Description William Sjöblom 2022-03-17 09:39:58 CET

When testing server upgrades for bug 4885, I stumbled on the following
backtrace when upgrading a 4.9.0 server or older to 4.13.0 or newer on 
DEB based systems (in my case Ubuntu 18.04 and 20.04):
> ┌────
> │ ~/tl-4.14.0-server/ $ ./install-server
> │ ...
> │ Run ThinLinc Setup now [Yes/no]? y
> │ 
> │ Traceback (most recent call last):
> │   File "/opt/thinlinc/sbin/../libexec/tl-setup.py", line 18, in <module>
> │     from thinlinc import sudorelaunch
> │   File "/opt/thinlinc/modules/thinlinc/sudorelaunch.py", line 14, in <module>
> │     import subprocess
> │ ImportError: bad magic number in 'subprocess': b'\x03\xf3\r\n'
> │ Error in sys.excepthook:
> │ Traceback (most recent call last):
> │   File "/usr/lib/python3/dist-packages/apport_python_hook.py", line 72, in apport_excepthook
> │     from apport.fileutils import likely_packaged, get_recent_crashes
> │   File "/usr/lib/python3/dist-packages/apport/__init__.py", line 5, in <module>
> │     from apport.report import Report
> │   File "/usr/lib/python3/dist-packages/apport/report.py", line 12, in <module>
> │     import subprocess, tempfile, os.path, re, pwd, grp, os, time, io
> │ ImportError: bad magic number in 'subprocess': b'\x03\xf3\r\n'
> │ 
> │ Original exception was:
> │ Traceback (most recent call last):
> │   File "/opt/thinlinc/sbin/../libexec/tl-setup.py", line 18, in <module>
> │     from thinlinc import sudorelaunch
> │   File "/opt/thinlinc/modules/thinlinc/sudorelaunch.py", line 14, in <module>
> │     import subprocess
> │ ImportError: bad magic number in 'subprocess': b'\x03\xf3\r\n'
> └────

This seems to stem from the assumption that Python 3 will not load
`*.pyc' files outside `__pycache__' directories made in r37238 along
with the following `*.pyc' file loading behavior:
https://peps.python.org/pep-3147/#case-4-legacy-pyc-files-and-source-less-imports

In version 4.9.0 or below, we shipped a local
`/opt/thinlinc/modules/subprocess.py'. When imported, this file got a
corresponding `subprocess.pyc' file. In 4.10.0 when the `subprocess.py'
file got removed, we have a lone `subprocess.pyc' file for which "Case
4" in PEP3147 applies when using Python 3. Thus, unless explicitly
removed (as was the case before r37238) a Python 3 ThinLinc server will
attempt to load the `subprocess.pyc' file when trying to import the 
system-wide `subprocess' and crash with the above magic mismatch.

Worth noting is that this `.pyc' loading behavior of Python 3 may have
further similar consequences when upgrading even older versions as there
has historically been more files residing at the top-level
ThinLinc-specific `PYTHONPATH'. In other words, this may only be the tip
of the iceberg if we consider upgrades from really old versions.