If a packet is heard multiple times, rebroadcast using the highest hop limit

Sometimes a packet will be in the TX queue waiting to be transmitted,
when it is overheard being rebroadcast by another node, with a higher
hop limit remaining. When this occurs, modify the pending packet in
the TX queue to avoid unnecessarily wasting hops.

Author: erayd

src/mesh/FloodingRouter.cpp

@@ -28,6 +28,10 @@ bool FloodingRouter::shouldFilterReceived(const meshtastic_MeshPacket *p)
             // cancel rebroadcast of this message *if* there was already one, unless we're a router/repeater!
             if (Router::cancelSending(p->from, p->id))
                 txRelayCanceled++;
+        } else if (iface && p->hop_limit > 0) {
+            // If we overhear a duplicate copy of the packet with more hops left than the one we are waiting to
+            // rebroadcast, then update the hop limit of the packet currently sitting in the TX queue.
+            iface->clampHopsToMax(getFrom(p), p->id, p->hop_limit - 1);
         }
 
         /* If the original transmitter is doing retransmissions (hopStart equals hopLimit) for a reliable transmission, e.g., when

src/mesh/MeshPacketQueue.cpp

@@ -93,6 +93,19 @@ meshtastic_MeshPacket *MeshPacketQueue::getFront()
     return p;
 }
 
+/** Attempt to find a packet in this queue. Returns a pointer to the found packet, or NULL if not found. */
+meshtastic_MeshPacket *MeshPacketQueue::find(NodeNum from, PacketId id)
+{
+    for (auto it = queue.begin(); it != queue.end(); it++) {
+        auto p = (*it);
+        if (getFrom(p) == from && p->id == id) {
+            return p;
+        }
+    }
+
+    return NULL;
+}
+
 /** Attempt to find and remove a packet from this queue.  Returns a pointer to the removed packet, or NULL if not found */
 meshtastic_MeshPacket *MeshPacketQueue::remove(NodeNum from, PacketId id)
 {

src/mesh/MeshPacketQueue.h

@@ -35,6 +35,9 @@ class MeshPacketQueue
 
     meshtastic_MeshPacket *getFront();
 
+    /** Attempt to find a packet in this queue. Returns a pointer to the found packet, or NULL if not found. */
+    meshtastic_MeshPacket *find(NodeNum from, PacketId id);
+
     /** Attempt to find and remove a packet from this queue.  Returns the packet which was removed from the queue */
     meshtastic_MeshPacket *remove(NodeNum from, PacketId id);
 };

src/mesh/RadioInterface.h

@@ -176,6 +176,9 @@ class RadioInterface
     /** The delay to use when we want to flood a message. Use a weighted scale based on SNR */
     uint32_t getTxDelayMsecWeighted(float snr);
 
+    /** Clamp the hop limit to the greater of the hop count provided, or the hop count in the queue */
+    virtual void clampHopsToMax(NodeNum from, PacketId id, uint32_t hop_limit) { return; }
+
     /**
      * Calculate airtime per
      * https://www.rs-online.com/designspark/rel-assets/ds-assets/uploads/knowledge-items/application-notes-for-the-internet-of-things/LoRa%20Design%20Guide.pdf

src/mesh/RadioLibInterface.cpp

@@ -336,6 +336,18 @@ void RadioLibInterface::startTransmitTimerSNR(float snr)
     }
 }
 
+/**
+ * Clamp the hop limit to the greater of the hop count provided, or the hop count in the queue
+ */
+void RadioLibInterface::clampHopsToMax(NodeNum from, PacketId id, uint32_t hop_limit)
+{
+    meshtastic_MeshPacket *p = txQueue.find(from, id);
+    if (p && p->hop_limit < hop_limit) {
+        LOG_DEBUG("Increasing hop limit for packet from %d to %d", p->hop_limit, hop_limit);
+        p->hop_limit = hop_limit;
+    }
+}
+
 void RadioLibInterface::handleTransmitInterrupt()
 {
     // This can be null if we forced the device to enter standby mode.  In that case

src/mesh/RadioLibInterface.h

@@ -200,4 +200,9 @@ class RadioLibInterface : public RadioInterface, protected concurrency::Notified
     virtual void setStandby();
 
     const char *radioLibErr = "RadioLib err=";
+
+    /**
+     * Clamp the hop limit to the greater of the hop count provided, or the hop count in the queue
+     */
+    void clampHopsToMax(NodeNum from, PacketId id, uint32_t hop_limit);
 };