fix: update ENR with actual port when bound to ephemeral port 0

[usmansaleem]

Summary

When `DiscoverySystem` is started with port `0` (e.g. Besu `--p2p-port=0`), the OS assigns an ephemeral port at bind time. Previously the local NodeRecord (ENR) was never updated with the actual bound port, so the advertised address was unreachable by peers.

Changes

`NettyDiscoveryServerImpl.getListenAddress()`
Returns the configured IP with the actual OS-assigned port after bind, but only when the configured port was `0`. Non-zero port behaviour is completely unchanged.

`LocalNodeRecordStore.onBoundPortResolved(InetSocketAddress)`
New method that updates the UDP or UDP6 port in the ENR after the OS assigns an actual port for an ephemeral bind. Unlike `onSocketAddressChanged`, this method bypasses `NewAddressHandler` because ephemeral port resolution is an internal OS operation, not an externally-reported address change. The advertised IP is preserved from the existing ENR; only the port is updated. No-op if the current ENR port is not `0`.

`DiscoveryManagerImpl.start()`
Calls `localNodeRecordStore.onBoundPortResolved(boundAddress)` in the per-server `thenAccept` callback after each server binds. The `NewAddressHandler` is intentionally bypassed here — callers such as Besu configure their handler to block all external address changes (to prevent peers from altering the advertised address), which would otherwise silently suppress the ephemeral port update.

Motivation

Fixes the ephemeral UDP port issue in Besu where `--p2p-port=0` (or `--p2p-discovery-port=0`) caused the ENR to advertise `udp=0` / `udp6=0`, making the node unreachable by peers.

The initial fix attempt used `onSocketAddressChanged` but that routes through `NewAddressHandler`. Besu's handler returns the old record unchanged (to prevent peers from overriding the advertised address), so the ENR update was silently dropped. `onBoundPortResolved` is the correct path for trusted internal updates.

After this fix, Besu's `warnIfEphemeralPortsWithDiscV5()` warning can be removed since ephemeral UDP ports now work correctly.

Test plan

• `NettyDiscoveryServerImplTest` — `getListenAddress()` returns port `0` before start, and the actual bound port after start with an ephemeral address
• `DiscoveryIntegrationTest.ephemeralPortIsReflectedInLocalNodeRecord` — starts a full `DiscoverySystem` with `listen("127.0.0.1", 0)` and asserts `getLocalNodeRecord().getUdpAddress().getPort()` is non-zero after startup
• `LocalNodeRecordStoreTest.onBoundPortResolvedUpdatesEphemeralUdpPort` — unit test: ephemeral port is resolved and listener is notified
• `LocalNodeRecordStoreTest.onBoundPortResolvedDoesNotUpdateNonEphemeralPort` — unit test: non-zero port nodes are unaffected
• Full test suite passes with no regressions (`./gradlew test`)

---

Note

Medium Risk
Touches discovery startup and ENR mutation logic; incorrect port/IP-family handling or concurrency bugs could make nodes advertise unreachable addresses, though changes are scoped and covered by new unit/integration tests.

Overview
Ensures nodes started with an ephemeral UDP port (port=0) advertise the actual OS-assigned port in their local ENR after binding.

NettyDiscoveryServerImpl.getListenAddress() now reports the bound port once started (while preserving the configured IP), and DiscoveryManagerImpl.start() uses that bound address to both select the channel’s IP family and to call the new LocalNodeRecordStore.onBoundPortResolved().

LocalNodeRecordStore is made thread-safe via AtomicReference and adds onBoundPortResolved() to update only the UDP/UDP6 port (bypassing NewAddressHandler) with a CAS loop to avoid losing concurrent dual-stack updates; new tests cover server listen address behavior, ENR port resolution in an integration run, and the new store logic including dual-stack concurrency.

^{Written by Cursor Bugbot for commit 8e5a224. This will update automatically on new commits. Configure here.}

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[usmansaleem]

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[usmansaleem]

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