Different threads using same instance of DbContext, called from within DelegatingHandler - .net-core

Question

I have .Net Core 3.1 application that is using EF Core 3.1.9. During a specific process I am getting the following error:
A second operation started on this context before a previous operation completed. This is usually caused by different threads using the same instance of DbContext. For more information on how to avoid threading issues with DbContext, see https://go.microsoft.com/fwlink/?linkid=2097913
I am using Dependency Injection for the DbContext and have gone through all the flows to make sure everything is properly and immediately await'ed.
The error occurs within LtiUserRepository.cs which will be shown below.
That process starts with an external http call using an HttpClient that has a custom MessageHandler, registered in Startup.cs:
services.AddHttpClient<MyRepository>("MyCustomUserClient", client =>
{
var canvasUrl = Configuration.GetSection("Urls:Removed").Value ?? "https://example.com/";
client.BaseAddress = new System.Uri(removed);
}).AddHttpMessageHandler<LtiUserApiAuthenticationHttpClientHandler>();
The code that initiates the HTTP Call is:
public async Task<PlatformQuizSubmissions> GetUserQuiz(string courseId, string quizId)
{
var path = $"api/v1/courses/{courseId}/quizzes/{quizId}/submission";
var response = await _myCustomUserClient.GetAsync(path);
// some stuff
var responseContent = await response.Content.ReadAsStringAsync();
// Some other stuff
}
The purpose of the custom MessageHandler is to check for a header, get some data, and append a query parameter to each request
public sealed class LtiUserApiAuthenticationHttpClientHandler : DelegatingHandler
{
private readonly IHttpContextAccessor _accessor;
private readonly ILtiUserService _userService;
public LtiUserApiAuthenticationHttpClientHandler(IHttpContextAccessor accessor, ILtiUserService ltiUserService)
{
_accessor = accessor;
_userService = ltiUserService;
}
protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
{
var obo = _accessor.HttpContext.Request.Headers["QT-OBO"];
// THIS IS THE PART THAT QUERIES THE DATABASE
var user = await _userService.Get(new Guid(obo));
var uriBuilder = new UriBuilder(request.RequestUri);
if (string.IsNullOrEmpty(uriBuilder.Query))
{
uriBuilder.Query = $"as_user_id={user.PlatformUserId}";
}
else
{
uriBuilder.Query = $"{uriBuilder.Query}&as_user_id={user.PlatformUserId}";
}
request.RequestUri = uriBuilder.Uri;
return await base.SendAsync(request, cancellationToken);
}
}
You can see above that the MessageHandler calls _userservice.Get, which is this:
public async Task<LtiUser> Get(Guid guid)
{
return await _ltiUserRepository.Get(guid);
}
That simply returns from the repository, which is this:
public class LtiUserRepository : ILtiUserRepository
{
private readonly SqlDbContext _db;
private readonly IMapper _mapper;
private readonly ILogger<LtiUserRepository> _logger;
public LtiUserRepository(SqlDbContext sqlDbContext, IMapper mapper, ILoggerFactory logger)
{
_db = sqlDbContext;
_mapper = mapper;
_logger = logger != null ? logger.CreateLogger<LtiUserRepository>() : throw new ArgumentNullException(nameof(logger));
}
public async Task<LtiUser> Get(Guid guid)
{
try
{
return await _db.LtiUsers
.AsNoTracking()
.Where(l => l.UUID == guid)
.ProjectTo<LtiUser>(_mapper.ConfigurationProvider)
.SingleOrDefaultAsync();
}
catch (Exception ex)
{
// This is where the error is caught.
_logger.LogCritical($"Could not get LtiUser via (UUID) {guid} : {ex.Message}");
return null;
}
}
}
The database is registered in Startup.cs with:
protected virtual void ConfigureDatabaseServices(IServiceCollection services)
{
services.AddDbContext<SqlDbContext>(
o => o.UseSqlServer(Configuration.GetConnectionString("DbConnectionString")),
ServiceLifetime.Transient);
}
When I hit this endpoint using ApacheBench with 20 requests, concurrency of 2 I get this error anywhere from 2 to 10 times. However, looking at the following snippet from the MessageHandler (LtiUserApiAuthenticationHttpClientHandler) again:
var user = await _userService.Get(new Guid(obo));
if (string.IsNullOrEmpty(uriBuilder.Query))
{
uriBuilder.Query = $"as_user_id={user.PlatformUserId}";
}
else
{
uriBuilder.Query = $"{uriBuilder.Query}&as_user_id={user.PlatformUserId}";
}
If I replace user.PlatformUserId with a hardcoded, known value, (and comment out the call to _userService.Get) I can use AB with 1000 requests and a concurrency of 20 and have 0 occurrences of the issue. That leads me to believe I have it narrowed down to the offending flow, but am not sure of the correct way to do this.