I have a set of integration tests that have been working great on Windows for quite some time. While troubleshooting an unrelated issue I was running my tests on Ubuntu 20.04 LTS via WSL, and about half of the tests were failing with this IOException.

The Error Message

System.IO.IOException : The configured user limit (128) on the number of inotify instances has been reached, or the per-process limit on the number of open file descriptors has been reached.

It seems that due to the way my tests work using WebApplicationFactory<Startup> and calling factory.WithWebHostBuilder(builder => ..., I'm creating too many instances of the HostBuilder, which by default installs a file system watcher on the appsettings.json file. All those instances tap out the default number of inotify instances allowed. Switching to a polling mechanism clears this up.

The Solution I Found

Configure the environment variable to have dotnet use polling instead.

export DOTNET_USE_POLLING_FILE_WATCHER=true

I don't know if there are unintended side effects of this change; however, since its only applying to test runs, I feel its a safe change to make.