I’m getting an intermittent compilation error when building a project in Docker:

== Compilation error in file lib/sentry/logger.ex ==
** (MatchError) no match of right hand side value: {:error, :enoent}
    (elixir) lib/kernel/parallel_compiler.ex:198: anonymous fn/4 in Kernel.ParallelCompiler.spawn_workers/6
could not compile dependency :sentry, "mix compile" failed. You can recompile this dependency with "mix deps.compile sentry", update it with "mix deps.update sentry" or clean it with "mix deps.clean sentry"

Kernel.ParallelCompiler.spawn_workers/6 calls :elixir_compiler.file/2, which calls file:read_file/1, which returns {error, enoent} if it can’t find a file.

:elixir_compiler.file/2 tries to pattern-match…

{ok, Bin} = file:read_file(File)

…fails, and raises the MatchError we see above.

Here are a few thoughts:

• I could just do what the instructions tell me to do …and recompile the dependency with mix deps.compile sentry, but this only happens intermittently, so I don’t think the problem is with how I’m compiling. This is part of an automated build, too, and recompiling one dependency occasionally isn’t ideal.
• I’m using the most recent version of Sentry, so mix deps.update sentry won’t help.
• I don’t think mix deps.clean sentry is the answer because I’m starting off with a clean slate; I’m building the project while building a Docker image. Both deps/ and _build/ are in my .dockerignore file.
• This only seems to happen with the Sentry library.
• The issue is intermittent. On most attempts to build the project and the Docker image, everything compiles correctly.
• Even though the error was in lib/sentry/logger.ex in the code snippet above, this error sometimes springs up in other files in the Sentry library.
• My Docker image is running Erlang/OTP 20, and Elixir 1.6.1 compiled with OTP 20, on Debian GNU/Linux 8.

Do you have any suggestions for how to debug this? I’m a bit flummoxed. There’s an existing issue on the Sentry project that describes the problem, but I thought it might actually be a bug in the Elixir compiler, so I posted here, just in case