use a better heuristic for the apt update last run time

As reported in prometheus-community#182, `pkgcache.bin` gets updated on more than just
`apt update`. In particular, it gets modified when a package is
installed, upgraded or removed.

So let's fallback on a better heuristic, which is the `APT::Periodic`
timestamp. This should give us a much more precise and deliberate
status.

We also fallback to the `lists` directory, which gets updated when new
mirror lists gets moved into place. This does run the risk of staying
silently unchanged if there's no change on mirrors but (a) that's
rather infrequent and (b) we might actually want to warn on that
anyway.

Signed-off-by: Antoine Beaupré <[email protected]>
Closes: prometheus-community#183

apt_info.py

@@ -21,6 +21,7 @@
 #          Daniel Swarbrick <[email protected]>
 
 import apt
+import apt_pkg
 import collections
 import os
 from prometheus_client import CollectorRegistry, Gauge, generate_latest
@@ -87,8 +88,16 @@ def _write_autoremove_pending(registry, cache):
 
 def _write_cache_timestamps(registry):
     g = Gauge('apt_package_cache_timestamp_seconds', "Apt update last run time.", registry=registry)
+    apt_pkg.init_config()
+    if apt_pkg.config.get("APT::Periodic::Update-Package-Lists") != "0":
+        # if we run updates automatically with APT::Periodic, we can
+        # check this timestamp file
+        stamp_file = "/var/lib/apt/periodic/update-success-stamp"
+    else:
+        # if not, let's just fallback on the lists directory
+        stamp_file = '/var/lib/apt/lists'
     try:
-        g.set(os.stat('/var/cache/apt/pkgcache.bin').st_mtime)
+        g.set(os.stat(stamp_file).st_mtime)
     except OSError:
         pass