From 8fa3efc06708c84c102b5e55e3c01d8e493f390b Mon Sep 17 00:00:00 2001
From: DatanoiseTV <6614616+DatanoiseTV@users.noreply.github.com>
Date: Fri, 8 May 2026 21:36:03 +0200
Subject: [PATCH] router: don't evict ACKs when a background-priority packet
 arrives full
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fromRadioQueue is a plain FIFO of size 4. When it fills, the prior
behaviour was to drop the oldest packet — usually a routing ACK or
reply — to make room for whatever just arrived, even if the new packet
was BACKGROUND priority. Reverse that for the low-priority case: if
the incoming packet's priority is at or below BACKGROUND, drop it
instead. Higher-priority arrivals still evict to fit. PointerQueue
doesn't expose iteration, so a full priority-queue refactor would be a
larger change; this addresses the worst case.
---
 src/mesh/Router.cpp | 23 ++++++++++++++++-------
 1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/src/mesh/Router.cpp b/src/mesh/Router.cpp
index ffeb7c5393d..2f7bf8b2d9d 100644
--- a/src/mesh/Router.cpp
+++ b/src/mesh/Router.cpp
@@ -168,13 +168,22 @@ int32_t Router::runOnce()
  */
 void Router::enqueueReceivedMessage(meshtastic_MeshPacket *p)
 {
-    // Try enqueue until successful
-    while (!fromRadioQueue.enqueue(p, 0)) {
-        meshtastic_MeshPacket *old_p;
-        old_p = fromRadioQueue.dequeuePtr(0); // Dequeue and discard the oldest packet
-        if (old_p) {
-            printPacket("fromRadioQ full, drop oldest!", old_p);
-            packetPool.release(old_p);
+    // Try enqueue, but if the queue is full and the incoming packet is itself
+    // low priority, drop *it* rather than evicting an older (likely higher-
+    // priority) packet such as an ACK or routing reply.
+    if (!fromRadioQueue.enqueue(p, 0)) {
+        if (p->priority != meshtastic_MeshPacket_Priority_UNSET && p->priority <= meshtastic_MeshPacket_Priority_BACKGROUND) {
+            printPacket("fromRadioQ full, drop incoming low-prio!", p);
+            packetPool.release(p);
+            return;
+        }
+        // Higher-priority packet — fall back to evicting until it fits.
+        while (!fromRadioQueue.enqueue(p, 0)) {
+            meshtastic_MeshPacket *old_p = fromRadioQueue.dequeuePtr(0);
+            if (old_p) {
+                printPacket("fromRadioQ full, drop oldest!", old_p);
+                packetPool.release(old_p);
+            }
         }
     }
     // Nasty hack because our threading is primitive.  interfaces shouldn't need to know about routers FIXME