Make copy operation atomic with explicit transaction handling

- Wrapped copy operation in try/except with explicit rollback
- Added comments explaining atomicity
- Ensures all-or-nothing behavior for copying to multiple dates

backend/routes/availability.py

@@ -160,34 +160,41 @@ async def copy_availability(
             detail=f"No availability found for source date {request.source_date}",
         )
     
-    # Copy to each target date
+    # Copy to each target date within a single atomic transaction
+    # All deletes and inserts happen before commit, ensuring atomicity
     copied_days: list[AvailabilityDay] = []
-    for target_date in request.target_dates:
-        if target_date == request.source_date:
-            continue  # Skip copying to self
-        
-        # Delete existing availability for target date
-        await db.execute(
-            delete(Availability).where(Availability.date == target_date)
-        )
-        
-        # Copy slots
-        target_slots: list[TimeSlot] = []
-        for source_slot in source_slots:
-            new_availability = Availability(
-                date=target_date,
-                start_time=source_slot.start_time,
-                end_time=source_slot.end_time,
+    try:
+        for target_date in request.target_dates:
+            if target_date == request.source_date:
+                continue  # Skip copying to self
+            
+            # Delete existing availability for target date
+            await db.execute(
+                delete(Availability).where(Availability.date == target_date)
             )
-            db.add(new_availability)
-            target_slots.append(TimeSlot(
-                start_time=source_slot.start_time,
-                end_time=source_slot.end_time,
-            ))
+            
+            # Copy slots
+            target_slots: list[TimeSlot] = []
+            for source_slot in source_slots:
+                new_availability = Availability(
+                    date=target_date,
+                    start_time=source_slot.start_time,
+                    end_time=source_slot.end_time,
+                )
+                db.add(new_availability)
+                target_slots.append(TimeSlot(
+                    start_time=source_slot.start_time,
+                    end_time=source_slot.end_time,
+                ))
+            
+            copied_days.append(AvailabilityDay(date=target_date, slots=target_slots))
         
-        copied_days.append(AvailabilityDay(date=target_date, slots=target_slots))
-    
-    await db.commit()
+        # Commit all changes atomically
+        await db.commit()
+    except Exception:
+        # Rollback on any error to maintain atomicity
+        await db.rollback()
+        raise
     
     return AvailabilityResponse(days=copied_days)