SaneDev.snap() raises SANE_STATUS_INVAL on Epson epsonscan2 backend at any depth — works with scanimage #107

Environment

• python-sane: 2.9.2 (both the distro package and a fresh pip install . from this repo)
• Python 3.13.13, openSUSE Leap 16.0
• SANE backend: epsonscan2:Perfection V39/GT-S650:esci2:usb:ES010D:317
• Scanner: EPSON Perfection V39 / GT-S650 (USB)

Summary

SaneDev.snap() fails with _sane.error: Invalid argument (SANE_STATUS_INVAL) on every scan through the Epson epsonscan2 backend. The failure is independent of scan mode and depth — reproduced at 1-bit lineart and 8-bit grayscale. The same device scans cleanly with scanimage at both depths, so hardware, USB permissions and the backend itself are all functional.

Minimal reproducer

import sane

sane.init()
dev = sane.open("epsonscan2:Perfection V39/GT-S650:esci2:usb:ES010D:317")
dev.mode = "Grayscale"
dev.resolution = 300
dev.start()
print(dev.get_parameters())   # ('gray', 1, (2280, 2280), 8, 2280)
img = dev.snap()              # raises _sane.error: Invalid argument

start() and get_parameters() return GOOD; the INVAL is raised inside SaneDev_snap's sane_read() loop. Cross-check: scanimage --mode Grayscale --resolution 300 > test.pnm completes and yields a valid 8-bit PGM.

Please see this issue in scantpaper for more details.