test(transport): add edge case tests for TransportManager

[groupsky]

Current Coverage

TransportManager has comprehensive basic test coverage (13 tests), but is missing some edge case scenarios that could help catch race conditions and error handling issues.

Current tests: packages/transport/src/manager.test.ts

• ✅ Transport pooling by configuration
• ✅ Mutex serialization for RTU operations
• ✅ Separate mutexes for different connections
• ✅ Basic error propagation
• ✅ Lifecycle management

Missing Edge Cases

1. Concurrent Operations on Shared RTU Transport

What to test: Verify that multiple concurrent operations on the same RTU bus are actually serialized by the mutex, not just pooled.

Test scenario:

test('should serialize concurrent read operations on shared RTU transport', async () => {
  const config: RTUConfig = { /* same config */ }
  const transport1 = await manager.getTransport(config)
  const transport2 = await manager.getTransport(config)
  
  // Start multiple operations concurrently
  const results = await Promise.all([
    transport1.readHoldingRegisters(0, 10),
    transport2.readHoldingRegisters(10, 10),
    transport1.readInputRegisters(20, 10),
  ])
  
  // Verify operations executed in sequence (mock can track call order)
  expect(executionOrder).toEqual(['op1-start', 'op1-end', 'op2-start', ...])
})

2. Transport Failure During Operation

What to test: What happens when the underlying transport throws an error mid-operation?

Test scenario:

test('should propagate transport errors through mutex wrapper', async () => {
  mockTransport.readHoldingRegisters.mockRejectedValue(new Error('Connection lost'))
  
  const transport = await manager.getTransport(config)
  await expect(transport.readHoldingRegisters(0, 10)).rejects.toThrow('Connection lost')
  
  // Verify mutex was released and subsequent operations can proceed
  mockTransport.readHoldingRegisters.mockResolvedValue(Buffer.alloc(20))
  await expect(transport.readHoldingRegisters(0, 10)).resolves.toBeDefined()
})

3. Close During Active Operation

What to test: Race condition when closeAll() is called while operations are in progress.

Test scenario:

test('should handle closeAll during active operations', async () => {
  const transport = await manager.getTransport(config)
  
  // Start long-running operation
  const slowOperation = transport.readHoldingRegisters(0, 100)
  
  // Close while operation is running
  const closePromise = manager.closeAll()
  
  // Both should complete without deadlock
  await Promise.race([
    slowOperation.catch(() => {}), // May fail or succeed
    closePromise
  ])
})

4. Multiple Devices Adding/Removing Concurrently

What to test: Concurrent getTransport() and closeAll() calls don't corrupt internal state.

Test scenario:

test('should handle concurrent getTransport calls safely', async () => {
  const promises = Array.from({ length: 10 }, (_, i) =>
    manager.getTransport({
      port: '/dev/ttyUSB0',
      baudRate: 9600,
      /* ... */
      slaveId: i // Different slave IDs, same bus
    })
  )
  
  const transports = await Promise.all(promises)
  
  // All should get the same pooled transport
  expect(new Set(transports).size).toBe(1)
  expect(manager.getStats().rtuTransports).toBe(1)
})

Benefits

1. Confidence in mutex behavior: Explicit tests prove operations are truly serialized
2. Error recovery: Ensures system recovers gracefully from transport failures
3. Race condition detection: Catches potential deadlocks or state corruption
4. Documentation: Tests serve as examples of expected behavior under stress

Priority

Low-Medium - Current tests are good, these are "nice to have" for increased confidence

Related

• Identified in PR feat(transport): implement TransportManager with RTU mutex to prevent bus collisions #125 review comments
• Complements existing tests in packages/transport/src/manager.test.ts
• Related to feat(transport): implement reference counting for automatic transport cleanup #126 (reference counting) - these tests will be valuable when implementing cleanup

---

From Claude Code Review of PR #125

[groupsky]

✅ VERIFIED FIXED: packages/transport/src/manager.test.ts (607 lines) contains comprehensive edge case tests including: concurrent operations on different TCP connections (line 287), concurrent operations on same TCP transport (line 367), concurrent operations on different slaves (line 429), error handling during closeAll (lines 562-581), and client cleanup edge cases (lines 583-605).