Stop the availability form failing in silence

Adding availability for 5:30-6:00 PM with the lesson length left on its
60-minute default saved nothing and said nothing. A window is stored as
consecutive lesson-length slots, so one that fits no lesson splits into
none: splitByDuration() returned [], createFromWindow() inserted
nothing, and addSlot() discarded the result and re-rendered the page
unchanged.

The REST endpoint already rejected that window with a 400. The admin
form checked the same rules separately, and its copy was both laxer and
mute — an unreadable date, an end before the start, and a two-day window
were bare `return`s, and it never checked offering ownership at all, so
a crafted POST could tie a slot to another instructor's offering and
inherit their price and payment routing.

Both callers now go through WindowValidator, which returns the window or
a WP_Error explaining the refusal. The endpoint returns that error as
is; the page renders its message as a notice. handleFormAction returns
a [notice, error] pair so deletes report themselves too, and a
successful add says how many slots it created.

Two failures could also go unnoticed underneath: wpdb::insert's result
was ignored, and insert_id still holds the previous statement's id after
a failed write, so a failure looked like a success — and could become
the recurrence group of a weekly series, orphaning every later
occurrence. weeks was unbounded server-side despite the form's max=52.

availability-admin.js narrows the lesson-length choices to those that
fit the window and blocks submission when none do, which is what makes
the original mistake hard to repeat. It is a convenience: the server
validates regardless.

Closes #130

src/Availability/AvailabilityController.php

@@ -13,6 +13,7 @@ class AvailabilityController {
 	public function __construct(
 		private AvailabilityRepository $repository,
 		private OfferingRepository $offerings,
+		private WindowValidator $validator,
 	) {}
 
 	public function renderPage(): void {
@@ -21,9 +22,11 @@ class AvailabilityController {
 		}
 
 		$instructorId = get_current_user_id();
+		$notice       = '';
+		$error        = '';
 
 		if ( isset( $_POST['usc_action'] ) && check_admin_referer( 'usc_availability_action' ) ) {
-			$this->handleFormAction( $instructorId );
+			[ $notice, $error ] = $this->handleFormAction( $instructorId );
 		}
 
 		$slots           = $this->repository->findByInstructor( $instructorId );
@@ -44,72 +47,144 @@ class AvailabilityController {
 		include USC_PLUGIN_DIR . 'templates/admin/availability.php';
 	}
 
-	private function handleFormAction( int $instructorId ): void {
+	/**
+	 * Run the submitted action and report what happened. Every branch returns a
+	 * message: a form that silently reloads leaves the instructor unable to tell
+	 * "saved 41 slots" from "saved nothing".
+	 *
+	 * @return array{string, string} Success notice and error message; each is
+	 *                               empty when it does not apply.
+	 */
+	private function handleFormAction( int $instructorId ): array {
 		// Nonce is verified by the caller (renderPage) before this method runs.
 		// phpcs:disable WordPress.Security.NonceVerification.Missing
 		$action = sanitize_key( Val::string( wp_unslash( $_POST['usc_action'] ?? '' ) ) );
 
 		if ( 'add' === $action ) {
-			$this->addSlot( $instructorId );
+			return $this->addSlot( $instructorId );
 		}
 
 		if ( 'delete' === $action ) {
-			$this->deleteOwnSlot( absint( Val::int( $_POST['slot_id'] ?? 0 ) ), $instructorId );
+			return $this->deleteOwnSlot( absint( Val::int( $_POST['slot_id'] ?? 0 ) ), $instructorId )
+				? [ __( 'Availability slot deleted.', 'unsupervised-schedular' ), '' ]
+				: [ '', __( 'That slot could not be deleted. It may already be booked, or belong to someone else.', 'unsupervised-schedular' ) ];
 		}
 
 		if ( 'bulk_delete' === $action ) {
 			// The array itself carries no data; each element is coerced and
 			// absint-sanitized individually below.
 			// phpcs:ignore WordPress.Security.ValidatedSanitizedInput
-			$rawIds = $_POST['slot_ids'] ?? [];
+			$rawIds  = $_POST['slot_ids'] ?? [];
+			$deleted = 0;
+			$failed  = 0;
+
 			foreach ( is_array( $rawIds ) ? $rawIds : [] as $rawId ) {
-				$this->deleteOwnSlot( absint( Val::int( $rawId ) ), $instructorId );
+				if ( $this->deleteOwnSlot( absint( Val::int( $rawId ) ), $instructorId ) ) {
+					++$deleted;
+					continue;
+				}
+
+				++$failed;
 			}
+
+			return $this->bulkDeleteResult( $deleted, $failed );
 		}
 		// phpcs:enable WordPress.Security.NonceVerification.Missing
+
+		return [ '', '' ];
+	}
+
+	/**
+	 * Wording for a bulk delete, which can partly succeed.
+	 *
+	 * @return array{string, string}
+	 */
+	private function bulkDeleteResult( int $deleted, int $failed ): array {
+		$notice = $deleted > 0
+			? sprintf(
+				/* translators: %d: number of availability slots deleted. */
+				_n( '%d slot deleted.', '%d slots deleted.', $deleted, 'unsupervised-schedular' ),
+				$deleted
+			)
+			: '';
+
+		$error = $failed > 0
+			? sprintf(
+				/* translators: %d: number of slots that could not be deleted. */
+				_n(
+					'%d slot could not be deleted — it may already be booked.',
+					'%d slots could not be deleted — they may already be booked.',
+					$failed,
+					'unsupervised-schedular'
+				),
+				$failed
+			)
+			: '';
+
+		if ( 0 === $deleted && 0 === $failed ) {
+			$error = __( 'No slots were selected.', 'unsupervised-schedular' );
+		}
+
+		return [ $notice, $error ];
 	}
 
 	/**
 	 * Delete a slot only when it exists and belongs to the given instructor.
-	 * The repository additionally refuses to delete booked slots.
+	 * The repository additionally refuses to delete booked slots. Returns whether
+	 * the row actually went away.
 	 */
-	private function deleteOwnSlot( int $slotId, int $instructorId ): void {
+	private function deleteOwnSlot( int $slotId, int $instructorId ): bool {
 		if ( $slotId <= 0 ) {
-			return;
+			return false;
 		}
 
 		$slot = $this->repository->findById( $slotId );
-		if ( $slot && $slot->instructorId === $instructorId ) {
-			$this->repository->delete( $slotId );
+
+		if ( null === $slot || $slot->instructorId !== $instructorId ) {
+			return false;
 		}
+
+		return $this->repository->delete( $slotId );
 	}
 
-	private function addSlot( int $instructorId ): void {
+	/**
+	 * Validate and persist a submitted window.
+	 *
+	 * @return array{string, string}
+	 */
+	private function addSlot( int $instructorId ): array {
 		// phpcs:disable WordPress.Security.NonceVerification.Missing
-		$startDt = AvailabilitySlot::normalizeDateTime( sanitize_text_field( Val::string( wp_unslash( $_POST['start_dt'] ?? '' ) ) ) );
-		$endDt   = AvailabilitySlot::normalizeDateTime( sanitize_text_field( Val::string( wp_unslash( $_POST['end_dt'] ?? '' ) ) ) );
-
-		// A window must start and end on the same day (weekly repeat covers longer
-		// ranges) and fit at least one lesson; it is stored as lesson-length slots.
-		if ( null === $startDt || null === $endDt || $endDt <= $startDt || substr( $startDt, 0, 10 ) !== substr( $endDt, 0, 10 ) ) {
-			return;
-		}
-
-		$offeringId = absint( Val::int( $_POST['offering_id'] ?? 0 ) );
-		$duration   = absint( Val::int( $_POST['duration_minutes'] ?? 0 ) );
-
-		$window = new AvailabilitySlot(
-			instructorId:    $instructorId,
-			startDt:         $startDt,
-			endDt:           $endDt,
-			durationMinutes: $duration > 0 ? $duration : 60,
-			offeringId:      $offeringId > 0 ? $offeringId : null,
+		$window = $this->validator->validate(
+			$instructorId,
+			sanitize_text_field( Val::string( wp_unslash( $_POST['start_dt'] ?? '' ) ) ),
+			sanitize_text_field( Val::string( wp_unslash( $_POST['end_dt'] ?? '' ) ) ),
+			absint( Val::int( $_POST['duration_minutes'] ?? 0 ) ),
+			absint( Val::int( $_POST['offering_id'] ?? 0 ) ),
 		);
 
+		if ( $window instanceof \WP_Error ) {
+			return [ '', $window->get_error_message() ];
+		}
+
 		$recurrence = sanitize_key( Val::string( wp_unslash( $_POST['recurrence'] ?? 'single' ) ) );
 		$weeks      = absint( Val::int( $_POST['weeks'] ?? 1 ) );
-
-		$this->repository->createFromWindow( $window, 'weekly' === $recurrence, $weeks );
 		// phpcs:enable WordPress.Security.NonceVerification.Missing
+
+		$ids = $this->repository->createFromWindow( $window, 'weekly' === $recurrence, $weeks );
+
+		// The window was valid, so it split into at least one slot — an empty
+		// result means every insert failed.
+		if ( [] === $ids ) {
+			return [ '', __( 'The availability could not be saved. Please try again.', 'unsupervised-schedular' ) ];
+		}
+
+		return [
+			sprintf(
+				/* translators: %d: number of bookable slots created. */
+				_n( 'Added %d bookable slot.', 'Added %d bookable slots.', count( $ids ), 'unsupervised-schedular' ),
+				count( $ids )
+			),
+			'',
+		];
 	}
 }