EF Core race condition: fixing check-then-act stock reservation (atomic update, RowVersion)

// ======================================================================
// Q: Spot the bug (also written by AI)
//
// It has a check-then-act race condition. Two concurrent orders both read
// Available = 5 and each reserves 5 units.
// Two fixes: an atomic update with ExecuteUpdateAsync (the best option here), or
// optimistic concurrency with a row version (catch DbUpdateConcurrencyException
// and retry or return false).
// ======================================================================

// AI-generated code with the bug
public async Task<bool> ReserveStock(string sku, int qty)
{
    var item = await _db.Stock.FirstAsync(s => s.Sku == sku);
    if (item.Available >= qty)
    {
        item.Available -= qty;
        await _db.SaveChangesAsync();
        return true;
    }
    return false;
}

// Fix 1: atomic update (best option here)
var affected = await _db.Stock
    .Where(s => s.Sku == sku && s.Available >= qty)
    .ExecuteUpdateAsync(s => s.SetProperty(x => x.Available, x => x.Available - qty), ct);
return affected == 1;

// Fix 2: optimistic concurrency
public class StockItem
{
    public string Sku { get; set; } = default!;
    public int Available { get; set; }
    [Timestamp] public byte[] RowVersion { get; set; } = default!; // SQL Server
    // PostgreSQL: modelBuilder.Entity<StockItem>().Property<uint>("xmin").IsRowVersion();
}
// ...
catch (DbUpdateConcurrencyException) { /* retry or return false */ }