From 38bd9828f75eb5785748656121e62da990d15ed7 Mon Sep 17 00:00:00 2001
From: Graham McIntire <graham@mcintire.me>
Date: Sun, 4 Jan 2026 13:28:53 -0600
Subject: [PATCH] Improve Counter64 decoder to handle 16-byte values

Some SNMP implementations return 16-byte binaries for Counter64 values
instead of the standard 8 bytes. Updated decoder to:
- Handle 8-byte standard Counter64
- Handle 16-byte values by reading last 8 bytes
- Handle any size > 8 bytes by reading last 8 bytes
- Add debug/warning logging for non-standard sizes

This fixes DBConnection.EncodeError when polling interface statistics
from devices that return non-standard Counter64 encodings.
---
 lib/towerops/snmp/poller_worker.ex | 23 ++++++++++++++++++++---
 1 file changed, 20 insertions(+), 3 deletions(-)

diff --git a/lib/towerops/snmp/poller_worker.ex b/lib/towerops/snmp/poller_worker.ex
index ed200279..c5e7c910 100644
--- a/lib/towerops/snmp/poller_worker.ex
+++ b/lib/towerops/snmp/poller_worker.ex
@@ -245,15 +245,32 @@ defmodule Towerops.Snmp.PollerWorker do
   defp decode_snmp_value(value) when is_number(value), do: value
 
   defp decode_snmp_value(value) when is_binary(value) do
-    # Try to decode as Counter64 (8 bytes, big-endian unsigned integer)
+    # Try to decode based on byte size
     case byte_size(value) do
       8 ->
+        # Standard Counter64 (8 bytes, big-endian unsigned integer)
         <<counter::unsigned-big-integer-size(64)>> = value
         counter
 
+      16 ->
+        # Some SNMP implementations return 16-byte values
+        # Try reading last 8 bytes as Counter64
+        <<_prefix::binary-size(8), counter::unsigned-big-integer-size(64)>> = value
+        counter
+
+      size when size > 8 ->
+        # Large binary, try reading last 8 bytes
+        offset = size - 8
+        <<_prefix::binary-size(offset), counter::unsigned-big-integer-size(64)>> = value
+        Logger.debug("Decoded #{size}-byte SNMP value as Counter64 from last 8 bytes: #{counter}")
+        counter
+
       _ ->
-        # Unknown binary format, return nil
-        Logger.debug("Unknown SNMP binary value format, size: #{byte_size(value)}")
+        # Unknown binary format or too small
+        Logger.warning(
+          "Unknown SNMP binary value format, size: #{byte_size(value)}, bytes: #{inspect(value)}"
+        )
+
         nil
     end
   end