type countUpReader struct {
	reader    io.Reader
	bytesRead int64
}

// Max bzip2 concurrency across calls. 50% of GOMAXPROCS.
var bz2Limiter = pbzip2.CreateConcurrencyPool((runtime.GOMAXPROCS(0) + 1) / 2)

func (r *countUpReader) Read(p []byte) (n int, err error) {
	n, err = r.reader.Read(p)
	atomic.AddInt64(&r.bytesRead, int64(n))
	return n, err
}

func (r *countUpReader) BytesRead() int64 {
	if r == nil {
		return 0
	}

	n, err := r.src.Read(p)
	r.bytesRead += int64(n)
	if r.sha256 != nil {
		r.sha256.Write(p[:n])
	}
	if r.contentHasher != nil {
		r.contentHasher.Write(p[:n])
	}

	if err == io.EOF { // Verify content SHA256, if set.
		if r.expectedMin > 0 {
			if r.bytesRead < r.expectedMin {
				return 0, SizeTooSmall{Want: r.expectedMin, Got: r.bytesRead}
			}
		}
		if r.expectedMax > 0 {