A supply nobody could read is unknown, not empty

plugins/printers/services/zabbix_service.py

@@ -208,18 +208,64 @@ class ZabbixService:
                 continue
             if str(item.get("state", "0")) != "0":
                 continue
-            try:
-                level = int(float(item.get("lastvalue", 0)))
-            except (ValueError, TypeError):
-                level = 0
+            level, lastseen = self._readlevel(item)
             supplies.append({
                 "name": item.get("name", "Unknown"),
                 "level": level,
+                "lastseen": lastseen,
                 "color": self._extract_color(item),
                 "itemid": item.get("itemid"),
             })
         return supplies
 
+    # How old the newest reading may be and still be presented as the level.
+    # Generous on purpose: supply items are polled in hours, not seconds, and
+    # calling a slow poller "unknown" would be its own lie. This exists for the
+    # host that stopped answering, not for one that answers rarely.
+    STALE_AFTER_SECONDS = 24 * 3600
+
+    @classmethod
+    def _readlevel(cls, item):
+        """(level, lastseen) for one supply item; level is None when unknown.
+
+        An item that has never collected returns lastvalue as an EMPTY STRING,
+        and one whose host went quiet keeps a lastvalue that is arbitrarily old.
+        Both used to become 0 - the empty string because float('') raises and
+        the handler substituted zero, the stale one because nothing ever read
+        lastclock, which was fetched and then ignored.
+
+        Zero is not a safe stand-in for "no reading". It is a legitimate value
+        meaning the cartridge is spent, so the substitution was indistinguishable
+        from a real empty - and it hit every item on the host at once, which is
+        what a printer that has gone dark looks like. Downstream that reached
+        `level <= EMPTY_LEVEL`, so the forecast gave it daysleft 0 and the order
+        list asked purchasing to buy a full set of cartridges for a printer
+        nobody had measured.
+
+        None says "unknown" and every consumer already handles it: the detail
+        page renders N/A, the alert loop skips it, and analyse() falls back to
+        the last real level in history rather than inventing one.
+        """
+        raw = item.get("lastvalue")
+        if raw is None or str(raw).strip() == "":
+            return None, None
+        try:
+            level = int(float(raw))
+        except (ValueError, TypeError):
+            return None, None
+
+        try:
+            lastseen = int(item.get("lastclock") or 0)
+        except (ValueError, TypeError):
+            lastseen = 0
+        if lastseen <= 0:
+            # A value with no clock cannot be aged. Trust the value and say the
+            # age is unknown rather than discarding a reading that may be fine.
+            return level, None
+        if time.time() - lastseen > cls.STALE_AFTER_SECONDS:
+            return None, lastseen
+        return level, lastseen
+
     # Per item, per call. Zabbix applies `limit` to the WHOLE result set, not
     # per item, so the budget is multiplied by the number of items asked for.
     HISTORY_PER_ITEM = 2000