[lukaszsamson]

Crash

Reported on Elixir 1.19.5:

** (FunctionClauseError) no function clause matching in ElixirLS.LanguageServer.Server.handle_info/2
  server.ex:391: ElixirLS.LanguageServer.Server.handle_info({#Reference<...>, {:error, [%{message: "** (MatchError) no match of right hand side value: {:error, :enoent} ... (elixir 1.19.5) Kernel.ParallelCompiler spawn_workers ...", stacktrace: [{:elixir_compiler, :file, 2, ...}], severity: :error, ...}], %{runtime_warnings: [], compile_warnings: [%{message: "redefining module Jason.Codegen ...", ...}, ...]}}}, %ElixirLS.LanguageServer.Server{...})

Root cause

The {ref, {:error, errors, %{runtime_warnings: ..., compile_warnings: ...}}} message is the final reply Mix's Elixir compiler worker sends to the process that called Mix.Task.run("compile"). So a compile ran inside the Server process, got aborted, and its late reply landed in the Server mailbox.

SourceFile.formatter_for/4 passes a non-compiling plugin_loader in the mix project branch, but not in the non mix project branch. There Mix.Tasks.ElixirLSFormat.formatter_for_file/2 falls back to its default loader, which runs mix loadpaths and mix compile whenever .formatter.exs lists plugins. The completion handler calls formatter_for/4 synchronously in the Server process, and the surrounding catch swallowed the aborted compile, leaving the stray message behind.

Fix

• Factor the non-compiling plugin loader out into SourceFile.plugin_loader/1 and use it in both branches. Plugins that are not loaded are skipped with a warning, the same as in mix projects.
• Add a catch-all handle_info/2 to ElixirLS.LanguageServer.Server that logs and ignores unexpected messages, so a stray message can no longer bring down the server.

Tests

• formatter_for/4 in a non mix project whose .formatter.exs lists one loaded and one missing plugin: it returns options, the loaded plugin is used and no compile runs. Before this change it went through the Mix loader and returned {:error, "Formatter plugin ... cannot be found"}.
• The Server survives unexpected messages, including a compiler worker style reply.