Add airtime of current received/transmitted packet to nextTx time of all pending retransmissions (#2309)

To avoid unnecessary retransmissions when sending them too early

Co-authored-by: Ben Meadors <benmmeadors@gmail.com>

src/mesh/RadioInterface.cpp

@@ -175,7 +175,7 @@ uint32_t RadioInterface::getPacketTime(uint32_t pl)
     return msecs;
 }
 
-uint32_t RadioInterface::getPacketTime(meshtastic_MeshPacket *p)
+uint32_t RadioInterface::getPacketTime(const meshtastic_MeshPacket *p)
 {
     uint32_t pl = 0;
     if (p->which_payload_variant == meshtastic_MeshPacket_encrypted_tag) {

src/mesh/RadioInterface.h

@@ -154,7 +154,7 @@ class RadioInterface
      *
      * @return num msecs for the packet
      */
-    uint32_t getPacketTime(meshtastic_MeshPacket *p);
+    uint32_t getPacketTime(const meshtastic_MeshPacket *p);
     uint32_t getPacketTime(uint32_t totalPacketLen);
 
     /**

src/mesh/ReliableRouter.cpp

@@ -24,6 +24,15 @@ ErrorCode ReliableRouter::send(meshtastic_MeshPacket *p)
         startRetransmission(copy);
     }
 
+    /* If we have pending retransmissions, add the airtime of this packet to it, because during that time we cannot receive an
+       (implicit) ACK. Otherwise, we might retransmit too early.
+     */
+    for (auto i = pending.begin(); i != pending.end(); i++) {
+        if (i->first.id != p->id) {
+            i->second.nextTxMsec += iface->getPacketTime(p);
+        }
+    }
+
     return FloodingRouter::send(p);
 }
 
@@ -53,6 +62,15 @@ bool ReliableRouter::shouldFilterReceived(const meshtastic_MeshPacket *p)
         }
     }
 
+    /* At this point we have already deleted the pending retransmission if this packet was an (implicit) ACK to it.
+       Now for all other pending retransmissions, we have to add the airtime of this received packet to the retransmission timer,
+       because while receiving this packet, we could not have received an (implicit) ACK for it.
+       If we don't add this, we will likely retransmit too early.
+    */
+    for (auto i = pending.begin(); i != pending.end(); i++) {
+        i->second.nextTxMsec += iface->getPacketTime(p);
+    }
+
     /* Resend implicit ACKs for repeated packets (assuming the original packet was sent with HOP_RELIABLE)
      * this way if an implicit ACK is dropped and a packet is resent we'll rebroadcast again.
      * Resending real ACKs is omitted, as you might receive a packet multiple times due to flooding and