf = bytes.Buffer{} }() // avoid pinning src
+
 		dec := &l.jsonDec
-		dec.Reset(bytes.NewBuffer(src))
+		dec.Reset(&l.bytesBuf)
 		if tok, err := dec.ReadToken(); tok.Kind() != '{' || err != nil {
 			return false
 		}