archive/tar: optimize nanosecond parsing in parsePAXTime

src/archive/tar/strconv.go

@@ -213,15 +213,17 @@ func parsePAXTime(s string) (time.Time, error) {
 	}
 
 	// Parse the nanoseconds.
-	if strings.Trim(sn, "0123456789") != "" {
-		return time.Time{}, ErrHeader
-	}
-	if len(sn) < maxNanoSecondDigits {
-		sn += strings.Repeat("0", maxNanoSecondDigits-len(sn)) // Right pad
-	} else {
-		sn = sn[:maxNanoSecondDigits] // Right truncate
+	// 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
+		}
 	}
-	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)
+				}
+			}
+		})
+	}
+}