b.Write(buf[0:1])
	var cap0 int
	for i := 0; i < 5<<10; i++ {
		b.Write(buf)
		b.Read(buf)
		if i == 0 {
			cap0 = b.Cap()
		}
	}
	cap1 := b.Cap()
	// (*Buffer).grow allows for 2x capacity slop before sliding,
	// so set our error threshold at 3x.
	if cap1 > cap0*3 {
		t.Errorf("buffer cap = %d; too big (grew from %d)", cap1, cap0)
	}
}

func BenchmarkWriteByte(b *testing.B) {

	f.UncompressedSize64 = uint64(f.UncompressedSize)
	filenameLen := int(b.uint16())
	extraLen := int(b.uint16())
	commentLen := int(b.uint16())
	b = b[4:] // skipped start disk number and internal attributes (2x uint16)
	f.ExternalAttrs = b.uint32()
	f.headerOffset = int64(b.uint32())
	d := make([]byte, filenameLen+extraLen+commentLen)
	if _, err := io.ReadFull(r, d); err != nil {
		return err
	}

	// we allocate buffers rounded up to the closest size class.
	c := len(b) + n // ensure enough space for n elements
	if c < 2*cap(b) {
		// The growth rate has historically always been 2x. In the future,
		// we could rely purely on append to determine the growth rate.
		c = 2 * cap(b)
	}
	b2 := append([]byte(nil), make([]byte, c)...)
	copy(b2, b)
	return b2[:len(b)]
}

			// zip64 extra header should be used.
			b.uint32(uint32max) // compressed size
			b.uint32(uint32max) // uncompressed size

			// append a zip64 extra block to Extra
			var buf [28]byte // 2x uint16 + 3x uint64
			eb := writeBuf(buf[:])
			eb.uint16(zip64ExtraID)
			eb.uint16(24) // size = 3x uint64
			eb.uint64(h.UncompressedSize64)
			eb.uint64(h.CompressedSize64)
			eb.uint64(h.offset)

			// walking the tree and copying it piecewise. If the directory tree
			// contains nested modules this could push a lot of unnecessary contents,
			// but for the golang.org/x repos it seems to be significantly (~2x)
			// faster than copying one file at a time (via filepath.WalkDir),
			// apparently due to high latency in 'adb' commands.
			if err := adb("push", modDir, deviceModDir); err != nil {
				return 0, err
			}