SNMP: add e2e diagnostic dump for test_e2e_v1_with_apc_ups_profile

When the assertion fails (currently failing in CI on every SNMP PR with
"Needed at least 1 candidates for 'snmp.device.reachable', got 0"), dump:

- docker ps state
- snmpsim container logs (tail 40)
- snmpsim mount sources/destinations
- whether apc_ups fixture is present + first lines of args_list.txt
- a direct snmpget probe against the snmpsim container

Diagnostic only, no test-logic change. Helps narrow whether the daemon
isn't routing the community, the fixture isn't mounted, or the agent
isn't reaching the daemon.

Author: ian28223

snmp/tests/test_e2e_core.py

@@ -22,7 +22,46 @@ def test_e2e_v1_with_apc_ups_profile(dd_agent_check):
             'community_string': 'apc_ups',
         }
     )
-    assert_apc_ups_metrics(dd_agent_check, config)
+    try:
+        assert_apc_ups_metrics(dd_agent_check, config)
+    except AssertionError:
+        _dump_snmp_e2e_diagnostics(instance, community='apc_ups')
+        raise
+
+
+def _dump_snmp_e2e_diagnostics(instance, community):
+    """Print everything we'd need to triage 'got 0 candidates' failures.
+
+    Triggers only when an assertion has already failed; keeps the test signal
+    intact while making CI logs self-explanatory for env flakes.
+    """
+    import subprocess
+
+    def _run(cmd, label):
+        print('\n----- DEBUG: {} -----'.format(label), flush=True)
+        print('$ {}'.format(' '.join(cmd)), flush=True)
+        try:
+            out = subprocess.run(cmd, capture_output=True, text=True, timeout=30)
+            print('exit={}\nstdout:\n{}\nstderr:\n{}'.format(out.returncode, out.stdout, out.stderr), flush=True)
+        except Exception as exc:
+            print('error: {}'.format(exc), flush=True)
+
+    print('\n========== SNMP E2E DIAGNOSTICS ==========', flush=True)
+    print('instance: ip={} port={} community={}'.format(
+        instance.get('ip_address'), instance.get('port'), community), flush=True)
+
+    _run(['docker', 'ps', '-a', '--format', '{{.Names}}\t{{.Image}}\t{{.Status}}'], 'docker ps')
+    _run(['docker', 'logs', '--tail', '40', SNMP_CONTAINER_NAME], 'snmpsim logs (tail 40)')
+    _run(['docker', 'inspect', SNMP_CONTAINER_NAME,
+          '--format', '{{range .Mounts}}{{.Source}} -> {{.Destination}}{{println}}{{end}}'],
+         'snmpsim mounts')
+    _run(['docker', 'exec', SNMP_CONTAINER_NAME, 'sh', '-c',
+          'ls /usr/snmpsim/data/ | wc -l; ls /usr/snmpsim/data/apc_ups* 2>&1; head -5 /usr/snmpsim/data/args_list.txt 2>&1'],
+         'snmpsim data dir + apc_ups fixture + args_list.txt head')
+    _run(['docker', 'exec', SNMP_CONTAINER_NAME, 'sh', '-c',
+          'snmpget -v1 -c apc_ups 127.0.0.1:1161 1.3.6.1.2.1.1.1.0 2>&1 || true'],
+         'snmpget apc_ups sysDescr from inside snmpsim container')
+    print('========== END SNMP E2E DIAGNOSTICS ==========\n', flush=True)
 
 
 def test_e2e_core_v3_no_auth_no_priv(dd_agent_check):