Add debug logging

This can help one in identifying what the cache filters identified in
each category, which is useful both for debugging the code itself and
for identifying what builtin command-line apt search filters may not be
identifying in a similar manner.

One example of this is the obsolete package filter, which can sometimes
catch more packages than `apt list "?obsolete"`

Author: Gabriel Filion

apt_info.py

@@ -25,7 +25,9 @@
 import apt
 import apt_pkg
 import collections
+import logging
 import os
+import sys
 from prometheus_client import CollectorRegistry, Gauge, generate_latest
 
 _UpgradeInfo = collections.namedtuple("_UpgradeInfo", ["labels", "count"])
@@ -58,10 +60,16 @@ def _convert_candidates_to_upgrade_infos(candidates):
     return changes_list
 
 
-def _write_pending_upgrades(registry, cache):
+def _write_pending_upgrades(registry, cache, logger):
     candidates = {
         p.candidate for p in cache if p.is_upgradable and not p.phasing_applied
     }
+    for candidate in candidates:
+        logger.debug(
+            "pending upgrade: %s / %s",
+            candidate.package,
+            candidate.architecture,
+        )
     upgrade_list = _convert_candidates_to_upgrade_infos(candidates)
 
     if upgrade_list:
@@ -71,7 +79,7 @@ def _write_pending_upgrades(registry, cache):
             g.labels(change.labels['origin'], change.labels['arch']).set(change.count)
 
 
-def _write_held_upgrades(registry, cache):
+def _write_held_upgrades(registry, cache, logger):
     held_candidates = {
         p.candidate for p in cache
         if (
@@ -80,6 +88,12 @@ def _write_held_upgrades(registry, cache):
             and not p.phasing_applied
         )
     }
+    for candidate in held_candidates:
+        logger.debug(
+            "held upgrade: %s / %s",
+            candidate.package,
+            candidate.architecture,
+        )
     upgrade_list = _convert_candidates_to_upgrade_infos(held_candidates)
 
     if upgrade_list:
@@ -89,22 +103,37 @@ def _write_held_upgrades(registry, cache):
             g.labels(change.labels['origin'], change.labels['arch']).set(change.count)
 
 
-def _write_obsolete_packages(registry, cache):
+def _write_obsolete_packages(registry, cache, logger):
     # This corresponds to the apt filter "?obsolete"
     obsoletes = [p for p in cache if p.is_installed and (
                   p.candidate is None or
                   not p.candidate.origins or
                   (len(p.candidate.origins) == 1 and
                    p.candidate.origins[0].origin in ['', "/var/lib/dpkg/status"])
                 )]
+    for package in obsoletes:
+        if package.candidate is None:
+            logger.debug("obsolete package with no candidate: %s", package)
+        else:
+            logger.debug(
+                "obsolete package: %s / %s",
+                package,
+                package.candidate.architecture,
+            )
 
     g = Gauge('apt_packages_obsolete_count', "Apt packages which are obsolete",
               registry=registry)
     g.set(len(obsoletes))
 
 
-def _write_autoremove_pending(registry, cache):
-    autoremovable_packages = {p for p in cache if p.is_auto_removable}
+def _write_autoremove_pending(registry, cache, logger):
+    autoremovable_packages = {p.candidate for p in cache if p.is_auto_removable}
+    for candidate in autoremovable_packages:
+        logger.debug(
+            "autoremovable package: %s / %s",
+            candidate.package,
+            candidate.architecture,
+        )
     g = Gauge('apt_autoremove_pending', "Apt packages pending autoremoval.",
               registry=registry)
     g.set(len(autoremovable_packages))
@@ -147,13 +176,18 @@ def _write_reboot_required(registry):
 
 
 def _main():
+    logging.basicConfig(stream=sys.stderr)
+    logger = logging.getLogger(__name__)
+    if os.environ.get("DEBUG"):
+        logger.setLevel(logging.DEBUG)
+
     cache = apt.cache.Cache()
 
     registry = CollectorRegistry()
-    _write_pending_upgrades(registry, cache)
-    _write_held_upgrades(registry, cache)
-    _write_obsolete_packages(registry, cache)
-    _write_autoremove_pending(registry, cache)
+    _write_pending_upgrades(registry, cache, logger)
+    _write_held_upgrades(registry, cache, logger)
+    _write_obsolete_packages(registry, cache, logger)
+    _write_autoremove_pending(registry, cache, logger)
     _write_installed_packages_per_origin(registry, cache)
     _write_cache_timestamps(registry)
     _write_reboot_required(registry)