Description

Refactor the discovery protocol architecture to use a single GenServer instead of three separate ones (DiscoveryMultiplexer + Discv4Server + Discv5Server). Since PeerTable is already shared and the multiplexer already discriminates packets, merging these GenServers simplifies the architecture and eliminates inter-GenServer communication overhead.

Current Architecture (4 GenServers)

┌─────────────────────┐
│ DiscoveryMultiplexer│ ← routes packets
└──────────┬──────────┘
           │
     ┌─────┴─────┐
     ▼           ▼
┌─────────┐ ┌─────────┐
│ Discv4  │ │ Discv5  │ ← separate GenServers
│ Server  │ │ Server  │
└────┬────┘ └────┬────┘
     │           │
     └─────┬─────┘
           ▼
    ┌───────────┐
    │ PeerTable │ ← shared state
    └───────────┘

Proposed Architecture (2 GenServers)

┌──────────────────────────────────┐
│        DiscoveryServer           │
│  ┌─────────────┬──────────────┐  │
│  │ Discv4State │ Discv5State  │  │ ← protocol-specific state
│  └─────────────┴──────────────┘  │
│  ┌────────────────────────────┐  │
│  │   Shared: socket, signer,  │  │ ← common fields
│  │   local_node, bootnodes    │  │
│  └────────────────────────────┘  │
│  • Single message loop           │
│  • Protocol dispatch via enum    │
│  • Unified timers                │
└───────────────┬──────────────────┘
                ▼
         ┌───────────┐
         │ PeerTable │
         └───────────┘

Benefits

• Eliminates inter-GenServer communication overhead (no more casting between multiplexer and protocol servers)
• Single lifecycle & shutdown coordination
• Unified logging with protocol tags
• Simpler architecture to reason about
• Single place for revalidate/lookup/prune logic with protocol parameter
• Reduced code duplication (~30-40% of current code is duplicated between servers)

Implementation Approach

1. Combine state structs: Create Discv4State and Discv5State structs for protocol-specific fields
2. Unified InMessage enum: Single enum with protocol-tagged variants

   enum InMessage {
       Discv4(Discv4Message),
       Discv5(Discv5Message),
       Revalidate(DiscoveryProtocol),
       Lookup(DiscoveryProtocol),
       Prune,
       Shutdown,
   }

3. Protocol dispatch: Route to handle_discv4_message() or handle_discv5_packet() based on variant
4. Unified timers: Single timer management with protocol parameter where intervals differ
5. Extract shared logic: Common implementations for revalidate(), lookup(), prune() with protocol parameter

Protocol-Specific State

Discv4:

• find_node_message: BytesMut (cached, re-signed every 5 seconds)
• Revalidation interval: 12 hours

Discv5:

• counter: u32 (nonce generation)
• pending_by_nonce: FxHashMap (messages awaiting WhoAreYou)
• pending_challenges: FxHashMap (WhoAreYou challenges)
• whoareyou_rate_limit: FxHashMap (per-IP rate limiting)
• ip_votes + voting state (external IP detection)
• Revalidation interval: 30 seconds

Dependencies

• Should be done after feat(l1): support dual discovery protocols (discv4 + discv5) with shared PeerTable #5962 (discovery multiplexer) is merged

Related Issues

• Remove "experimental-discv5" feature flag and enable discv5 by default #5971 - Remove "experimental-discv5" feature flag
• Detect and address performance degradation in discovery operations with large contact tables #5972 - Detect and address performance degradation in discovery operations