From 9df53b1629e78c69e8168be544e06d5e9b086588 Mon Sep 17 00:00:00 2001
From: Graham McIntire <graham@mcintire.me>
Date: Sun, 8 Feb 2026 14:09:26 -0600
Subject: [PATCH] fix: suppress repetitive SNMP MIB resolution errors

Add logger filter to drop repetitive 'Cannot find module' errors
from net-snmp library. These errors occur when snmptranslate can't
find vendor-specific MIB modules but don't affect functionality
since the system falls back to numeric OIDs.

The errors were flooding production logs (hundreds of identical
messages) during polling of Morningstar devices that reference
TRISTAR MIB modules.

Changes:
- Add drop_snmp_mib_errors/2 filter to LoggerFilters module
- Register filter in production logger configuration
- Filter matches exact error format from snmptranslate stderr

The root cause (missing NIF binary) will be addressed separately
by compiling the towerops_nif.so during Docker build.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
---
 config/prod.exs                |  6 +++++-
 lib/towerops/logger_filters.ex | 21 +++++++++++++++++++++
 2 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/config/prod.exs b/config/prod.exs
index 5e0f75d4..2ce803df 100644
--- a/config/prod.exs
+++ b/config/prod.exs
@@ -1,11 +1,15 @@
 import Config
 
-# Filter out harmless Oban shutdown messages during deployments
+# Filter out harmless Oban shutdown messages and repetitive SNMP MIB errors
 config :logger, :default_handler,
   filters: [
     drop_oban_shutdown: {
       &Towerops.LoggerFilters.drop_oban_shutdown/2,
       []
+    },
+    drop_snmp_mib_errors: {
+      &Towerops.LoggerFilters.drop_snmp_mib_errors/2,
+      []
     }
   ]
 
diff --git a/lib/towerops/logger_filters.ex b/lib/towerops/logger_filters.ex
index 47c0c015..b7e1732f 100644
--- a/lib/towerops/logger_filters.ex
+++ b/lib/towerops/logger_filters.ex
@@ -42,4 +42,25 @@ defmodule Towerops.LoggerFilters do
       String.contains?(msg, "EXIT") and
       String.contains?(msg, ":shutdown")
   end
+
+  @doc """
+  Drops repetitive SNMP MIB resolution errors from net-snmp library.
+
+  These errors occur when snmptranslate can't find vendor-specific MIB modules.
+  The errors are logged repeatedly during polling but don't affect functionality
+  since the system falls back to numeric OIDs.
+
+  Example messages filtered:
+  - "Cannot find module (TRISTAR): At line 1 in (none)"
+  - "Cannot find module (VENDOR-MIB): At line 1 in (none)"
+  """
+  def drop_snmp_mib_errors(log_event, _opts) do
+    if snmp_mib_error?(log_event), do: :stop, else: :ignore
+  end
+
+  defp snmp_mib_error?(%{msg: {:string, msg}}) when is_binary(msg) do
+    String.contains?(msg, "Cannot find module") and String.contains?(msg, "At line 1 in (none)")
+  end
+
+  defp snmp_mib_error?(_), do: false
 end