Z13: Runtime handles RENDEZVOUS/PUSH_DIRECT_PATHS

[JKamsker]

Implements handling of root RENDEZVOUS + peer PUSH_DIRECT_PATHS in ZeroTierDataplaneRuntime and sends UDP hole punches. Adds unit tests and marks the final MVP checklist item complete.

Summary by CodeRabbit

• New Features

  • Added support for UDP direct path establishment between peers with rendezvous and hole punch signaling capabilities.

• Tests

  • Added comprehensive integration tests verifying direct path discovery and hole punch message handling.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

Configuration used: defaults

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between 4608072 and a460dc0.

📒 Files selected for processing (5)

• ZTSharp.Tests/ZeroTierDataplaneRuntimeDirectPathTests.cs
• ZTSharp/ZeroTier/Internal/ZeroTierDataplanePeerPacketHandler.cs
• ZTSharp/ZeroTier/Internal/ZeroTierDataplaneRuntime.cs
• ZTSharp/ZeroTier/Internal/ZeroTierDataplaneRxLoops.cs
• docs/Tasks/zerotier-real-stack-mvp.md

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📝 Walkthrough

Walkthrough

This PR implements UDP hole punch functionality for ZeroTier direct path establishment. It adds control message handlers (Rendezvous, PushDirectPaths) to the dataplane runtime, introduces per-peer endpoint management infrastructure, and includes integration tests verifying hole punch packet exchanges.

Changes

Cohort / File(s)	Summary
Test Suite ZTSharp.Tests/ZeroTierDataplaneRuntimeDirectPathTests.cs	New integration test class with two async tests: one for handling Rendezvous messages and another for PushDirectPaths, both verifying hole punch payload reception within 2 seconds. Includes internal helpers to build test payloads and configure runtime/transport/identity setup.
Runtime Control Handlers ZTSharp/ZeroTier/Internal/ZeroTierDataplaneRuntime.cs	Introduces per-peer direct endpoint cache (ConcurrentDictionary), wires control handlers (HandleRootControlPacketAsync, HandlePeerControlPacketAsync) to packet processors, and adds GetOrCreateDirectEndpointManager helper. Routes Rendezvous and PushDirectPaths messages to respective endpoint managers.
Peer Packet Handler ZTSharp/ZeroTier/Internal/ZeroTierDataplanePeerPacketHandler.cs	Adds optional async control handler field and constructor parameter. Introduces handling case for PushDirectPaths verb that delegates to the control handler when provided, enabling peer-initiated direct path updates.
RX Loop Handler ZTSharp/ZeroTier/Internal/ZeroTierDataplaneRxLoops.cs	Adds optional root-control handler hook with constructor parameter. Converts root payload handling from ReadOnlySpan to ReadOnlyMemory, and invokes handler on dispatch failures to process Rendezvous and other root-directed control messages.
Documentation docs/Tasks/zerotier-real-stack-mvp.md	Marks Milestone Z13 (Rendezvous and PushDirectPaths handling) as completed.

Sequence Diagram(s)

sequenceDiagram
    participant Root as ZeroTier Root
    participant RxLoop as ZeroTierDataplaneRxLoops
    participant Runtime as ZeroTierDataplaneRuntime
    participant Manager as DirectEndpointManager

    Root->>RxLoop: Rendezvous packet
    RxLoop->>RxLoop: TryDispatchResponse fails
    RxLoop->>Runtime: HandleRootControlPacketAsync(Rendezvous, ...)
    Runtime->>Runtime: GetOrCreateDirectEndpointManager(peerId)
    Runtime->>Manager: HandleRendezvousFromRoot(...)
    Manager->>Manager: Parse rendezvous endpoint
    Manager->>Root: Send hole punch packet

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sequenceDiagram
    participant Peer as Remote Peer
    participant Handler as ZeroTierDataplanePeerPacketHandler
    participant Runtime as ZeroTierDataplaneRuntime
    participant Manager as DirectEndpointManager

    Peer->>Handler: PushDirectPaths packet
    Handler->>Handler: Dispatch to control handler
    Handler->>Runtime: HandlePeerControlPacketAsync(peerId, PushDirectPaths, ...)
    Runtime->>Runtime: GetOrCreateDirectEndpointManager(peerId)
    Runtime->>Manager: HandleDirectPathsFromPeer(...)
    Manager->>Manager: Update direct endpoints
    Manager->>Peer: Send hole punch packet

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Estimated Code Review Effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Poem

🐰 Bouncing through the network hops,
Direct paths spring where firewall stops,
Rendezvous whispers, punch cards fly,
Peers find peers beneath the sky! 🌐✨

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and specifically identifies the main change: implementing handling of RENDEZVOUS and PUSH_DIRECT_PATHS messages in the ZeroTier dataplane runtime, matching the PR's primary objective.

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✨ Finishing Touches

• 📝 Generate docstrings (stacked PR)
• 📝 Generate docstrings (commit on current branch)

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch z13-runtime-direct-paths

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