X fstail: Reduce allocations

name      old time/op    new time/op    delta
FsTail-4     166µs ± 0%     107µs ± 1%  -35.54%  (p=0.008 n=5+5)

name      old alloc/op   new alloc/op   delta
FsTail-4    28.1kB ± 0%    12.6kB ± 0%  -54.95%  (p=0.008 n=5+5)

name      old allocs/op  new allocs/op  delta
FsTail-4       671 ± 0%        78 ± 0%  -88.38%  (p=0.008 n=5+5)

t/neo/storage/fs1/cmd/fstail/fstail.go

@@ -120,28 +120,18 @@ func fsTail(w io.Writer, path string, ntxn int) (err error) {
 
 		// print information about read txn record
 		xbuf.Reset()
-		xbuf .S(txnh.Tid.Time()) .S("hash=") .X(sha1.Sum(data))
+
+		dataSha1 := sha1.Sum(data)
+		xbuf .V(txnh.Tid.Time()) .S(": hash=") .Xb(dataSha1[:])
 
 		// fstail.py uses repr to print user/description:
-		// https://github.com/zopefoundation/ZODB/blob/5.2.0-4-g359f40ec7/src/ZODB/scripts/fstail.py#L39
-		xbuf .S("\nuser=") .Qpyb(txnh.User) .S(" description=") .Qpyb(txnh.Description)
+		// https://github.com/zopefoundation/ZODB/blob/5.2.0-5-g6047e2fae/src/ZODB/scripts/fstail.py#L39
+		xbuf .S("\nuser=") .Qbpy(txnh.User) .S(" description=") .Qbpy(txnh.Description)
 
 		// NOTE in zodb/py .length is len - 8, in zodb/go - whole txn record length
-		xbuf .S(" length=") .D(txnh.Len - 8)
-		xbuf .S(" offset=") .D(txnh.Pos) .S(" (+") .D(txnh.HeaderLen()) .S(")\n\n")
+		xbuf .S(" length=") .D64(txnh.Len - 8)
+		xbuf .S(" offset=") .D64(txnh.Pos) .S(" (+") .D64(txnh.HeaderLen()) .S(")\n\n")
 
-//		// print information about read txn record
-//		_, err = fmt.Fprintf(w, "%s: hash=%x\nuser=%s description=%s length=%d offset=%d (+%d)\n\n",
-//				txnh.Tid.Time(), sha1.Sum(data),
-//
-//				// fstail.py uses repr to print user/description:
-//				// https://github.com/zopefoundation/ZODB/blob/5.2.0-4-g359f40ec7/src/ZODB/scripts/fstail.py#L39
-//				pyQuoteBytes(txnh.User), pyQuoteBytes(txnh.Description),
-//
-//				// NOTE in zodb/py .length is len - 8, in zodb/go - whole txn record length
-//				txnh.Len - 8,
-//
-//				txnh.Pos, txnh.HeaderLen())
 		_, err = w.Write(xbuf.Bytes())
 		if err != nil {
 			break

t/neo/xcommon/xfmt/fmt.go

@@ -73,7 +73,7 @@ func Append(b []byte, x Stringer) []byte {
 }
 
 // V, similarly to %v, adds x formatted by default rules
-// XXX -> v(interface {}) ?
+// XXX -> V(interface {}) ?
 func (b *Buffer) V(x Stringer) *Buffer {
 	*b = Append(*b, x)
 	return b
@@ -118,6 +118,12 @@ func (b *Buffer) D(i int) *Buffer {
 	return b
 }
 
+// D64 appends int64 formatted by %d
+func (b *Buffer) D64(i int64) *Buffer {
+	*b = strconv.AppendInt(*b, i, 10)
+	return b
+}
+
 // X appends int formatted by %x
 func (b *Buffer) X(i int) *Buffer {
 	*b = strconv.AppendInt(*b, int64(i), 16)

t/neo/xcommon/xfmt/fmt_test.go

@@ -23,13 +23,25 @@ var testv = []struct {format, xformatMeth string; value interface{}} {
 	{"%x",		"Xb",		[]byte("hexstring")},
 	{"%x",		"Xs",		"stringhex"},
 	{"%d",		"D",		12765},
+	{"%d",		"D64",		int64(12764)},
 	{"%x",		"X",		12789},
 	{"%016x",	"X016",		uint64(124)},
 
-	// TODO .V
+	{"%v",		"V",		&stringerTest{}},
 }
 
 
+type stringerTest struct {
+}
+
+func (s *stringerTest) String() string {
+	return string(s.XFmtString(nil))
+}
+
+func (s *stringerTest) XFmtString(b []byte) []byte {
+	return append(b, `stringer test`...)
+}
+
 // verify formatting result is the same in between std fmt and xfmt
 func TestXFmt(t *testing.T) {
 	buf := &Buffer{}
@@ -111,8 +123,11 @@ func BenchmarkXFmt(b *testing.B) {
 		case string:	methProxy = func(buf *Buffer, v interface{}) { meth.(func (*Buffer, string) *Buffer)(buf, v.(string)) }
 		case []byte:	methProxy = func(buf *Buffer, v interface{}) { meth.(func (*Buffer, []byte) *Buffer)(buf, v.([]byte)) }
 		case int:	methProxy = func(buf *Buffer, v interface{}) { meth.(func (*Buffer, int) *Buffer)(buf, v.(int)) }
+		case int64:	methProxy = func(buf *Buffer, v interface{}) { meth.(func (*Buffer, int64) *Buffer)(buf, v.(int64)) }
 		case uint64:	methProxy = func(buf *Buffer, v interface{}) { meth.(func (*Buffer, uint64) *Buffer)(buf, v.(uint64)) }
 
+		case *stringerTest:	methProxy = func(buf *Buffer, v interface{}) { meth.(func (*Buffer, Stringer) *Buffer)(buf, v.(Stringer)) }
+
 		default:
 			b.Fatalf("TODO add support for %T", tt.value)
 		}

t/neo/zodb/time.go

@@ -15,8 +15,12 @@ type TimeStamp struct {
 }
 
 func (t TimeStamp) String() string {
+	return string(t.XFmtString(nil))
+}
+
+func (t TimeStamp) XFmtString(b []byte) []byte {
 	// NOTE UTC() in case we get TimeStamp with modified from-outside location
-	return t.UTC().Format("2006-01-02 15:04:05.000000")
+	return t.UTC().AppendFormat(b, "2006-01-02 15:04:05.000000")
 }