t.Fatalf("got %d headers, want %d headers", len(hdrs), len(v.headers))
			}

			for i, sum := range chksums {
				if i >= len(v.chksums) {
					t.Fatalf("entry %d: unexpected sum: got %s", i, sum)
				}
				if sum != v.chksums[i] {
					t.Fatalf("entry %d: incorrect checksum: got %s, want %s", i, sum, v.chksums[i])
				}
			}

			if err != v.err {
				t.Fatalf("unexpected error: got %v, want %v", err, v.err)

// If the checksum fails, then FormatUnknown is returned.
func (b *block) getFormat() Format {
	// Verify checksum.
	var p parser
	value := p.parseOctal(b.toV7().chksum())
	chksum1, chksum2 := b.computeChecksum()
	if p.err != nil || (value != chksum1 && value != chksum2) {
		return FormatUnknown
	}

	// Guess the magic values.
	magic := string(b.toUSTAR().magic())
	version := string(b.toUSTAR().version())