test: add reservation concurrency tests

src/test/java/com/slotline/reservation/ReservationConcurrencyTest.java

@@ -0,0 +1,102 @@
+package com.slotline.reservation;
+
+import com.slotline.organization.Organization;
+import com.slotline.organization.OrganizationRepository;
+import com.slotline.reservation.dto.ReservationRequest;
+import com.slotline.resource.Resource;
+import com.slotline.resource.ResourceRepository;
+import com.slotline.security.AuthenticatedUser;
+import com.slotline.user.Role;
+import com.slotline.user.User;
+import com.slotline.user.UserRepository;
+import org.junit.jupiter.api.Test;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.dao.ConcurrencyFailureException;
+import org.springframework.dao.DataIntegrityViolationException;
+import org.springframework.security.crypto.password.PasswordEncoder;
+import org.springframework.test.context.ActiveProfiles;
+
+import com.slotline.common.SlotConflictException;
+
+import java.time.Instant;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicInteger;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+@SpringBootTest
+@ActiveProfiles("test")
+class ReservationConcurrencyTest {
+
+    @Autowired
+    private ReservationService reservationService;
+
+    @Autowired
+    private ResourceRepository resourceRepository;
+
+    @Autowired
+    private OrganizationRepository organizationRepository;
+
+    @Autowired
+    private UserRepository userRepository;
+
+    @Autowired
+    private ReservationRepository reservationRepository;
+
+    @Autowired
+    private PasswordEncoder passwordEncoder;
+
+    @Test
+    void onlyOneBookingWinsForTheSameSlotUnderConcurrency() throws Exception {
+        Organization organization = organizationRepository.save(
+                new Organization("Concurrency Org", "concurrency-org", Instant.now()));
+        Resource court = resourceRepository.save(
+                new Resource(organization.getId(), "Court 1", "Outdoor tennis court", Instant.now()));
+        User booker = userRepository.save(
+                new User(organization.getId(), "racer", passwordEncoder.encode("password123"), Role.USER, Instant.now()));
+        AuthenticatedUser actor =
+                new AuthenticatedUser(booker.getId(), booker.getUsername(), booker.getRole(), organization.getId());
+
+        Instant start = Instant.parse("2026-09-01T10:00:00Z");
+        Instant end = Instant.parse("2026-09-01T11:00:00Z");
+        ReservationRequest sameSlot = new ReservationRequest(court.getId(), start, end);
+
+        int attempts = 12;
+        ExecutorService pool = Executors.newFixedThreadPool(attempts);
+        CountDownLatch startGate = new CountDownLatch(1);
+        AtomicInteger successes = new AtomicInteger();
+        AtomicInteger conflicts = new AtomicInteger();
+
+        List<Future<?>> futures = new ArrayList<>();
+        for (int i = 0; i < attempts; i++) {
+            futures.add(pool.submit(() -> {
+                startGate.await();
+                try {
+                    reservationService.create(actor, sameSlot);
+                    successes.incrementAndGet();
+                } catch (SlotConflictException | ConcurrencyFailureException | DataIntegrityViolationException conflict) {
+                    conflicts.incrementAndGet();
+                }
+                return null;
+            }));
+        }
+
+        startGate.countDown();
+        for (Future<?> future : futures) {
+            future.get(20, TimeUnit.SECONDS);
+        }
+        pool.shutdownNow();
+
+        assertThat(successes.get()).isEqualTo(1);
+        assertThat(conflicts.get()).isEqualTo(attempts - 1);
+        assertThat(reservationRepository.findByResourceIdAndStatusOrderByStartTimeAsc(
+                court.getId(), ReservationStatus.ACTIVE)).hasSize(1);
+    }
+}