fix: Use a better upgradability measure

Fixes #35

upgradable.sh

@@ -2,8 +2,8 @@
 # SPDX-License-Identifier: CC-BY-SA-4.0
 # SPDX-FileCopyrightText: © 2023 Olaf Meeuwissen
 
-# Make sure apt and apt-get output is in the expected locale so we can
-# parse output reliably.  For sort purposes it is nice to have a known
+# Make sure apt-get output is in the expected locale so we can parse
+# output reliably.  For sorting purposes, it is nice to have a known
 # collation order as well.
 
 LC_ALL=C.UTF-8
@@ -11,19 +11,13 @@ export LC_ALL
 
 apt-get --quiet update
 
-# This count has been used to decide whether container images need to
-# be upgraded so far.  However, because the apt CLI is not stable, we
-# should deprecate this in favour of something that *is* stable.
-
-upgradable_count=$(apt list --upgradable 2>/dev/null | grep -c upgradable)
-
 out=$(mktemp)
 trap 'rm $out' 0 1 2 15
 
-# The apt-get CLI is stable and the results of parsing its output will
-# be used to replace the upgradable_count.  Parse results are reported
-# for informational purposes.  Only the number of upgradable packages
-# will be used to decide upgradability.
+# Simulate an upgrade that intelligently handles changing dependencies
+# with new versions of packages and get rid of any packages that are
+# no longer needed.  Collect command output for parsing and feedback
+# purposes.
 
 apt-get dist-upgrade --auto-remove --simulate > "$out"
 
@@ -47,11 +41,4 @@ if test -n "$removable"; then
     echo "$removable" | sed 's/^/- /' | sort
 fi
 
-# We expect both counts to be identical but report any discrepancies.
-
-if test "$upgradable_count" -ne "$dist_upgrade_count"; then
-    echo "apt list --upgradable reports $upgradable_count packages"
-    echo "apt-get dist-upgrade reports $dist_upgrade_count packages"
-fi
-
-test 0 -lt "$upgradable_count"
+test 0 -lt "$dist_upgrade_count"