Update value under concurrency fails when a cancellation token is registered.

[troydai]

Scenario:

1. Set up a memory cache.
2. Add an entry with a cancellation token registered with it.
3. Start a child thread which keep reading the memory cache with the same key.
4. Reset the value of the entry from main thread.
5. Exception InvalidOperationException is thrown.

Issue repo:

https://github.com/troydai/CachingIssues/blob/2a85107b9b572c47d66d2a0303deafecaeaa3166/TestActions/ConcurrencyIssue184.cs#L17

Run the sample follow exception will be thrown from child thread after the value is updated by main thread.

Unhandled Exception: System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
   at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
   at System.Collections.Generic.List`1.Enumerator.MoveNextRare()
   at Microsoft.Extensions.Caching.Memory.CacheEntry.PropageOptions(CacheEntry parent)
   at Microsoft.Extensions.Caching.Memory.MemoryCache.TryGetValue(Object key, Object& result)
   at Microsoft.Extensions.Caching.Memory.CacheExtensions.TryGetValue[TItem](IMemoryCache cache, Object key, TItem& value)
   at Microsoft.Extensions.Caching.Memory.CacheExtensions.Get[TItem](IMemoryCache cache, Object key)
   at ConsoleApplication.Program.<>c__DisplayClass5_0.<Monitor>b__0(Int32 round, DateTime beginning)
   at ConsoleApplication.Program.<>c__DisplayClass4_0.<Polling>b__0()
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)

[Eilon]

@troydai thanks - can you add a bit more info to this bug?

[troydai]

Yes. I'm adding except stack to the description. I did talk with @Tratcher and @sebastienros before I file the issue. He's has a rough idea what probably break this scneario.

[kevinchalet]

Since this ticket is marked for 1.0.0, I guess it was not fixed for RC2?

An unhandled exception occurred while processing the request.

InvalidOperationException: La collection a été modifiée ; l'opération d'énumération peut ne pas s'exécuter.
à System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
à System.Collections.Generic.List`1.Enumerator.MoveNextRare()
à System.Collections.Generic.List`1.Enumerator.MoveNext()
à Microsoft.Extensions.Caching.Memory.CacheEntry.PropageOptions(CacheEntry parent)
à Microsoft.Extensions.Caching.Memory.CacheEntry.Dispose()
à Microsoft.Extensions.Caching.Memory.CacheExtensions.Set[TItem](IMemoryCache cache, Object key, TItem value, MemoryCacheEntryOptions options)
à Microsoft.AspNetCore.Mvc.Razor.RazorViewEngine.OnCacheMiss(ViewLocationExpanderContext expanderContext, ViewLocationCacheKey cacheKey)
à Microsoft.AspNetCore.Mvc.Razor.RazorViewEngine.LocatePageFromViewLocations(ActionContext actionContext, String pageName, Boolean isMainPage)
à Microsoft.AspNetCore.Mvc.Razor.RazorViewEngine.FindView(ActionContext context, String viewName, Boolean isMainPage)
à Microsoft.AspNetCore.Mvc.ViewEngines.CompositeViewEngine.FindView(ActionContext context, String viewName, Boolean isMainPage)
à Microsoft.AspNetCore.Mvc.ViewFeatures.Internal.ViewResultExecutor.FindView(ActionContext actionContext, ViewResult viewResult)
à Microsoft.AspNetCore.Mvc.ViewResult.<ExecuteResultAsync>d__26.MoveNext()

[Tratcher]

Correct. You managed to reproduce it in the wild? We only managed in contrived background threading scenarios.

[kevinchalet]

You managed to reproduce it in the wild?

Yep. Adding some "DB initialization" code at the end of Startup.Configure in a whole new MVC RC2 app seems enough to reproduce it almost consistently on my machine:

var factory = app.ApplicationServices.GetRequiredService<IServiceScopeFactory>();
using (var scope = factory.CreateScope()) {
    var context = scope.ServiceProvider.GetRequiredService<ApplicationDbContext>();
    context.Database.EnsureCreated();

    if (!context.Users.Any()) {
        // Skipped.
    }

    context.SaveChanges();
}

For reasons I can't explain (yet), this alternative pattern causes fewer exceptions:

var options = app.ApplicationServices.GetRequiredService<DbContextOptions<ApplicationDbContext>>();
using (var context = new ApplicationDbContext(options)) {
    context.Database.EnsureCreated();

    if (!context.Users.Any()) {
        // Skipped.
    }

    context.SaveChanges();
}