fix: Wonder Ink prints without crashing, and prints worksheets at all
Little Waders, 0.2.2: SIGSEGV in QPrinter::resolution() every time a teacher confirmed a print job. QWebEngineView.print() keeps the raw QPrinter pointer and returns; Qt's didPrintPage() reads printer->resolution() once Chromium has rendered the page, and by then PyQt had deleted the printer, which print_view() held only as a local. Reproduced in the lab with the 0.2.2 code, same backtrace, no job. Keeping the printer alive fixed the crash and was measured, and Qt's path was still not a print path: a worksheet PDF tab printed as one blank page, a lesson went as 24.6 MB of 300 dpi pictures with no text, and a queue refusing jobs was reported as printed. Print is now: make a PDF (a worksheet already is one; a lesson page is printToPdf'd onto the printer's own paper; an inline PDF is downloaded and opened as a worksheet), show it in a preview with printer, pages, copies and colour, and hand it to CUPS with lp — Chrome's route in the same rooms. Nothing asynchronous holds a printer. Black and white uses the driver's ColorModel=Grayscale, because Elmer's generic PCL 6 driver ignores print-color-mode. An exception escaping any slot no longer aborts Wonder Ink: PyQt6's default is to kill the process, and it is now logged instead. tests/test-classroom-printing.sh: 50 checks. tools/lab/wonderink-print- check: 39 checks over 13 scenarios in the lab, against a queue built from Elmer's PPD, judged by what CUPS received. Not yet on paper. Co-Authored-By: Claude Opus 5.5 <[email protected]>
This commit is contained in:
ekklesia
co-authored by
Claude Opus 5.5
Claude Opus 5.5
parent
690ef95aff
commit
10433b850d
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"""Printing a PDF to the room's printer, and showing what will be printed.
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Wonder Ink's printing, without Qt's printing in it. Every document Wonder
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Ink prints is a PDF by the time it gets here — a worksheet already is
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one, and a lesson page is turned into one by the engine — so what is left
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is the part the rest of the classroom already trusts: CUPS.
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WHY NOT QPRINTER.
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Measured in the lab on Debian 13 (QtWebEngine 6.8.2), against a queue
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built from Elmer's own PPD with a file in place of the printer:
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QWebEngineView.print() a lesson page arrived as 24.6 MB of
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300 dpi pictures, no text left in it;
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a worksheet PDF tab arrived as ONE BLANK
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PAGE; a queue rejecting jobs was reported
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as printed successfully; and on 0.2.2 it
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crashed, because the printer was deleted
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before Qt had finished with it.
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printToPdf() + lp the same lesson was an 18 KB PDF with its
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text, on Letter, through Elmer's filters
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in 0.43 s; a rejected job is an error
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with CUPS's own words in it.
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The second is what Chrome does, and Chrome is what the rooms already
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print from without trouble.
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"""
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import re
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import subprocess
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# lp hands the file to the scheduler and returns; it does not wait for
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# the printer. A scheduler that has not answered in this long is not
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# going to, and a teacher is standing at the dialog.
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SUBMIT_TIMEOUT = 30
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QUERY_TIMEOUT = 10
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# What a worksheet can reasonably be asked for. A spin box that goes to
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# 999 is one slip of a finger away from a ream of paper.
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MAX_COPIES = 50
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# Paper names a driver uses, in the terms Qt's page sizes are named by.
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# Anything else is Letter, which is what the church's printer is.
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PAPER = {"Letter": "Letter", "A4": "A4", "Legal": "Legal"}
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# The driver's own words for "no colour", in the order they are tried.
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# Elmer's generic PCL 6 driver ignores the IPP print-color-mode attribute
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# altogether — measured, it logs "Unknown option" and prints in colour —
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# so black and white has to be asked for in the PPD's terms.
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GRAY_CHOICES = ("Grayscale", "Gray", "Mono", "Monochrome", "BlackWhite")
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def _run(command, timeout=QUERY_TIMEOUT):
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try:
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return subprocess.run(
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command, capture_output=True, text=True, timeout=timeout
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)
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except (OSError, subprocess.TimeoutExpired) as error:
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return subprocess.CompletedProcess(command, 1, "", str(error))
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def printers():
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"""(names, default) for every printer CUPS will accept a job for.
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lpstat -e is the list a teacher can print to: in a classroom that is
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Elmer and nothing else, because discovery is switched off.
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"""
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names = [
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line.strip()
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for line in _run(["lpstat", "-e"]).stdout.splitlines()
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if line.strip()
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]
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default = None
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match = re.search(
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r"system default destination:\s*(\S+)", _run(["lpstat", "-d"]).stdout
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)
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if match and match.group(1) in names:
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default = match.group(1)
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elif names:
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default = names[0]
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return names, default
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def options(printer):
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"""What this printer's driver offers, parsed from lpoptions -l.
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{"paper": "Letter", "gray": ("ColorModel", "Grayscale") or None}
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"""
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found = {"paper": "Letter", "gray": None}
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for line in _run(["lpoptions", "-p", printer, "-l"]).stdout.splitlines():
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if ":" not in line:
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continue
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key, choices = line.split(":", 1)
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key = key.split("/", 1)[0].strip()
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choices = choices.split()
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default = next((c[1:] for c in choices if c.startswith("*")), None)
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plain = [c.lstrip("*") for c in choices]
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if key == "PageSize" and default in PAPER:
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found["paper"] = PAPER[default]
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if key in ("ColorModel", "ColorMode", "OutputMode"):
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gray = next((c for c in GRAY_CHOICES if c in plain), None)
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if gray is not None and found["gray"] is None:
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found["gray"] = (key, gray)
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return found
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def page_count(pdf):
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"""How many pages the PDF has, or 0 if it cannot be read."""
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result = _run(["pdfinfo", str(pdf)])
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match = re.search(r"^Pages:\s+(\d+)", result.stdout, re.MULTILINE)
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return int(match.group(1)) if match else 0
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def render_page(pdf, number, width):
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"""PNG bytes of one page, WIDTH pixels wide, or None."""
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try:
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result = subprocess.run(
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[
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"pdftoppm", "-f", str(number), "-l", str(number),
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"-scale-to-x", str(int(width)), "-scale-to-y", "-1",
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"-png", "-singlefile", str(pdf), "-",
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],
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capture_output=True,
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timeout=QUERY_TIMEOUT,
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)
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except (OSError, subprocess.TimeoutExpired):
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return None
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if result.returncode != 0 or not result.stdout:
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return None
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return result.stdout
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def parse_pages(text, count):
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"""A teacher's page list, as CUPS wants it, or ValueError saying why.
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"" or "all" is every page (None). Otherwise numbers and ranges
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separated by commas — "2", "1-3", "1, 3, 5-6" — each inside the
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document. Returned normalised: "1-3,5".
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"""
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text = (text or "").strip().lower()
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if text in ("", "all"):
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return None
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parts = []
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for piece in re.split(r"\s*,\s*", text):
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match = re.fullmatch(r"(\d+)(?:\s*-\s*(\d+))?", piece)
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if not match:
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raise ValueError(
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f"“{piece}” is not a page number. "
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"Use numbers like 2 or 1-3, separated by commas."
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)
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first = int(match.group(1))
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last = int(match.group(2) or first)
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if first < 1 or last < first:
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raise ValueError(f"“{piece}” is not a range of pages.")
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if last > count:
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raise ValueError(
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f"This document has {count} page{'s' if count != 1 else ''}."
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)
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parts.append(str(first) if first == last else f"{first}-{last}")
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return ",".join(parts)
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def submit(pdf, printer, copies=1, pages=None, gray=None, title="Wonder Ink"):
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"""Hand a PDF to CUPS. (True, job id) or (False, what went wrong).
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Success means the scheduler has the job and its id — which is the
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thing Qt's own print path could not say: it reported a rejected job
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as printed.
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"""
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copies = max(1, min(int(copies), MAX_COPIES))
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command = ["lp", "-d", printer, "-n", str(copies), "-t", title]
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if pages:
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command += ["-P", pages]
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if gray:
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command += ["-o", f"{gray[0]}={gray[1]}"]
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command += ["--", str(pdf)]
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result = _run(command, timeout=SUBMIT_TIMEOUT)
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match = re.search(r"request id is (\S+)", result.stdout)
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if result.returncode == 0 and match:
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return True, match.group(1)
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message = (result.stderr or result.stdout).strip()
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# "lp: The printer or class is not accepting jobs." reads better
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# without the program's name in front of it.
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message = re.sub(r"^lp:\s*", "", message) or "The printer did not accept the job."
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return False, message
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