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Add Reverse Connect support#1716

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feature/reverse-connect
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Add Reverse Connect support#1716
kevinherron wants to merge 31 commits intomainfrom
feature/reverse-connect

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@kevinherron kevinherron commented Mar 25, 2026 •
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Closes #1715

Summary

  • Implement OPC UA Reverse Connect across all three layers: protocol messages (ReverseHelloMessage), transport (client listener + server connector with FSMs), and SDK (server ReverseConnectManager, client createReverseConnect() factory)
  • Server maintains the idle socket invariant per spec: at least one idle connection to each registered client at all times, spawning a replacement when a connection becomes active
  • Client transport uses a Netty ServerBootstrap to accept inbound connections, validates ReverseHello against an optional allowedServerUris whitelist, then completes the standard Hello/Ack/OpenSecureChannel handshake
  • Server transport uses exponential backoff with configurable initial interval, max delay, and reject backoff for retry behavior

Architecture

See docs/architecture/reverse-connect.md for the full design document covering protocol flow, layering decisions, FSM state diagrams, and configuration options.

Implementation details

Protocol (stack-core)

  • ReverseHelloMessage — encode/decode with 4096-byte field limits per spec
  • MessageType.ReverseHello (RHE) enum value

Transport layer

  • Client: OpcTcpReverseConnectTransport, ReverseConnectChannelFsm (NotConnected → Handshaking → Connected → Reconnecting), UascClientReverseHelloHandler
  • Server: OpcTcpReverseConnectServerTransport, ReverseConnectConnectionFsm (Idle → Connecting → RheSent → Active, with ConnectWait for backoff), UascServerReverseHelloHandler
  • ChannelStateObservable interface for transport-agnostic session reconnection

SDK layer

  • Server: ReverseConnectManager with dynamic add/remove, idle socket invariant enforcement, and pre-start registration support; OpcUaServer integration via setReverseConnectManager()/addReverseConnect()/removeReverseConnect()
  • Client: OpcUaClient.createReverseConnect() with two-pass discovery; DiscoveryClient reverse connect overloads

Test plan

  • ReverseHelloMessageTest — encode/decode round-trip and validation edge cases
  • UascClientReverseHelloHandlerTest — handshake sequence, server URI filtering, error handling
  • ReverseConnectChannelFsmTest — client FSM state transitions and reconnection
  • UascServerReverseHelloHandlerTest — RHE send on channel active, Hello/Ack flow
  • OpcTcpReverseConnectServerTransportTest — server outbound connection behavior
  • ReverseConnectConnectionFsmTest — server FSM transitions, backoff, and idle socket invariant
  • OpcTcpReverseConnectTransportTest — client transport end-to-end (listen, accept, handshake)
  • ReverseConnectTest — SDK-level integration test (full client ↔ server reverse connect session)
  • ReverseConnectDiscoveryTest — discovery via reverse connect
  • mvn clean verify passes

@kevinherron
Add .mcp.json to .gitignore
5f80e80
@kevinherron
Add ReverseHello message type and transport layer support
a5221ea 
Implement OPC UA Reverse Connect at the stack and transport layers.
Add ReverseHelloMessage encoding/decoding, client-side reverse connect
transport with channel FSM, and server-side reverse connect transport
that initiates connections to clients behind firewalls.
@kevinherron
Add Reverse Connect support to SDK client and server APIs
0da25f2 
Extend OpcUaClient and DiscoveryClient with reverse connect factory
methods that accept a pre-established transport. Add ReverseConnectManager
and ReverseConnectHandle to OpcUaServer for managing outbound connections
to clients. Update SessionFsm to support reverse connect lifecycle.
@kevinherron
Add Reverse Connect tests and examples
fae23ff 
Add unit tests for ReverseHelloMessage encoding/decoding and
ReverseConnectChannelFsm state transitions. Add integration tests
for reverse connect client sessions and discovery. Include a Prosys
reverse connect client example.
@kevinherron
Add Reverse Connect architecture documentation
d3a1549
@kevinherron kevinherron requested a review from Copilot March 25, 2026 02:53
Copilot AI reviewed Mar 25, 2026
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Pull request overview

This PR adds OPC UA Reverse Connect support end-to-end (stack-core protocol message + transport implementations/FSMs + SDK server/client APIs), including documentation, examples, and a comprehensive set of unit/integration tests to validate the handshake, reconnection behavior, and discovery flows.

Changes:

  • Introduces ReverseHelloMessage (RHE) support in stack-core encoding/decoding and message type mapping.
  • Adds reverse-connect transports + FSMs for client (listening) and server (outbound connector/backoff + idle-socket invariant signaling).
  • Adds SDK integration (ReverseConnectManager, OpcUaServer APIs, OpcUaClient.createReverseConnect() and DiscoveryClient overloads) plus docs/examples/tests.

Reviewed changes

Copilot reviewed 37 out of 38 changed files in this pull request and generated 6 comments.

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File Description
opc-ua-stack/transport/src/test/java/org/eclipse/milo/opcua/stack/transport/server/uasc/UascServerReverseHelloHandlerTest.java Unit tests for server-side RHE handler behavior (RHE send, Hello/Ack, deadline).
opc-ua-stack/transport/src/test/java/org/eclipse/milo/opcua/stack/transport/server/tcp/ReverseConnectConnectionFsmTest.java Unit tests for server reverse-connect connection FSM (backoff, stop, transitions).
opc-ua-stack/transport/src/test/java/org/eclipse/milo/opcua/stack/transport/server/tcp/OpcTcpReverseConnectServerTransportTest.java Unit tests for outbound reverse-connect server transport connect success/failure.
opc-ua-stack/transport/src/test/java/org/eclipse/milo/opcua/stack/transport/client/uasc/UascClientReverseHelloHandlerTest.java Unit tests for client-side RHE handler (whitelist, timeouts, error handling).
opc-ua-stack/transport/src/test/java/org/eclipse/milo/opcua/stack/transport/client/tcp/ReverseConnectChannelFsmTest.java Unit tests for client reverse-connect channel FSM (connect/get/disconnect/reconnect).
opc-ua-stack/transport/src/main/java/org/eclipse/milo/opcua/stack/transport/server/uasc/UascServerReverseHelloHandler.java Server Netty handler that sends ReverseHello on channel activation.
opc-ua-stack/transport/src/main/java/org/eclipse/milo/opcua/stack/transport/server/uasc/UascServerHelloHandler.java Adjusts visibility to enable clean subclassing for reverse-connect handshake.
opc-ua-stack/transport/src/main/java/org/eclipse/milo/opcua/stack/transport/server/uasc/UascServerAsymmetricHandler.java Fires a user event after OpenSecureChannelResponse to signal SecureChannel opened.
opc-ua-stack/transport/src/main/java/org/eclipse/milo/opcua/stack/transport/server/uasc/SecureChannelOpenedEvent.java Defines the Netty user event used by reverse-connect server FSM.
opc-ua-stack/transport/src/main/java/org/eclipse/milo/opcua/stack/transport/server/tcp/ReverseConnectConnectionFsm.java Server reverse-connect connection lifecycle FSM (connect/backoff/active/stop).
opc-ua-stack/transport/src/main/java/org/eclipse/milo/opcua/stack/transport/server/tcp/ReverseConnectConfigBuilder.java Builder for server reverse-connect timing/backoff configuration.
opc-ua-stack/transport/src/main/java/org/eclipse/milo/opcua/stack/transport/server/tcp/ReverseConnectConfig.java Interface for server reverse-connect config values.
opc-ua-stack/transport/src/main/java/org/eclipse/milo/opcua/stack/transport/server/tcp/OpcTcpReverseConnectServerTransport.java Outbound Netty connector wiring server pipeline for reverse-connect sockets.
opc-ua-stack/transport/src/main/java/org/eclipse/milo/opcua/stack/transport/client/uasc/UascClientReverseHelloHandler.java Client reverse-connect handshake handler (RHE → Hello → Ack → UASC handler install).
opc-ua-stack/transport/src/main/java/org/eclipse/milo/opcua/stack/transport/client/tcp/ReverseConnectChannelFsm.java Client reverse-connect channel FSM (accepted connection → handshake → connected/reconnect).
opc-ua-stack/transport/src/main/java/org/eclipse/milo/opcua/stack/transport/client/tcp/OpcTcpReverseConnectTransportConfigBuilder.java Builder for client reverse-connect transport config (listen addr, whitelist, timeouts).
opc-ua-stack/transport/src/main/java/org/eclipse/milo/opcua/stack/transport/client/tcp/OpcTcpReverseConnectTransportConfig.java Client reverse-connect transport config interface.
opc-ua-stack/transport/src/main/java/org/eclipse/milo/opcua/stack/transport/client/tcp/OpcTcpReverseConnectTransport.java Client transport that listens and accepts inbound server reverse-connect sockets.
opc-ua-stack/transport/src/main/java/org/eclipse/milo/opcua/stack/transport/client/tcp/OpcTcpClientTransport.java Adds ChannelStateObservable support for forward-connect transport.
opc-ua-stack/transport/src/main/java/org/eclipse/milo/opcua/stack/transport/client/ChannelStateObservable.java Transport-agnostic connection state observer interface for session reconnection.
opc-ua-stack/stack-core/src/test/java/org/eclipse/milo/opcua/stack/core/channel/messages/ReverseHelloMessageTest.java Unit tests for ReverseHello message encode/decode and limits.
opc-ua-stack/stack-core/src/main/java/org/eclipse/milo/opcua/stack/core/channel/messages/TcpMessageEncoder.java Adds encoding support for ReverseHello.
opc-ua-stack/stack-core/src/main/java/org/eclipse/milo/opcua/stack/core/channel/messages/TcpMessageDecoder.java Adds decoding support for ReverseHello.
opc-ua-stack/stack-core/src/main/java/org/eclipse/milo/opcua/stack/core/channel/messages/ReverseHelloMessage.java New protocol message type for reverse-connect initiation.
opc-ua-stack/stack-core/src/main/java/org/eclipse/milo/opcua/stack/core/channel/messages/MessageType.java Adds ReverseHello message type mapping (“RHE”).
opc-ua-sdk/sdk-server/src/main/java/org/eclipse/milo/opcua/sdk/server/ReverseConnectManager.java SDK manager enforcing idle socket invariant and dynamic registration lifecycle.
opc-ua-sdk/sdk-server/src/main/java/org/eclipse/milo/opcua/sdk/server/ReverseConnectHandle.java Handle type for reverse-connect registrations.
opc-ua-sdk/sdk-server/src/main/java/org/eclipse/milo/opcua/sdk/server/OpcUaServer.java Server integration: configure/start/stop reverse-connect manager + add/remove APIs.
opc-ua-sdk/sdk-client/src/main/java/org/eclipse/milo/opcua/sdk/client/session/SessionFsmFactory.java Uses ChannelStateObservable for transport-agnostic connection loss detection.
opc-ua-sdk/sdk-client/src/main/java/org/eclipse/milo/opcua/sdk/client/session/SessionFsm.java Stores transport listener with new ChannelStateObservable key.
opc-ua-sdk/sdk-client/src/main/java/org/eclipse/milo/opcua/sdk/client/OpcUaClient.java Adds createReverseConnect() factory (two-pass discovery, keep listener open).
opc-ua-sdk/sdk-client/src/main/java/org/eclipse/milo/opcua/sdk/client/DiscoveryClient.java Adds reverse-connect overloads for getEndpoints() and findServers().
opc-ua-sdk/integration-tests/src/test/java/org/eclipse/milo/opcua/stack/transport/client/tcp/OpcTcpReverseConnectTransportTest.java Transport integration tests for reverse-connect client transport against real server.
opc-ua-sdk/integration-tests/src/test/java/org/eclipse/milo/opcua/sdk/client/ReverseConnectTest.java End-to-end SDK integration tests for reverse-connect sessions and reconnection.
opc-ua-sdk/integration-tests/src/test/java/org/eclipse/milo/opcua/sdk/client/ReverseConnectDiscoveryTest.java Integration tests for reverse-connect discovery helpers on a fixed listen port.
milo-examples/client-examples/src/main/java/org/eclipse/milo/examples/client/ReverseConnectExampleProsys.java Example showing reverse-connect client usage with Prosys Simulation Server.
docs/architecture/reverse-connect.md Architecture/design document for reverse-connect layering, FSMs, and config.
.gitignore Ignores .mcp.json.

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kevinherron and others added 7 commits March 24, 2026 20:12
@kevinherron
Update docs/architecture/reverse-connect.md
cb302dd 
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
@kevinherron
Update opc-ua-stack/stack-core/src/main/java/org/eclipse/milo/opcua/s…
a05753c 
…tack/core/channel/messages/ReverseHelloMessage.java

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
@kevinherron
Update opc-ua-stack/stack-core/src/main/java/org/eclipse/milo/opcua/s…
dfb1e0e 
…tack/core/channel/messages/ReverseHelloMessage.java

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
@kevinherron
Handle keep-alive channel close for reverse-connect transport
60ee5b5
@kevinherron
Fix thread safety and deadlock in reverse connect shutdown
179c15d 
Make ReverseConnectManager thread-safe with a ReentrantLock that
protects the connections and handlesByClientUrl maps, firing FSM
events outside the lock to prevent lock-ordering deadlocks.

Change OpcUaServer.shutdown() to stop the reverse connect manager
non-blockingly so shutdown can be called from I/O or FSM executor
threads without deadlocking.

In ReverseConnectChannelFsm, introduce a ConnectFuture wrapper to
eliminate raw-type warnings, execute pipeline operations on the
channel's event loop, and pass the pre-decoded ReverseHelloMessage
directly to the handler instead of re-encoding and replaying bytes.

In OpcUaClient, use the FSM Disconnect event instead of closing the
raw channel, and add an orTimeout to disconnectAsync's closeSession.
@kevinherron
Add reverse connect unit and integration tests
c54e3c9 
Add shutdownCompletesWithoutDeadlock integration test that verifies
OpcUaServer.shutdown() completes when called from a single-threaded
executor, catching the deadlock fixed in the previous commit.

Add ReverseConnectChannelFsm tests for Disconnect event transitions.

Add ReverseConnectManagerTest with unit tests for lifecycle, handle
management, idle connection spawning, and concurrent stop safety.
@kevinherron
Document Maven module targeting flags
7003848 
Add a section to testing.md explaining -pl, -am, and -amd flags
with examples and a rule of thumb for when to use each.
Copilot AI mentioned this pull request Mar 25, 2026
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Copilot AI commented Mar 25, 2026

@kevinherron I've opened a new pull request, #1717, to work on those changes. Once the pull request is ready, I'll request review from you.

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Copilot AI commented Mar 25, 2026

@kevinherron I've opened a new pull request, #1718, to work on those changes. Once the pull request is ready, I'll request review from you.

kevinherron and others added 7 commits March 25, 2026 06:13
@kevinherron
Fix inaccuracies in reverse connect architecture documentation
7be168d 
Corrects 6 discrepancies found by cross-referencing the doc against the
current implementation: RHE dispatch mechanism (direct call, not pipeline
re-encode), shutdown order, connect() signature, KEY_CF type, missing
test entries, and missing test classes.
@kevinherron
Fire SecureChannelOpenedEvent only after successful write of OpenSecu…
3dda5de 
…reChannelResponse (#1717)

* Initial plan

* Fire SecureChannelOpenedEvent from writeAndFlush success listener

Co-authored-by: kevinherron <340273+kevinherron@users.noreply.github.com>
Agent-Logs-Url: https://github.com/eclipse-milo/milo/sessions/4b9455b2-5b0d-443e-a0b7-4cb32f655663

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: kevinherron <340273+kevinherron@users.noreply.github.com>
@kevinherron
Replace assert with runtime checks in TcpMessageDecoder (#1718)
778977d 
* Initial plan

* Replace assert with runtime UaException checks in TcpMessageDecoder

Co-authored-by: kevinherron <340273+kevinherron@users.noreply.github.com>
Agent-Logs-Url: https://github.com/eclipse-milo/milo/sessions/6f91137a-a9a1-424e-a62a-ebea7b8ddf6d

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: kevinherron <340273+kevinherron@users.noreply.github.com>
@kevinherron
Convert ReverseHelloMessage to a Java record
669d136
@kevinherron
Fix bind failure handling and disconnect pending future in reverse co…
940e56d 
…nnect transport

- Fail pending ConnectFuture on Disconnect in NotConnected state in
  ReverseConnectChannelFsm, preventing orphaned futures when
  disconnectAsync() is called before a server connects.

- Reset serverBootstrap/serverChannel on bind failure in
  OpcTcpReverseConnectTransport.startListening() so subsequent
  connect() calls can retry instead of being permanently stuck.

- Return a failed CompletableFuture from connect() on bind failure
  instead of throwing synchronously, ensuring downstream future
  chains (disconnect cleanup, timeouts) are properly constructed.
@kevinherron
Add connect timeout for reverse connect discovery and client flows
b178cb2 
- Add configurable connectTimeout (default 60s) to
  OpcTcpReverseConnectTransportConfig for bounding one-shot
  discovery flows without affecting long-lived connections.

- Apply .orTimeout() to connectAsync() in DiscoveryClient.getEndpoints,
  DiscoveryClient.findServers, and OpcUaClient.createReverseConnect
  so callers don't hang indefinitely waiting for a server to connect.

- Chain disconnectAsync() to future completion in DiscoveryClient
  using handle/thenCompose instead of fire-and-forget whenComplete,
  ensuring the listening socket is fully closed before the result
  future completes.
@kevinherron
Fix thread safety and restart robustness in ReverseConnectManager
e43b5dc 
- Move the !running check inside synchronized(pendingRegistrations) in
  addReverseConnect to close a TOCTOU race with start() that could
  orphan registrations.

- Clean pendingRegistrations in removeReverseConnect so handles removed
  before start() don't get materialized into FSMs.

- Rebuild stale FSMs on restart: snapshot handles from the prior
  start/stop cycle and replace their terminal-state FSMs with fresh
  ones, restoring connectivity without requiring re-registration.
@kevinherron kevinherron requested a review from Copilot March 25, 2026 20:07
Copilot AI reviewed Mar 25, 2026
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Pull request overview

Copilot reviewed 42 out of 43 changed files in this pull request and generated 6 comments.

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kevinherron and others added 7 commits March 25, 2026 13:17
@kevinherron
Update opc-ua-sdk/sdk-client/src/main/java/org/eclipse/milo/opcua/sdk…
6da156d 
…/client/OpcUaClient.java

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
@kevinherron
Update opc-ua-stack/transport/src/main/java/org/eclipse/milo/opcua/st…
561679e 
…ack/transport/client/uasc/UascClientReverseHelloHandler.java

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
@kevinherron
Update opc-ua-sdk/sdk-server/src/main/java/org/eclipse/milo/opcua/sdk…
8191f72 
…/server/ReverseConnectManager.java

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
@kevinherron
Update test to match per-handle removal behavior in removeReverseConnect
93e4523
@kevinherron
Address PR review feedback for reverse connect cleanup and logging
5911667 
Clear KEY_RETRY_DELAY_MS in Disconnected state cleanup to avoid stale
backoff state. Log serverChannel.localAddress() instead of config
listen address so the actual bound port is visible when using ephemeral
port 0.
@kevinherron
Reject duplicate inbound connections during reverse connect handshake
58dff21 
Add an internal transition in the Handshaking state that closes channels
from duplicate ConnectionAccepted events. Without this, a second inbound
connection during handshake would be silently dropped by the FSM, leaving
the child channel orphaned (never closed).
@kevinherron
Add multiplexed reverse connect transport for shared-port multi-serve…
9c41b0a 
…r support

Introduce a new transport model that allows multiple OPC UA servers to
reverse-connect through a single client listening port, complementing the
existing 1-1 transport where each server requires its own port.

Stack transport layer:
- OpcTcpMultiplexedReverseConnectTransport: per-server transport that
  receives channels from a shared listener instead of owning a socket
- ChannelConsumerRegistry: dispatches inbound channels by ServerUri
- InboundChannelTransport: abstraction for inbound channel delivery
- EndpointResolver: on-demand endpoint discovery (1-shot and 2-shot)
- Refactor ReverseConnectChannelFsm to accept a ChannelFsmConfig record,
  decoupling it from any specific transport type

SDK client layer:
- MultiplexedReverseConnectListener: owns the ServerBootstrap, decodes
  ReverseHello, and dispatches to per-server transports
- ClientCustomizer/ClientListener: hooks for on-demand client creation
- Config and builder classes for both listener and transport

Tests and docs:
- Unit tests for EndpointResolver, InboundChannelTransport, transport,
  and listener
- Integration test for end-to-end multiplexed reverse connect
- Example client for Prosys OPC UA Simulation Server
- Update architecture doc with multiplexed transport model
@kevinherron kevinherron requested a review from Copilot March 28, 2026 00:21
Copilot AI reviewed Mar 28, 2026
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Pull request overview

Copilot reviewed 59 out of 60 changed files in this pull request and generated 4 comments.

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...src/main/java/org/eclipse/milo/opcua/stack/transport/client/tcp/InboundChannelTransport.java
Comment on lines +81 to +89
var handler =
new UascClientReverseHelloHandler(
transportConfig,
applicationContext,
requestId::getAndIncrement,
hf,
Set.of(), // allowedServerUris: empty = accept all (already validated)
0L); // reverseHelloTimeoutMs: 0 = no timeout (already received)

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Passing reverseHelloTimeoutMs = 0L here relies on UascClientReverseHelloHandler interpreting 0 as "no timeout". Currently the handler schedules a ReverseHello timeout in channelActive()/handlerAdded(), so a 0ms timeout can race and fail the handshake before onReverseHello() cancels it. Either pass a positive timeout and/or update the handler to explicitly disable the ReverseHello timer when the ReverseHello is pre-decoded.

Copilot uses AI. Check for mistakes.
@kevinherron
Fix flaky rateLimitingHandlerInstalled test
3dd49a5 
AbstractRemoteAddressFilter removes itself from the pipeline during
fireChannelRegistered(), racing with the test thread's assertion.
Check for the handler inside the customizer callback instead, where
initChannel() guarantees it is still present.
@kevinherron
Fix reverse connect timeout race and add multiplexed transport to kee…
bf6e61e 
…p-alive close

Skip scheduling the ReverseHello timeout when reverseHelloTimeoutMs <= 0
to prevent a racy 0ms wheel-timer task on channels where the ReverseHello
was already pre-decoded. Add OpcTcpMultiplexedReverseConnectTransport to
closeTransportChannel so keep-alive failure can force-close its channel.
@kevinherron
Fix ReDoS vulnerability in endpoint URL regex
9339084 
Replace \[.*] with \[[^]]*] in ENDPOINT_URL_PATTERN to prevent
polynomial backtracking on crafted inputs. The greedy .* could
match ']' characters, creating ambiguity exploitable for denial
of service (CodeQL java/polynomial-redos).
@kevinherron
Update reverse connect architecture doc
2da99db 
Add coverage for the ReverseHello timeout race guard (AtomicBoolean
dual-path start), keep-alive channel close handling across all three
transport types, and the single-argument createTransport overload.
@kevinherron
Fix flaky reconnectionAfterChannelDrop secure channel race
745151e 
The Handshaking entry action set KEY_CHANNEL to the raw TCP channel
before the OPN handshake completed, allowing getChannel()'s fast path
to return a partially initialized channel with no UASC message handler.
During reconnection this caused ActivateSession write failures and
retry storms that widened the window for the server-side session to be
bound to a stale secure channel id, producing Bad_SecureChannelIdInvalid
on the subsequent Read.

Only set KEY_CHANNEL in the Connected entry action, after the handshake
succeeds and the pipeline is fully initialized. During Handshaking,
getChannel() now correctly falls through to the GetChannel event path
which chains to KEY_CF and resolves when Connected is reached.

Also replace the fragile random-port-with-retry allocation in
TestServer.create() with ephemeral port 0 on the loopback address.
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Add Reverse Connect support

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