Nowadays it’s a standard practice to cache dependencies and compiled artifacts to improve CI build time. For Elixir apps, this would mean caching the contents of _build and deps. The compiler is smart enough to figure out what needs to be recompiled. This setup was working well except for one annoying issue.

Sometimes our test suite would pass, but the final code coverage computation part would fail. We were just using --cover, which is a standard coverage tool supported by the Elixir.

** (MatchError) no match of right hand side value: {:error, {:no_source_code_found, Myapp.Repo}}
    (mix 1.13.4) lib/mix/tasks/test.coverage.ex:290: anonymous fn/3 in Mix.Tasks.Test.Coverage.html/2
    (elixir 1.13.4) lib/enum.ex:2396: Enum."-reduce/3-lists^foldl/2-0-"/3
    (mix 1.13.4) lib/mix/tasks/test.coverage.ex:289: Mix.Tasks.Test.Coverage.html/2

I initially thought there was something wrong with the specific module because each time, it would fail with the same module name. Usually, clearing the CI cache would solve the problem. Initial attempts to reproduce the issue locally were not successful. The problem was pretty annoying though, as if dealing with non deterministic test failures in CI is not enough, now, we have to deal with test coverage failures as well.

I copied the contents of _build and deps to my dev machine and ran the same command. Sure enough, the test passed, but the coverage failed. Finally, I found a reliable way to reproduce the issue locally. I started to trace the function calls using recon_trace and ended up on the line in the otp cover module that would explain the whole thing.

iex(2)> GenServer.module_info(:compile)
  version: '8.0.4',
  options: [:no_spawn_compiler_process, :from_core, :no_core_prepare,
  source: '/build/source/lib/elixir/lib/gen_server.ex'

Each compiled beam module stores some information that gets exposed via module_info/1 function. The absolute path of the source file is one of them, and this is where our problem starts. I mentioned in the beginning that we cache compiled beam files. So the compilation would happen on one machine, and the beam file might get used for another build on another machine. We use Gitlab for our CI build server, and Gitlab by default checkout the source code into a folder named {builds_dir}/$RUNNER_TOKEN_KEY/$CONCURRENT_ID/$NAMESPACE/$PROJECT_NAME (example /builds/2mn-ncv-/0/user/playground). The cover module would try to fetch the source code and of course, the source wouldn’t be there because the source files were compiled on a different machine with a different folder name.

The fix for the issue was simple once the cause is understood. Gitlab allows the GIT_CLONE_PATH to be configured to a static path and it is safe to do as we were already using k8s runners. So even if we build the cache on a different machine, the source code path would be the same since we checkout the source to a static path.