Increment multithreading issue

[MarkoPielic]

Increment operation should be thread safe and atomic, but it is not - at least not when the key you are trying to increment does not exist.

If you try to do this:

 var result = memcachedClient.Increment(key, 1, 1, TimeSpan.FromDays(2));
 Console.WriteLine("Increment value: " + result);

From the single thread it will work as expected.
But if you try to do it from multiple threads it will sometimes work as expected and sometimes not (roughly about 50-50).

Full program to reproduce the issue (running on .NET 6) is below.

using Enyim.Caching;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;

IConfigurationBuilder builder = new ConfigurationBuilder().AddJsonFile("appsettings.json");
IConfiguration configuration = builder.Build();

var serviceCollection = new ServiceCollection();
serviceCollection.AddSingleton(configuration);
serviceCollection.AddLogging();
serviceCollection.AddEnyimMemcached();

var serviceProvider = serviceCollection.BuildServiceProvider();
var memcachedClient = serviceProvider.GetService<IMemcachedClient>();

string key = Guid.NewGuid().ToString();
List<Task> tasks = Enumerable.Range(1, 10)
    .Select(item => Task.Run(() =>
    {
        var result = memcachedClient.Increment(key, 1, 1, TimeSpan.FromDays(2));
        Console.WriteLine("Increment value: " + result);
    }))
    .ToList();

await Task.WhenAll(tasks);

var result = await memcachedClient.GetAsync<string>(key);
Console.WriteLine("Final result: " + result.Value);

CURRENT BEHAVIOR

• increment defaults do 0 or 1 when there is no key (50-50)
• each subsequent increment increases by 1
• final result is 9 or 10 (50-50)

EXPECTED BEHAVIOR

• increment should alawys default to 1 if key does not exist
• increment should always increment by 1
• the output of the increment operation should never be 0
• final result should be 10