archive/tar: optimize nanosecond parsing in parsePAXTime

Modified parsePAXTime to use a byte array for nanosecond parsing, providing a more straightforward implementation with better performance when handling decimal fraction part.
Here are benchmark results:
goos: darwin
goarch: amd64
pkg: archive/tar
cpu: Intel(R) Core(TM) i7-8569U CPU @ 2.80GHz
                                │   old.txt    │               new.txt                │
                                │    sec/op    │    sec/op     vs base                │
ParsePAXTIme/NoNanos-8            20.55n ±  4%   20.45n ± 12%        ~ (p=1.000 n=10)
ParsePAXTIme/ExactNanos-8         52.42n ±  2%   42.16n ±  3%  -19.57% (p=0.000 n=10)
ParsePAXTIme/WithNanoPadding-8    99.33n ±  2%   39.58n ±  2%  -60.16% (p=0.000 n=10)
ParsePAXTIme/WithNanoTruncate-8   54.78n ±  1%   43.64n ±  4%  -20.34% (p=0.000 n=10)
ParsePAXTIme/TrailingError-8      31.87n ±  4%   17.55n ±  2%  -44.94% (p=0.000 n=10)
ParsePAXTIme/LeadingError-8       31.03n ±  2%   15.81n ±  6%  -49.03% (p=0.000 n=10)

Change-Id: If05ef512137d0115db9cb6d3ab432335230628bb
GitHub-Last-Rev: 106d25e5cf
GitHub-Pull-Request: golang/go#73164
Reviewed-on: https://go-review.googlesource.com/c/go/+/662835
Auto-Submit: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

src/archive/tar/strconv.go

@@ -213,15 +213,17 @@ func parsePAXTime(s string) (time.Time, error) {
 	}
 
 	// Parse the nanoseconds.
-	if strings.Trim(sn, "0123456789") != "" {
+	// Initialize an array with '0's to handle right padding automatically.
+	nanoDigits := [maxNanoSecondDigits]byte{'0', '0', '0', '0', '0', '0', '0', '0', '0'}
+	for i := range len(sn) {
+		switch c := sn[i]; {
+		case c < '0' || c > '9':
 			return time.Time{}, ErrHeader
+		case i < len(nanoDigits):
+			nanoDigits[i] = c
 		}
-	if len(sn) < maxNanoSecondDigits {
-		sn += strings.Repeat("0", maxNanoSecondDigits-len(sn)) // Right pad
-	} else {
-		sn = sn[:maxNanoSecondDigits] // Right truncate
 	}
-	nsecs, _ := strconv.ParseInt(sn, 10, 64) // Must succeed
+	nsecs, _ := strconv.ParseInt(string(nanoDigits[:]), 10, 64) // Must succeed after validation
 	if len(ss) > 0 && ss[0] == '-' {
 		return time.Unix(secs, -1*nsecs), nil // Negative correction
 	}

src/archive/tar/strconv_test.go

@@ -439,3 +439,66 @@ func TestFormatPAXRecord(t *testing.T) {
 		}
 	}
 }
+
+func BenchmarkParsePAXTIme(b *testing.B) {
+	tests := []struct {
+		name string
+		in   string
+		want time.Time
+		ok   bool
+	}{
+		{
+			name: "NoNanos",
+			in:   "123456",
+			want: time.Unix(123456, 0),
+			ok:   true,
+		},
+		{
+			name: "ExactNanos",
+			in:   "1.123456789",
+			want: time.Unix(1, 123456789),
+			ok:   true,
+		},
+		{
+			name: "WithNanoPadding",
+			in:   "1.123",
+			want: time.Unix(1, 123000000),
+			ok:   true,
+		},
+		{
+			name: "WithNanoTruncate",
+			in:   "1.123456789123",
+			want: time.Unix(1, 123456789),
+			ok:   true,
+		},
+		{
+			name: "TrailingError",
+			in:   "1.123abc",
+			want: time.Time{},
+			ok:   false,
+		},
+		{
+			name: "LeadingError",
+			in:   "1.abc123",
+			want: time.Time{},
+			ok:   false,
+		},
+	}
+	for _, tt := range tests {
+		b.Run(tt.name, func(b *testing.B) {
+			b.ReportAllocs()
+			for b.Loop() {
+				ts, err := parsePAXTime(tt.in)
+				if (err == nil) != tt.ok {
+					if err != nil {
+						b.Fatal(err)
+					}
+					b.Fatal("expected error")
+				}
+				if !ts.Equal(tt.want) {
+					b.Fatalf("time mismatch: got %v, want %v", ts, tt.want)
+				}
+			}
+		})
+	}
+}