feat(snmp): a NAS's disk and volume I/O, CPU waiting on disk, available memory, interface errors (#5997)

- Synology: what each disk and volume reads and writes, bytes and requests
  a second and the load DSM gives it (SYNOLOGY-STORAGEIO- and -SPACEIO-MIB,
  64-bit byte counters), and each disk's reconnects and sectors not found.
- Linux server: CPU waiting on disk, the wait tick counter's share of the
  CPU's, and memory available (free + buffers + cache). A reading may now be
  worked out from others: the poll asks for its inputs, rates each counter,
  and stores only the result.
- Interface traffic: errors and discards in and out, per second.
- byte_count unit: a byte counter shown as bytes a second.

Asustor's MIB has nothing more for this; its Linux server readings gain the
same CPU wait and available memory.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/plugins/snmp/test_scheduler.py

@@ -4,16 +4,24 @@ import asyncio
 from contextlib import asynccontextmanager
 from types import SimpleNamespace as NS
 
+import pytest
+
 import plugins.snmp.scheduler as scheduler
 import plugins.snmp.service as service
 from plugins.snmp.poller import Credentials, PollResult
 
 
-def _device():
-    return NS(id="d1", name="nas", address="10.0.0.5", port=161, version="2c", presets=["system"],
+def _device(presets=("system",)):
+    return NS(id="d1", name="nas", address="10.0.0.5", port=161, version="2c", presets=list(presets),
               interfaces="", custom=None, tables={}, tables_at=None, last_error=None, texts={})
 
 
+@asynccontextmanager
+async def _hung_session():
+    await asyncio.Event().wait()                       # never answers: the device list comes from memory
+    yield None
+
+
 def test_polls_go_on_while_the_database_hangs_and_are_stored_in_order_after(monkeypatch):
     async def main():
         stalled = asyncio.Event()
@@ -53,3 +61,50 @@ def test_polls_go_on_while_the_database_hangs_and_are_stored_in_order_after(monk
     assert [[(p.name, p.values) for p in r] for r in rounds] == [
         [("nas", [("uptime", 100.0)])], [("nas", [("uptime", 200.0)])]]
     assert rounds[0][0].at < rounds[1][0].at              # each stored at the time it was collected
+
+
+def test_a_reading_worked_out_from_others_is_asked_for_by_its_inputs_and_kept_alone(monkeypatch):
+    """#5997: CPU waiting on disk from the tick counters' rates, and available
+    memory from three gauges; the inputs are neither stored nor named missing."""
+    mem = {"memory_free_kb": 1000.0, "memory_buffers_kb": 200.0, "memory_cached_kb": 3000.0}
+    answers = iter([
+        {"cpu_ticks_user": 1000.0, "cpu_ticks_nice": 0.0, "cpu_ticks_system": 500.0,
+         "cpu_ticks_idle": 8000.0, "cpu_ticks_wait": 500.0} | mem,
+        {"cpu_ticks_user": 1100.0, "cpu_ticks_nice": 0.0, "cpu_ticks_system": 550.0,
+         "cpu_ticks_idle": 8800.0, "cpu_ticks_wait": 550.0} | mem,
+        {"cpu_ticks_user": 1200.0, "cpu_ticks_nice": 0.0, "cpu_ticks_system": 600.0,
+         "cpu_ticks_idle": 9600.0} | mem])                 # this time it doesn't answer the wait counter
+    asked, stored = [], []
+
+    async def poll(address, port, creds, oids):
+        asked.append(dict((k, o) for o, k in oids))
+        got = next(answers)
+        return PollResult(readings=got, missing=[k for _, k in oids if k not in got])
+
+    async def store(app_, rounds):
+        stored.extend(rounds)
+
+    async def main():
+        monkeypatch.setattr(scheduler, "DEVICES", [_device(["linux"])])
+        monkeypatch.setattr(scheduler, "DEVICES_READ_S", 0.05)
+        monkeypatch.setattr(scheduler, "_store", store)
+        monkeypatch.setattr(service, "credentials", lambda d: Credentials(community="public"))
+        scheduler.RATES = None
+        app = NS(db_sessionmaker=_hung_session)
+        for _ in range(3):
+            await scheduler._do_poll(app, poll=poll, walk=_no_tables)
+            await asyncio.sleep(0.01)
+
+    asyncio.run(main())
+    assert asked[0]["cpu_ticks_wait"] == "1.3.6.1.4.1.2021.11.54.0" and "cpu_iowait" not in asked[0]
+    assert "" not in asked[0].values()
+    first, second, third = (dict(r[0].values) for r in stored)
+    assert first == {"memory_available": 4200.0}           # a counter needs two samples
+    assert second == {"cpu_iowait": pytest.approx(5.0), "memory_available": 4200.0}   # 50 of 1,000 ticks waited
+    assert third == {"memory_available": 4200.0}
+    assert "cpu_iowait" in stored[2][0].missing and not any(k.startswith("cpu_ticks") for k in stored[2][0].missing)
+
+
+async def _no_tables(address, port, creds, tables):
+    from plugins.snmp.poller import TableScan
+    return TableScan(rows={})