Unit 14 — Mix integration

Copy Markdown View Source

Objectives

Build from mix test, see a no-op second compile, and load the NIF from an app that does not mention mojo.

Why

A shell command in test/test_helper.exs was enough for Stage 0 and invisible to mix compile in another project. The compiler task is the path a dependency actually runs.

Model

mix compile
  -> Elixir compiler
  -> Compile.Mojoler
       fingerprint match and .so files exist -> {:noop, []}
       else build into staging, rename, write FINGERPRINT

Theory

The compiler is listed after Elixir (Mix.compilers() ++ [:mojoler]) so the task module is compiled before it runs. MOJOLER_OUT points at priv/native/staging. File.rename replaces the destination on the same filesystem. The fingerprint file is written after the rename. A failed build deletes staging and leaves the old .so.

If this VM has already marked a library loaded (Mojoler.Nif.mark!/1 from the module that called load_nif) and the fingerprint changed, the task errors instead of renaming over a mapped file.

examples/consumer depends on {:mojoler, path: "../.."}. examples/umbrella/apps/child uses the same path from four directories down. Both call Mojoler.POC.add/2. Neither passes -Xlinker.

Lab

mix compile.mojoler
cd examples/consumer && mix test

Expected

The second compiler invocation is a no-op when nothing changed (test/build_test.exs asserts {:noop, []}). The consumer test passes.

Code

lib/mix/tasks/compile/mojoler.ex and lib/mojoler/build.ex. decide/3 is the table: match and outputs -> :noop; loaded and stale -> :loaded; else :build.

Failure

A stale .so after a Mojo edit: the fingerprint includes the .mojo bytes, so the next mix compile rebuilds. If you overwrite the .so while a VM has it mapped, the next call can crash. Restart the VM. The compiler's error is the reminder.

Exercise

What does mix clean in the consumer prove?

Answer

E23 ran mix test, then mix clean, then mix test again. The second run passed, so the dependency compiler ran again and produced a loadable library. It does not prove a Linux release.