g-ir-scanner doesn't work (so can't build gnome packages any more)

It appears to me to be happening because gobject-introspection only ships a python file for python3.8 not python3.7 which is the default in focal.

$ python3.7 /usr/bin/g-ir-scanner
Traceback (most recent call last):
  File "/usr/bin/g-ir-scanner", line 99, in <module>
    from giscanner.scannermain import scanner_main
  File "/usr/lib/x86_64-linux-gnu/gobject-introspection/giscanner/scannermain.py", line 35, in <module>
    from giscanner.ast import Include, Namespace
  File "/usr/lib/x86_64-linux-gnu/gobject-introspection/giscanner/ast.py", line 29, in <module>
    from .sourcescanner import CTYPE_TYPEDEF, CSYMBOL_TYPE_TYPEDEF
  File "/usr/lib/x86_64-linux-gnu/gobject-introspection/giscanner/sourcescanner.py", line 33, in <module>
    from giscanner._giscanner import SourceScanner as CSourceScanner
ModuleNotFoundError: No module named 'giscanner._giscanner'

$ python3.8 /usr/bin/g-ir-scanner
ERROR: Need at least one filename

$ ls -alh /usr/bin/python3
lrwxrwxrwx 1 root root 9 Oct 18 10:23 /usr/bin/python3 -> python3.7

Upgrading python3-minimal to 3.8.0-3 which has been stuck in proposed fixes this issue, but gobject-introspection doesn't reflect a relationship upon this.

# apt show gobject-introspection |grep Dep
Depends: libc6 (>= 2.29), libffi7 (>= 3.3~20180313), libgirepository-1.0-1 (= 1.62.0-4ubuntu1), libglib2.0-0 (>= 2.58.0), python3:any, build-essential, python3-distutils, python3-mako, python3-markdown

gobject-introspection needs proper dependencies. See the Debian report.

Status changed to 'Confirmed' because the bug affects multiple users.

Can anyone suggest a safe workaround?

I tried comment #3 and then got broken dependencies. apt decided most of my OS was no longer required and was uninstalled :/

Then I tried downgrading just gobject-introspection to 1.62.0-2ubuntu1. That works long enough to actually complete builds, but breaks other dependencies and causes gdm3 to crash.

I am keen to see a fix or a safe workaround.

a quick hack might be to build the package against the release pocket, and then copy it to the release pocket.

This bug was fixed in the package gobject-introspection - 1.62.0-4ubuntu2

---------------
gobject-introspection (1.62.0-4ubuntu2) focal; urgency=medium

  * rules: Pass the private module directory to dh_python3. We only build for
    the default python version. Previously dh_python3 wasn't finding the
    directory containing the private modules, so it didn't know this. This
    meant that no versioned python3 dependencies were generated so we could,
    for example, be migrated without the corresponding version of python3
    migrating. That results in programs which don't run. Once the directory is
    passed, dh_python3 generates dependencies on the right python versions.
    For future transitions gobject-introspection will need to be binNMUed.
    See Debian #950267.
  * Uploading this to a PPA without focal-proposed and then binary copying to
    the real focal-proposed to hopefully let this migrate on its own, to
    unbreak g-ir-scanner in focal release. After this migrates, the plan is to
    do a no-change rebuild normal upload to focal-proposed to get a binary
    with the correct dependencies. (LP: #1861556)

 -- Iain Lane <email address hidden> Mon, 03 Feb 2020 11:22:32 +0000