/usr/share/python3/runtime.d/uwsgi-plugin-asyncio-python3.rtupdate is in uwsgi-plugin-asyncio-python3 2.0.12-5ubuntu3.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 | #!/bin/sh
set -e
if [ "x$1" = "xrtupdate" ]; then
  # Transform new default Python runtime name from 'pythonX.Y' to 'pythonXY'.
  NEW_RTNAME_DOTLESS="$(echo "$3" | tr -d .)"
  # Flavors of Python interpreter which was available at packaging moment
  # (and for which appropriate asyncio_*_plugin.so was built).
  KNOWN_FLAVORS_DOTLESS="python35"
  DEFAULT_PLUGIN_ALTERNATIVE_PRIORITY=75
  ALT_PLUGIN_ALTERNATIVE_PRIORITY=35
  for maybe_new_defaultflavor_dotless in $KNOWN_FLAVORS_DOTLESS; do
    # Check if * runtime (where * is a new default Python
    # flavor) was available at packaging moment.
    #
    # If it wasn't available, there is no point in changing default alternative
    # (as appropriate asyncio_*_plugin.so wasn't built and packaged).
    if [ "x$NEW_RTNAME_DOTLESS" = "x$maybe_new_defaultflavor_dotless" ]
    then
      # Reset prorities of all packaged alternatives of
      # asyncio_python3_plugin.so to lower value.
      for flavor_dotless in $KNOWN_FLAVORS_DOTLESS; do
        update-alternatives --quiet \
          --install \
            /usr/lib/uwsgi/plugins/asyncio_python3_plugin.so \
            uwsgi-plugin-asyncio-python3 \
            /usr/lib/uwsgi/plugins/asyncio_${flavor_dotless}_plugin.so \
            $ALT_PLUGIN_ALTERNATIVE_PRIORITY
      done
      # Set higher priority for asyncio_*_plugin.so (where * is
      # a new default Python version).
      update-alternatives --quiet \
        --install \
          /usr/lib/uwsgi/plugins/asyncio_python3_plugin.so \
          uwsgi-plugin-asyncio-python3 \
          "/usr/lib/uwsgi/plugins/asyncio_${NEW_RTNAME_DOTLESS}_plugin.so" \
          $DEFAULT_PLUGIN_ALTERNATIVE_PRIORITY
      break # for maybe_new_defaultflavor_dotless ...
    fi
  done
fi
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