The tracer cannot handle multiple threads

[swebb]

This is a limitation with the current implementation. Not a bug.

The tracer is explicitly designed to run within one thread. For example here. Ideally the tracer could be made threadsafe so that multiple threads could write to it, possibly similar to the Buildkite::TestCollector::Session.

This issue effects both RSpec and Minitest.

This limitation is most noticeable in System/Feature tests. Capybara spins up the web server (Puma/Unicorn/etc.) in a separate thread. As a result the tracer only collects SQL queries from the RSpec thread (i.e., test data getting created or verified). The SQL performed by the application is not captured.

Here's an example of the span data from a system test. The server is performing a User.where(email: email) but it is not captured.

As this data is not captured by the tracer it also won't be populated in the "slowest sql queries" section.

Note that since the tracer isn't available to the server thread it means HTTP requests will also not be captured.