test(infrastructure): write RelayLabelRepository trait tests

Add reusable test suite with 18 test functions covering get_label(),
save_label(), delete_label(), and get_all_labels() methods. Tests
verify contract compliance for any repository implementation.

Added delete_label() method to trait interface and implemented it in
MockRelayLabelRepository to support complete CRUD operations.

TDD phase: RED - Tests written before SQLite implementation (T036)

Ref: T035 (specs/001-modbus-relay-control/tasks.md)

backend/src/domain/relay/repository/label.rs

@@ -29,6 +29,15 @@ pub trait RelayLabelRepository: Send + Sync {
     /// Returns `RepositoryError::DatabaseError` if the database operation fails.
     async fn save_label(&self, id: RelayId, label: RelayLabel) -> Result<(), RepositoryError>;
 
+    /// Deletes the label for a specific relay.
+    ///
+    /// If no label exists for the relay, this operation succeeds without error.
+    ///
+    /// # Errors
+    ///
+    /// Returns `RepositoryError::DatabaseError` if the database operation fails.
+    async fn delete_label(&self, id: RelayId) -> Result<(), RepositoryError>;
+
     /// Retrieves all relay labels from the repository.
     ///
     /// Returns a vector of tuples containing relay IDs and their corresponding labels.

backend/src/infrastructure/persistence/label_repository.rs

@@ -57,6 +57,11 @@ impl RelayLabelRepository for MockRelayLabelRepository {
         Ok(())
     }
 
+    async fn delete_label(&self, id: RelayId) -> Result<(), RepositoryError> {
+        self.labels().await.remove(&id.as_u8());
+        Ok(())
+    }
+
     async fn get_all_labels(&self) -> Result<Vec<(RelayId, RelayLabel)>, RepositoryError> {
         let mut result: Vec<(RelayId, RelayLabel)> = self
             .labels()

backend/src/infrastructure/persistence/label_repository_tests.rs

@@ -0,0 +1,449 @@
+//! Comprehensive tests for `RelayLabelRepository` trait contract.
+//!
+//! This module provides a reusable test suite that verifies any implementation
+//! of the `RelayLabelRepository` trait meets the expected contract. These tests
+//! can be run against different implementations (mock, SQLite, PostgreSQL, etc.)
+//! to ensure they all behave correctly.
+//!
+//! **T035**: Write tests for RelayLabelRepository trait
+//! - Test: `get_label(RelayId(1)) → Option<RelayLabel>`
+//! - Test: `save_label(RelayId(1), label) → Result<(), RepositoryError>`
+//! - Test: `delete_label(RelayId(1)) → Result<(), RepositoryError>`
+
+#[cfg(test)]
+mod relay_label_repository_contract_tests {
+    use crate::domain::relay::{
+        repository::RelayLabelRepository,
+        types::{RelayId, RelayLabel},
+    };
+
+    // =========================================================================
+    // get_label() Tests
+    // =========================================================================
+
+    /// Test: `get_label` returns None for non-existent relay
+    ///
+    /// Verifies that querying a relay ID that has no label returns None
+    /// rather than an error.
+    pub async fn test_get_label_returns_none_for_non_existent_relay<R: RelayLabelRepository>(
+        repo: &R,
+    ) {
+        let relay_id = RelayId::new(1).expect("Valid relay ID");
+
+        let result = repo.get_label(relay_id).await;
+
+        assert!(result.is_ok(), "get_label should succeed");
+        assert!(
+            result.unwrap().is_none(),
+            "get_label should return None for non-existent relay"
+        );
+    }
+
+    /// Test: `get_label` retrieves previously saved label
+    ///
+    /// Verifies that after saving a label, `get_label` returns the same label.
+    pub async fn test_get_label_retrieves_saved_label<R: RelayLabelRepository>(repo: &R) {
+        let relay_id = RelayId::new(2).expect("Valid relay ID");
+        let label = RelayLabel::new("Heater".to_string()).expect("Valid label");
+
+        // Save the label
+        repo.save_label(relay_id, label.clone())
+            .await
+            .expect("save_label should succeed");
+
+        // Retrieve the label
+        let result = repo.get_label(relay_id).await;
+
+        assert!(result.is_ok(), "get_label should succeed");
+        let retrieved = result.unwrap();
+        assert!(retrieved.is_some(), "get_label should return Some");
+        assert_eq!(
+            retrieved.unwrap().as_str(),
+            "Heater",
+            "Retrieved label should match saved label"
+        );
+    }
+
+    /// Test: `get_label` returns None after label is deleted
+    ///
+    /// Verifies that after deleting a label, `get_label` returns None.
+    pub async fn test_get_label_returns_none_after_delete<R: RelayLabelRepository>(repo: &R) {
+        let relay_id = RelayId::new(3).expect("Valid relay ID");
+        let label = RelayLabel::new("ToBeDeleted".to_string()).expect("Valid label");
+
+        // Save and then delete the label
+        repo.save_label(relay_id, label)
+            .await
+            .expect("save_label should succeed");
+        repo.delete_label(relay_id)
+            .await
+            .expect("delete_label should succeed");
+
+        // Verify it's gone
+        let result = repo.get_label(relay_id).await;
+        assert!(result.is_ok(), "get_label should succeed");
+        assert!(
+            result.unwrap().is_none(),
+            "get_label should return None after delete"
+        );
+    }
+
+    // =========================================================================
+    // save_label() Tests
+    // =========================================================================
+
+    /// Test: `save_label` successfully saves a label
+    ///
+    /// Verifies that `save_label` returns Ok and stores the label.
+    pub async fn test_save_label_succeeds<R: RelayLabelRepository>(repo: &R) {
+        let relay_id = RelayId::new(1).expect("Valid relay ID");
+        let label = RelayLabel::new("Pump".to_string()).expect("Valid label");
+
+        let result = repo.save_label(relay_id, label).await;
+
+        assert!(result.is_ok(), "save_label should succeed");
+    }
+
+    /// Test: `save_label` overwrites existing label
+    ///
+    /// Verifies that calling `save_label` multiple times for the same relay ID
+    /// replaces the old label with the new one.
+    pub async fn test_save_label_overwrites_existing_label<R: RelayLabelRepository>(repo: &R) {
+        let relay_id = RelayId::new(4).expect("Valid relay ID");
+        let label1 = RelayLabel::new("First".to_string()).expect("Valid label");
+        let label2 = RelayLabel::new("Second".to_string()).expect("Valid label");
+
+        // Save first label
+        repo.save_label(relay_id, label1)
+            .await
+            .expect("First save should succeed");
+
+        // Overwrite with second label
+        repo.save_label(relay_id, label2)
+            .await
+            .expect("Second save should succeed");
+
+        // Verify only the second label is present
+        let result = repo.get_label(relay_id).await.expect("get_label should succeed");
+        assert!(result.is_some(), "Label should exist");
+        assert_eq!(
+            result.unwrap().as_str(),
+            "Second",
+            "Label should be updated to second value"
+        );
+    }
+
+    /// Test: `save_label` works for all valid relay IDs (1-8)
+    ///
+    /// Verifies that all relay IDs in the valid range can have labels saved.
+    pub async fn test_save_label_for_all_valid_relay_ids<R: RelayLabelRepository>(repo: &R) {
+        for id in 1..=8 {
+            let relay_id = RelayId::new(id).expect("Valid relay ID");
+            let label = RelayLabel::new(format!("Relay {id}")).expect("Valid label");
+
+            let result = repo.save_label(relay_id, label).await;
+            assert!(
+                result.is_ok(),
+                "save_label should succeed for relay ID {id}"
+            );
+        }
+
+        // Verify all labels were saved
+        let all_labels = repo
+            .get_all_labels()
+            .await
+            .expect("get_all_labels should succeed");
+        assert_eq!(all_labels.len(), 8, "Should have all 8 relay labels");
+    }
+
+    /// Test: `save_label` accepts maximum length labels
+    ///
+    /// Verifies that labels at the maximum allowed length (50 characters)
+    /// can be saved successfully.
+    pub async fn test_save_label_accepts_max_length_labels<R: RelayLabelRepository>(repo: &R) {
+        let relay_id = RelayId::new(5).expect("Valid relay ID");
+        let max_label = RelayLabel::new("A".repeat(50)).expect("Valid max-length label");
+
+        let result = repo.save_label(relay_id, max_label).await;
+        assert!(
+            result.is_ok(),
+            "save_label should succeed with max-length label"
+        );
+
+        // Verify it was saved correctly
+        let retrieved = repo
+            .get_label(relay_id)
+            .await
+            .expect("get_label should succeed");
+        assert!(retrieved.is_some(), "Label should be saved");
+        assert_eq!(
+            retrieved.unwrap().as_str().len(),
+            50,
+            "Label should have correct length"
+        );
+    }
+
+    /// Test: `save_label` accepts minimum length labels
+    ///
+    /// Verifies that labels at the minimum allowed length (1 character)
+    /// can be saved successfully.
+    pub async fn test_save_label_accepts_min_length_labels<R: RelayLabelRepository>(repo: &R) {
+        let relay_id = RelayId::new(6).expect("Valid relay ID");
+        let min_label = RelayLabel::new("X".to_string()).expect("Valid min-length label");
+
+        let result = repo.save_label(relay_id, min_label).await;
+        assert!(
+            result.is_ok(),
+            "save_label should succeed with min-length label"
+        );
+
+        // Verify it was saved correctly
+        let retrieved = repo
+            .get_label(relay_id)
+            .await
+            .expect("get_label should succeed");
+        assert!(retrieved.is_some(), "Label should be saved");
+        assert_eq!(retrieved.unwrap().as_str(), "X", "Label should match");
+    }
+
+    // =========================================================================
+    // delete_label() Tests
+    // =========================================================================
+
+    /// Test: `delete_label` succeeds for existing label
+    ///
+    /// Verifies that `delete_label` returns Ok when deleting an existing label.
+    pub async fn test_delete_label_succeeds_for_existing_label<R: RelayLabelRepository>(repo: &R) {
+        let relay_id = RelayId::new(7).expect("Valid relay ID");
+        let label = RelayLabel::new("ToDelete".to_string()).expect("Valid label");
+
+        // Save the label first
+        repo.save_label(relay_id, label)
+            .await
+            .expect("save_label should succeed");
+
+        // Delete it
+        let result = repo.delete_label(relay_id).await;
+        assert!(result.is_ok(), "delete_label should succeed");
+    }
+
+    /// Test: `delete_label` succeeds for non-existent label
+    ///
+    /// Verifies that `delete_label` returns Ok even when no label exists
+    /// (idempotent operation).
+    pub async fn test_delete_label_succeeds_for_non_existent_label<R: RelayLabelRepository>(
+        repo: &R,
+    ) {
+        let relay_id = RelayId::new(8).expect("Valid relay ID");
+
+        // Delete without saving first
+        let result = repo.delete_label(relay_id).await;
+        assert!(
+            result.is_ok(),
+            "delete_label should succeed even if label doesn't exist"
+        );
+    }
+
+    /// Test: `delete_label` removes label from repository
+    ///
+    /// Verifies that after deleting a label, it no longer appears in `get_label`
+    /// or `get_all_labels` results.
+    pub async fn test_delete_label_removes_label_from_repository<R: RelayLabelRepository>(
+        repo: &R,
+    ) {
+        let relay1 = RelayId::new(1).expect("Valid relay ID");
+        let relay2 = RelayId::new(2).expect("Valid relay ID");
+        let label1 = RelayLabel::new("Keep".to_string()).expect("Valid label");
+        let label2 = RelayLabel::new("Remove".to_string()).expect("Valid label");
+
+        // Save two labels
+        repo.save_label(relay1, label1)
+            .await
+            .expect("save should succeed");
+        repo.save_label(relay2, label2)
+            .await
+            .expect("save should succeed");
+
+        // Delete one label
+        repo.delete_label(relay2)
+            .await
+            .expect("delete should succeed");
+
+        // Verify deleted label is gone
+        let get_result = repo.get_label(relay2).await.expect("get_label should succeed");
+        assert!(get_result.is_none(), "Deleted label should not exist");
+
+        // Verify other label still exists
+        let other_result = repo.get_label(relay1).await.expect("get_label should succeed");
+        assert!(other_result.is_some(), "Other label should still exist");
+
+        // Verify get_all_labels only returns the remaining label
+        let all_labels = repo
+            .get_all_labels()
+            .await
+            .expect("get_all_labels should succeed");
+        assert_eq!(all_labels.len(), 1, "Should only have one label remaining");
+        assert_eq!(all_labels[0].0.as_u8(), 1, "Should be relay 1");
+    }
+
+    /// Test: `delete_label` is idempotent
+    ///
+    /// Verifies that calling `delete_label` multiple times succeeds without error.
+    pub async fn test_delete_label_is_idempotent<R: RelayLabelRepository>(repo: &R) {
+        let relay_id = RelayId::new(3).expect("Valid relay ID");
+        let label = RelayLabel::new("Idempotent".to_string()).expect("Valid label");
+
+        // Save, then delete twice
+        repo.save_label(relay_id, label)
+            .await
+            .expect("save should succeed");
+        repo.delete_label(relay_id)
+            .await
+            .expect("First delete should succeed");
+        let second_delete = repo.delete_label(relay_id).await;
+
+        assert!(
+            second_delete.is_ok(),
+            "Second delete should succeed (idempotent)"
+        );
+    }
+
+    // =========================================================================
+    // get_all_labels() Tests
+    // =========================================================================
+
+    /// Test: `get_all_labels` returns empty vector when no labels exist
+    ///
+    /// Verifies that `get_all_labels` returns an empty vector rather than
+    /// an error when the repository is empty.
+    pub async fn test_get_all_labels_returns_empty_when_no_labels<R: RelayLabelRepository>(
+        repo: &R,
+    ) {
+        let result = repo.get_all_labels().await;
+
+        assert!(result.is_ok(), "get_all_labels should succeed");
+        assert!(
+            result.unwrap().is_empty(),
+            "get_all_labels should return empty vector"
+        );
+    }
+
+    /// Test: `get_all_labels` returns all saved labels
+    ///
+    /// Verifies that `get_all_labels` returns all labels that have been saved,
+    /// and only those relays with labels.
+    pub async fn test_get_all_labels_returns_all_saved_labels<R: RelayLabelRepository>(repo: &R) {
+        let relay1 = RelayId::new(1).expect("Valid relay ID");
+        let relay3 = RelayId::new(3).expect("Valid relay ID");
+        let relay5 = RelayId::new(5).expect("Valid relay ID");
+
+        let label1 = RelayLabel::new("Pump".to_string()).expect("Valid label");
+        let label3 = RelayLabel::new("Heater".to_string()).expect("Valid label");
+        let label5 = RelayLabel::new("Fan".to_string()).expect("Valid label");
+
+        // Save labels
+        repo.save_label(relay1, label1.clone())
+            .await
+            .expect("Save should succeed");
+        repo.save_label(relay3, label3.clone())
+            .await
+            .expect("Save should succeed");
+        repo.save_label(relay5, label5.clone())
+            .await
+            .expect("Save should succeed");
+
+        // Retrieve all labels
+        let result = repo
+            .get_all_labels()
+            .await
+            .expect("get_all_labels should succeed");
+
+        assert_eq!(result.len(), 3, "Should return exactly 3 labels");
+
+        // Verify the labels are present (order may vary by implementation)
+        let has_relay1 = result
+            .iter()
+            .any(|(id, label)| id.as_u8() == 1 && label.as_str() == "Pump");
+        let has_relay3 = result
+            .iter()
+            .any(|(id, label)| id.as_u8() == 3 && label.as_str() == "Heater");
+        let has_relay5 = result
+            .iter()
+            .any(|(id, label)| id.as_u8() == 5 && label.as_str() == "Fan");
+
+        assert!(has_relay1, "Should contain relay 1 with label 'Pump'");
+        assert!(has_relay3, "Should contain relay 3 with label 'Heater'");
+        assert!(has_relay5, "Should contain relay 5 with label 'Fan'");
+    }
+
+    /// Test: `get_all_labels` excludes relays without labels
+    ///
+    /// Verifies that only relays with labels are returned, not all possible
+    /// relay IDs (1-8).
+    pub async fn test_get_all_labels_excludes_relays_without_labels<R: RelayLabelRepository>(
+        repo: &R,
+    ) {
+        let relay2 = RelayId::new(2).expect("Valid relay ID");
+        let label2 = RelayLabel::new("Only This One".to_string()).expect("Valid label");
+
+        repo.save_label(relay2, label2)
+            .await
+            .expect("Save should succeed");
+
+        let result = repo
+            .get_all_labels()
+            .await
+            .expect("get_all_labels should succeed");
+
+        assert_eq!(
+            result.len(),
+            1,
+            "Should return only the one relay with a label"
+        );
+        assert_eq!(result[0].0.as_u8(), 2, "Should be relay 2");
+    }
+
+    /// Test: `get_all_labels` excludes deleted labels
+    ///
+    /// Verifies that deleted labels don't appear in `get_all_labels` results.
+    pub async fn test_get_all_labels_excludes_deleted_labels<R: RelayLabelRepository>(repo: &R) {
+        let relay1 = RelayId::new(1).expect("Valid relay ID");
+        let relay2 = RelayId::new(2).expect("Valid relay ID");
+        let relay3 = RelayId::new(3).expect("Valid relay ID");
+
+        let label1 = RelayLabel::new("Keep1".to_string()).expect("Valid label");
+        let label2 = RelayLabel::new("Delete".to_string()).expect("Valid label");
+        let label3 = RelayLabel::new("Keep2".to_string()).expect("Valid label");
+
+        // Save all three labels
+        repo.save_label(relay1, label1)
+            .await
+            .expect("save should succeed");
+        repo.save_label(relay2, label2)
+            .await
+            .expect("save should succeed");
+        repo.save_label(relay3, label3)
+            .await
+            .expect("save should succeed");
+
+        // Delete the middle one
+        repo.delete_label(relay2)
+            .await
+            .expect("delete should succeed");
+
+        // Verify get_all_labels only returns the two remaining labels
+        let result = repo
+            .get_all_labels()
+            .await
+            .expect("get_all_labels should succeed");
+        assert_eq!(result.len(), 2, "Should have 2 labels after deletion");
+
+        let has_relay1 = result.iter().any(|(id, _)| id.as_u8() == 1);
+        let has_relay2 = result.iter().any(|(id, _)| id.as_u8() == 2);
+        let has_relay3 = result.iter().any(|(id, _)| id.as_u8() == 3);
+
+        assert!(has_relay1, "Relay 1 should be present");
+        assert!(!has_relay2, "Relay 2 should NOT be present (deleted)");
+        assert!(has_relay3, "Relay 3 should be present");
+    }
+}

backend/src/infrastructure/persistence/mod.rs

@@ -6,5 +6,9 @@
 /// Mock repository implementation for testing.
 pub mod label_repository;
 
+/// Comprehensive tests for `RelayLabelRepository` trait contract (T035).
+#[cfg(test)]
+pub mod label_repository_tests;
+
 /// `SQLite` repository implementation for relay labels.
 pub mod sqlite_repository;

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

@@ -531,7 +531,7 @@
   - **Complexity**: Medium | **Uncertainty**: High
   - **Note**: Use #[ignore] attribute, run with `cargo test -- --ignored`
 
-- [ ] **T035** [P] [US4] [TDD] Write tests for RelayLabelRepository trait
+- [x] **T035** [P] [US4] [TDD] Write tests for RelayLabelRepository trait
   - Test: `get_label(RelayId(1)) → Option<RelayLabel>`
   - Test: `set_label(RelayId(1), label) → Result<(), RepositoryError>`
   - Test: `delete_label(RelayId(1)) → Result<(), RepositoryError>`