Support debugging the PID in slow compilation

Author: josevalim

lib/elixir/lib/kernel/parallel_compiler.ex

@@ -876,7 +876,12 @@ defmodule Kernel.ParallelCompiler do
     compiling = System.convert_time_unit(data.compiling, :native, :millisecond)
 
     if not data.warned and compiling >= state.long_compilation_threshold do
-      state.each_long_compilation.(data.file)
+      if is_function(state.each_long_compilation, 2) do
+        state.each_long_compilation.(data.file, data.pid)
+      else
+        state.each_long_compilation.(data.file)
+      end
+
       %{data | warned: true}
     else
       data

lib/mix/lib/mix/compilers/elixir.ex

@@ -1020,9 +1020,10 @@ defmodule Mix.Compilers.Elixir do
           each_module: fn file, module, _binary ->
             compiler_call(parent, ref, {:each_module, file, module, System.os_time(:second)})
           end,
-          each_long_compilation: fn file ->
+          each_long_compilation: fn file, pid ->
             Mix.shell().info(
-              "Compiling #{Path.relative_to(file, File.cwd!())} (it's taking more than #{threshold}s)"
+              "Compiling #{Path.relative_to(file, File.cwd!())} (it's taking more than #{threshold}s)" <>
+                "\n#{debug_stacktrace(pid)}"
             )
           end,
           long_compilation_threshold: threshold,
@@ -1282,6 +1283,15 @@ defmodule Mix.Compilers.Elixir do
     end
   end
 
+  defp debug_stacktrace(pid) do
+    with true <- Mix.debug?(),
+         {:current_stacktrace, stacktrace} <- Process.info(pid, :current_stacktrace) do
+      Exception.format_stacktrace(stacktrace)
+    else
+      _ -> ""
+    end
+  end
+
   ## Consolidation
 
   @doc """