fix issue

apps/edr-freight-api/src/modules/train-scheduling/booking-batch.service.ts

@@ -769,7 +769,54 @@ export class BookingBatchService implements OnModuleInit {
     bookings: Booking[],
     scheduleId: string,
   ): Promise<void> {
-    for (const b of bookings) await this.reserve(b, scheduleId);
+    // H8: the capacity check (pickExportSchedule → budget.fits) and the
+    // reservation writes below are not atomic on their own — two concurrent
+    // export accepts can each see the same train as fitting and both reserve,
+    // overshooting the train's capacity. Serialize reservations against this
+    // schedule: take a pessimistic_write lock on the TrainSchedule row
+    // (SELECT … FOR UPDATE), then RE-VERIFY budget.fits for these bookings'
+    // combined need from freshly-committed state INSIDE the lock before the
+    // reserve writes run. A loser (another accept took the space first) gets a
+    // ConflictException — the staff accept fails and reverts, exactly as an
+    // up-front full train does. Covered: the fits-vs-reserve overshoot on the
+    // export FCFS path; the lock is held for the duration of the reserve writes.
+    await this.dataSource.transaction(async (manager) => {
+      const locked = await manager.findOne(TrainSchedule, {
+        where: { id: scheduleId },
+        lock: { mode: "pessimistic_write" },
+      });
+      if (!locked) {
+        throw new ConflictException(
+          "Export train is no longer available for reservation",
+        );
+      }
+      const schedule =
+        await this.trainSchedulesRepository.findByIdWithFullGraph(scheduleId);
+      const locomotive = schedule?.trainSet?.locomotive;
+      if (!schedule || !locomotive) {
+        throw new ConflictException(
+          "Export train is no longer available for reservation",
+        );
+      }
+      const wagonDims = await this.loadWagonDims();
+      const limits = await this.capacityLimits(locomotive);
+      const budget = await this.remainingBudget(schedule, limits, wagonDims);
+      const leg = budget.legOf(
+        bookings[0].originYardId,
+        bookings[0].destinationYardId,
+      );
+      const need =
+        bookings.length >= 2
+          ? this.combinedNeed(bookings[0], bookings[1], wagonDims)
+          : this.needFor(bookings[0], wagonDims);
+      if (!leg || !budget.fits(need, leg)) {
+        throw new ConflictException(
+          "Train is full — no export capacity left for this day",
+        );
+      }
+
+      for (const b of bookings) await this.reserve(b, scheduleId);
+    });
     this.armSettle(scheduleId);
     const schedule =
       await this.trainSchedulesRepository.findByIdWithFullGraph(scheduleId);