Classroom_Desktop/lib/classroom_printing.py

"""Printing a PDF to the room's printer, and showing what will be printed.

Wonder Ink's printing, without Qt's printing in it. Every document Wonder
Ink prints is a PDF by the time it gets here — a worksheet already is
one, and a lesson page is turned into one by the engine — so what is left
is the part the rest of the classroom already trusts: CUPS.

WHY NOT QPRINTER.

Measured in the lab on Debian 13 (QtWebEngine 6.8.2), against a queue
built from Elmer's own PPD with a file in place of the printer:

    QWebEngineView.print()      a lesson page arrived as 24.6 MB of
                                300 dpi pictures, no text left in it;
                                a worksheet PDF tab arrived as ONE BLANK
                                PAGE; a queue rejecting jobs was reported
                                as printed successfully; and on 0.2.2 it
                                crashed, because the printer was deleted
                                before Qt had finished with it.

    printToPdf() + lp           the same lesson was an 18 KB PDF with its
                                text, on Letter, through Elmer's filters
                                in 0.43 s; a rejected job is an error
                                with CUPS's own words in it.

The second is what Chrome does, and Chrome is what the rooms already
print from without trouble.
"""

import re
import subprocess

# lp hands the file to the scheduler and returns; it does not wait for
# the printer. A scheduler that has not answered in this long is not
# going to, and a teacher is standing at the dialog.
SUBMIT_TIMEOUT = 30
QUERY_TIMEOUT = 10

# What a worksheet can reasonably be asked for. A spin box that goes to
# 999 is one slip of a finger away from a ream of paper.
MAX_COPIES = 50

# Paper names a driver uses, in the terms Qt's page sizes are named by.
# Anything else is Letter, which is what the church's printer is.
PAPER = {"Letter": "Letter", "A4": "A4", "Legal": "Legal"}

# The driver's own words for "no colour", in the order they are tried.
# Elmer's generic PCL 6 driver ignores the IPP print-color-mode attribute
# altogether — measured, it logs "Unknown option" and prints in colour —
# so black and white has to be asked for in the PPD's terms.
GRAY_CHOICES = ("Grayscale", "Gray", "Mono", "Monochrome", "BlackWhite")


def _run(command, timeout=QUERY_TIMEOUT):
    try:
        return subprocess.run(
            command, capture_output=True, text=True, timeout=timeout
        )
    except (OSError, subprocess.TimeoutExpired) as error:
        return subprocess.CompletedProcess(command, 1, "", str(error))


def printers():
    """(names, default) for every printer CUPS will accept a job for.

    lpstat -e is the list a teacher can print to: in a classroom that is
    Elmer and nothing else, because discovery is switched off.
    """
    names = [
        line.strip()
        for line in _run(["lpstat", "-e"]).stdout.splitlines()
        if line.strip()
    ]

    default = None
    match = re.search(
        r"system default destination:\s*(\S+)", _run(["lpstat", "-d"]).stdout
    )

    if match and match.group(1) in names:
        default = match.group(1)
    elif names:
        default = names[0]

    return names, default


def options(printer):
    """What this printer's driver offers, parsed from lpoptions -l.

    {"paper": "Letter", "gray": ("ColorModel", "Grayscale") or None}
    """
    found = {"paper": "Letter", "gray": None}

    for line in _run(["lpoptions", "-p", printer, "-l"]).stdout.splitlines():
        if ":" not in line:
            continue

        key, choices = line.split(":", 1)
        key = key.split("/", 1)[0].strip()
        choices = choices.split()
        default = next((c[1:] for c in choices if c.startswith("*")), None)
        plain = [c.lstrip("*") for c in choices]

        if key == "PageSize" and default in PAPER:
            found["paper"] = PAPER[default]

        if key in ("ColorModel", "ColorMode", "OutputMode"):
            gray = next((c for c in GRAY_CHOICES if c in plain), None)

            if gray is not None and found["gray"] is None:
                found["gray"] = (key, gray)

    return found


def page_count(pdf):
    """How many pages the PDF has, or 0 if it cannot be read."""
    result = _run(["pdfinfo", str(pdf)])
    match = re.search(r"^Pages:\s+(\d+)", result.stdout, re.MULTILINE)
    return int(match.group(1)) if match else 0


def render_page(pdf, number, width):
    """PNG bytes of one page, WIDTH pixels wide, or None."""
    try:
        result = subprocess.run(
            [
                "pdftoppm", "-f", str(number), "-l", str(number),
                "-scale-to-x", str(int(width)), "-scale-to-y", "-1",
                "-png", "-singlefile", str(pdf), "-",
            ],
            capture_output=True,
            timeout=QUERY_TIMEOUT,
        )
    except (OSError, subprocess.TimeoutExpired):
        return None

    if result.returncode != 0 or not result.stdout:
        return None

    return result.stdout


def parse_pages(text, count):
    """A teacher's page list, as CUPS wants it, or ValueError saying why.

    "" or "all" is every page (None). Otherwise numbers and ranges
    separated by commas — "2", "1-3", "1, 3, 5-6" — each inside the
    document. Returned normalised: "1-3,5".
    """
    text = (text or "").strip().lower()

    if text in ("", "all"):
        return None

    parts = []

    for piece in re.split(r"\s*,\s*", text):
        match = re.fullmatch(r"(\d+)(?:\s*-\s*(\d+))?", piece)

        if not match:
            raise ValueError(
                f"“{piece}” is not a page number. "
                "Use numbers like 2 or 1-3, separated by commas."
            )

        first = int(match.group(1))
        last = int(match.group(2) or first)

        if first < 1 or last < first:
            raise ValueError(f"“{piece}” is not a range of pages.")

        if last > count:
            raise ValueError(
                f"This document has {count} page{'s' if count != 1 else ''}."
            )

        parts.append(str(first) if first == last else f"{first}-{last}")

    return ",".join(parts)


def submit(pdf, printer, copies=1, pages=None, gray=None, title="Wonder Ink"):
    """Hand a PDF to CUPS. (True, job id) or (False, what went wrong).

    Success means the scheduler has the job and its id — which is the
    thing Qt's own print path could not say: it reported a rejected job
    as printed.
    """
    copies = max(1, min(int(copies), MAX_COPIES))
    command = ["lp", "-d", printer, "-n", str(copies), "-t", title]

    if pages:
        command += ["-P", pages]

    if gray:
        command += ["-o", f"{gray[0]}={gray[1]}"]

    command += ["--", str(pdf)]
    result = _run(command, timeout=SUBMIT_TIMEOUT)
    match = re.search(r"request id is (\S+)", result.stdout)

    if result.returncode == 0 and match:
        return True, match.group(1)

    message = (result.stderr or result.stdout).strip()
    # "lp: The printer or class is not accepting jobs." reads better
    # without the program's name in front of it.
    message = re.sub(r"^lp:\s*", "", message) or "The printer did not accept the job."
    return False, message