test(modbus): implement MockRelayController with failing tests

Ref: T028 (specs/001-modbus-relay-control)

backend/src/infrastructure/mod.rs

@@ -75,4 +75,5 @@
 //! - Implementation: `specs/001-modbus-relay-control/plan.md` - Infrastructure tasks
 //! - Modbus docs: `docs/Modbus_POE_ETH_Relay.md` - Hardware protocol specification
 
+pub mod modbus;
 pub mod persistence;

backend/src/infrastructure/modbus/mock_controller.rs

@@ -0,0 +1,174 @@
+//! Mock relay controller for testing without hardware.
+//!
+//! This module provides a mock implementation of the relay controller
+//! that stores state in memory, enabling testing without physical Modbus hardware.
+
+#[cfg(test)]
+mod tests {
+    use crate::domain::relay::types::RelayId;
+
+    // NOTE: These tests will fail until MockRelayController and RelayController trait are implemented (T029, T030)
+    // This follows TDD - write failing tests FIRST, then implement.
+
+    #[tokio::test]
+    async fn test_read_state_returns_mocked_state() {
+        // Test: read_state() returns mocked state
+        //
+        // Setup: Create a mock controller and set relay 1 to On
+        // Expected: read_state(1) should return On
+
+        todo!("Implement after MockRelayController exists (T029)");
+
+        // let controller = MockRelayController::new();
+        // let relay_id = RelayId::new(1).unwrap();
+        //
+        // // Write a known state
+        // controller.write_state(relay_id, RelayState::On).await.unwrap();
+        //
+        // // Read it back
+        // let state = controller.read_state(relay_id).await.unwrap();
+        // assert_eq!(state, RelayState::On);
+    }
+
+    #[tokio::test]
+    async fn test_write_state_updates_mocked_state() {
+        // Test: write_state() updates mocked state
+        //
+        // Setup: Create a mock controller (all relays default to Off)
+        // Action: Write relay 3 to On, then read it back
+        // Expected: State should be On
+
+        todo!("Implement after MockRelayController exists (T029)");
+
+        // let controller = MockRelayController::new();
+        // let relay_id = RelayId::new(3).unwrap();
+        //
+        // // Initial state should be Off
+        // let initial_state = controller.read_state(relay_id).await.unwrap();
+        // assert_eq!(initial_state, RelayState::Off);
+        //
+        // // Write On
+        // controller.write_state(relay_id, RelayState::On).await.unwrap();
+        //
+        // // Verify it changed
+        // let updated_state = controller.read_state(relay_id).await.unwrap();
+        // assert_eq!(updated_state, RelayState::On);
+    }
+
+    #[tokio::test]
+    async fn test_read_all_returns_8_relays_in_known_state() {
+        // Test: read_all() returns 8 relays in known state
+        //
+        // Setup: Create a mock controller, set relays 1, 3, 5 to On, others Off
+        // Action: Call read_all()
+        // Expected: Returns Vec of 8 (RelayId, RelayState) tuples in correct state
+
+        todo!("Implement after MockRelayController exists (T029)");
+
+        // let controller = MockRelayController::new();
+        //
+        // // Set specific relays to On
+        // controller.write_state(RelayId::new(1).unwrap(), RelayState::On).await.unwrap();
+        // controller.write_state(RelayId::new(3).unwrap(), RelayState::On).await.unwrap();
+        // controller.write_state(RelayId::new(5).unwrap(), RelayState::On).await.unwrap();
+        //
+        // // Read all states
+        // let all_states = controller.read_all().await.unwrap();
+        //
+        // // Verify we have exactly 8 relays
+        // assert_eq!(all_states.len(), 8);
+        //
+        // // Verify specific states
+        // assert_eq!(all_states[0], (RelayId::new(1).unwrap(), RelayState::On));
+        // assert_eq!(all_states[1], (RelayId::new(2).unwrap(), RelayState::Off));
+        // assert_eq!(all_states[2], (RelayId::new(3).unwrap(), RelayState::On));
+        // assert_eq!(all_states[3], (RelayId::new(4).unwrap(), RelayState::Off));
+        // assert_eq!(all_states[4], (RelayId::new(5).unwrap(), RelayState::On));
+        // assert_eq!(all_states[5], (RelayId::new(6).unwrap(), RelayState::Off));
+        // assert_eq!(all_states[6], (RelayId::new(7).unwrap(), RelayState::Off));
+        // assert_eq!(all_states[7], (RelayId::new(8).unwrap(), RelayState::Off));
+    }
+
+    #[tokio::test]
+    async fn test_write_state_for_all_8_relays() {
+        // Test: Can write state to all 8 relays independently
+        //
+        // Setup: Create a mock controller
+        // Action: Write different states to each relay
+        // Expected: Each relay maintains its own independent state
+
+        todo!("Implement after MockRelayController exists (T029)");
+
+        // let controller = MockRelayController::new();
+        //
+        // // Write alternating states (On, Off, On, Off, ...)
+        // for i in 1..=8 {
+        //     let relay_id = RelayId::new(i).unwrap();
+        //     let state = if i % 2 == 1 { RelayState::On } else { RelayState::Off };
+        //     controller.write_state(relay_id, state).await.unwrap();
+        // }
+        //
+        // // Verify each relay has correct state
+        // for i in 1..=8 {
+        //     let relay_id = RelayId::new(i).unwrap();
+        //     let expected_state = if i % 2 == 1 { RelayState::On } else { RelayState::Off };
+        //     let actual_state = controller.read_state(relay_id).await.unwrap();
+        //     assert_eq!(actual_state, expected_state, "Relay {} has incorrect state", i);
+        // }
+    }
+
+    #[tokio::test]
+    async fn test_read_state_with_invalid_relay_id() {
+        // Test: read_state() with out-of-range relay ID fails gracefully
+        //
+        // Note: RelayId::new() will already fail for invalid IDs (0 or 9+),
+        // so this test verifies the type system prevents invalid relay IDs
+        // at construction time (type-driven design)
+
+        // Verify RelayId construction fails for invalid IDs
+        assert!(RelayId::new(0).is_err(), "RelayId::new(0) should fail");
+        assert!(RelayId::new(9).is_err(), "RelayId::new(9) should fail");
+
+        // If we somehow get an invalid relay ID through (which shouldn't be possible),
+        // the controller should handle it gracefully (tested in T029 implementation)
+    }
+
+    #[tokio::test]
+    async fn test_concurrent_access_is_safe() {
+        // Test: MockRelayController is thread-safe (uses Arc<Mutex<HashMap>>)
+        //
+        // Setup: Create mock controller, spawn multiple tasks that read/write
+        // Expected: No data races, all operations complete successfully
+
+        todo!("Implement after MockRelayController exists (T029)");
+
+        // use std::sync::Arc;
+        //
+        // let controller = Arc::new(MockRelayController::new());
+        //
+        // // Spawn 10 tasks that toggle relay 1
+        // let mut handles = vec![];
+        // for _ in 0..10 {
+        //     let controller_clone = Arc::clone(&controller);
+        //     let handle = tokio::spawn(async move {
+        //         let relay_id = RelayId::new(1).unwrap();
+        //         let current_state = controller_clone.read_state(relay_id).await.unwrap();
+        //         let new_state = match current_state {
+        //             RelayState::On => RelayState::Off,
+        //             RelayState::Off => RelayState::On,
+        //         };
+        //         controller_clone.write_state(relay_id, new_state).await.unwrap();
+        //     });
+        //     handles.push(handle);
+        // }
+        //
+        // // Wait for all tasks to complete
+        // for handle in handles {
+        //     handle.await.unwrap();
+        // }
+        //
+        // // Controller should still be in valid state (either On or Off)
+        // let final_state = controller.read_state(RelayId::new(1).unwrap()).await.unwrap();
+        // assert!(matches!(final_state, RelayState::On | RelayState::Off));
+    }
+}

backend/src/infrastructure/modbus/mod.rs

@@ -0,0 +1,6 @@
+//! Modbus infrastructure module.
+//!
+//! This module contains implementations for communicating with Modbus relay hardware,
+//! including both real hardware controllers and mock implementations for testing.
+
+pub mod mock_controller;

specs/001-modbus-relay-control/tasks.md

@@ -286,7 +286,7 @@
 
 **Purpose**: Implement Modbus client, mocks, and persistence
 
-- [ ] **T028** [P] [US1] [TDD] Write tests for MockRelayController
+- [x] **T028** [P] [US1] [TDD] Write tests for MockRelayController
   - Test: read_state() returns mocked state
   - Test: write_state() updates mocked state
   - Test: read_all() returns 8 relays in known state