FAIL: curtin command apt-config (KeyError: 'RELEASE')

[Michael Weiss]

I'm getting the following error since I've updated the installer ISO image from the Ubuntu 20.04.3 LTS to Ubuntu 20.04.4 LTS:
Edit: Posted as attachment to avoid the formatting issues: https://bugs.launchpad.net/curtin/+bug/1964115/+attachment/5567000/+files/subiquity_log.txt
```
finish: cmd-apt-config: FAIL: curtin command apt-config
Traceback (most recent call last):
  File "/snap/subiquity/3119/lib/python3.8/site-packages/curtin/commands/main.py", line 202, in main
    ret = args.func(args)
  File "/snap/subiquity/3119/lib/python3.8/site-packages/curtin/commands/apt_config.py", line 663, in apt_command
    handle_apt(apt_cfg, target)
  File "/snap/subiquity/3119/lib/python3.8/site-packages/curtin/commands/apt_config.py", line 77, in handle_apt
    generate_sources_list(cfg, release, mirrors, target)
  File "/snap/subiquity/3119/lib/python3.8/site-packages/curtin/commands/apt_config.py", line 354, in generate_sources_list
    entries = update_mirrors(entries, mirrors)
  File "/snap/subiquity/3119/lib/python3.8/site-packages/curtin/commands/apt_config.py", line 255, in update_mirrors
    entry.uri = util.render_string(entry.uri, mirrors)
  File "/snap/subiquity/3119/lib/python3.8/site-packages/curtin/util.py", line 1203, in render_string
    return basic_template_render(content, params)
  File "/snap/subiquity/3119/lib/python3.8/site-packages/curtin/util.py", line 1193, in basic_template_render
    return BASIC_MATCHER.sub(replacer, content)
  File "/snap/subiquity/3119/lib/python3.8/site-packages/curtin/util.py", line 1191, in replacer
    return str(selected_params[key])
KeyError: 'RELEASE'
'RELEASE'
```

I'm using Packer (https://www.packer.io/) to perform an automated installation so nothing else has changed (and if I downgrade to Ubuntu 20.04.3 LTS everything works again).

The relevant part of my Ubuntu autoinstall configuration:
```
  apt:
    primary:
      - arches: [default]
        uri: "http://ubuntu.my-company.tld"
    sources_list: | # written by curtin custom template
      deb $MIRROR/$RELEASE $RELEASE main restricted
      deb $MIRROR/$RELEASE-updates $RELEASE-updates main restricted
      deb $MIRROR/$RELEASE-security $RELEASE-security main restricted

      deb $MIRROR/$RELEASE $RELEASE universe
      deb $MIRROR/$RELEASE-updates $RELEASE-updates universe
      deb $MIRROR/$RELEASE-security $RELEASE-security universe
```

I've based that on the following example configuration:
https://github.com/canonical/curtin/blob/600d6a19e26d773d1c4efae874e8c8785d0e3196/examples/apt-source.yaml#L118

[Michael Weiss (primeos-work)]

The problem persists with Ubuntu 22.04 LTS (tested with one of the final RCs: https://iso.qa.ubuntu.com/qatracker/milestones/432/builds/247146/downloads).

I did some analysis and the issue seems to be the "$MIRROR/$RELEASE" part of the custom template.

The relevant source code part (the generate_sources_list function in curtin/commands/apt_config.py):
```
   333 def generate_sources_list(cfg, release, mirrors, target=None, arch=None):
   334 """ generate_sources_list
   335 create a source.list file based on a custom or default template
   336 by replacing mirrors and release in the template
   337 """
   338 aptsrc = "/etc/apt/sources.list"
   339
   340 tmpl = cfg.get('sources_list', None)
   341 from_file = False
   342 if tmpl is None:
   343 LOG.info("No custom template provided, fall back to modify"
   344 "mirrors in %s on the target system", aptsrc)
   345 tmpl = util.load_file(paths.target_path(target, aptsrc))
   346 from_file = True
   347
   348 entries = [SourceEntry(line) for line in tmpl.splitlines(True)]
   349 if from_file:
   350 # when loading from an existing file, we also replace default
   351 # URIs with configured mirrors
   352 entries = update_default_mirrors(entries, mirrors, target, arch)
   353
   354 entries = update_mirrors(entries, mirrors)
   355 entries = update_dist(entries, release)
```

After SourceEntry from python-apt ("from aptsources.sourceslist import SourceEntry") parses the custom template (s. apt.sources_list above), the data ("entries") looks as follows:
```
{'invalid': False, 'disabled': False, 'type': 'deb', 'architectures': [], 'trusted': None, 'uri': '$MIRROR/$RELEASE', 'dist': '$RELEASE', 'comps': ['main', 'restricted'], 'comment': '', 'line': 'deb $MIRROR/$RELEASE $RELEASE main restricted\n', 'file': '/etc/apt/sources.list', 'template': None, 'children': []}
{'invalid': False, 'disabled': False, 'type': 'deb', 'architectures': [], 'trusted': None, 'uri': '$MIRROR/$RELEASE-updates', 'dist': '$RELEASE-updates', 'comps': ['main', 'restricted'], 'comment': '', 'line': 'deb $MIRROR/$RELEASE-updates $RELEASE-updates main restricted\n', 'file': '/etc/apt/sources.list', 'template': None, 'children': []}
{'invalid': False, 'disabled': False, 'type': 'deb', 'architectures': [], 'trusted': None, 'uri': '$MIRROR/$RELEASE-security', 'dist': '$RELEASE-security', 'comps': ['main', 'restricted'], 'comment': '', 'line': 'deb $MIRROR/$RELEASE-security $RELEASE-security main restricted\n', 'file': '/etc/apt/sources.list', 'template': None, 'children': []}
{'invalid': True, 'disabled': False, 'type': '', 'architectures': [], 'trusted': None, 'uri': '', 'dist': '', 'comps': [], 'comment': '', 'line': '\n', 'file': '/etc/apt/sources.list', 'template': None, 'children': []}
{'invalid': False, 'disabled': False, 'type': 'deb', 'architectures': [], 'trusted': None, 'uri': '$MIRROR/$RELEASE', 'dist': '$RELEASE', 'comps': ['universe'], 'comment': '', 'line': 'deb $MIRROR/$RELEASE $RELEASE universe\n', 'file': '/etc/apt/sources.list', 'template': None, 'children': []}
{'invalid'...

[Launchpad Janitor (janitor)]

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

[Andrei (foxfluffy)]

This bug affects me too as well. Crash has gone after I removed $RELEASE from uri and codename sections.