Load modules lazily and only if needed

lib/elixir/lib/kernel/error_handler.ex

@@ -41,7 +41,15 @@ defmodule Kernel.ErrorHandler do
     :erlang.garbage_collect(self())
 
     receive do
-      {^ref, value} -> value
+      {^ref, {:loading, pid}} ->
+        ref = :erlang.monitor(:process, pid)
+
+        receive do
+          {:DOWN, ^ref, _, _, _} -> :found
+        end
+
+      {^ref, value} ->
+        value
     end
   end
 end

lib/elixir/lib/kernel/parallel_compiler.ex

@@ -198,7 +198,7 @@ defmodule Kernel.ParallelCompiler do
           {:ok, [atom], [warning] | info()}
           | {:error, [error] | [Code.diagnostic(:error)], [warning] | info()}
   def compile_to_path(files, path, options \\ []) when is_binary(path) and is_list(options) do
-    spawn_workers(files, {:compile, path}, options)
+    spawn_workers(files, {:compile, path}, Keyword.put(options, :dest, path))
   end
 
   @doc """
@@ -338,8 +338,11 @@ defmodule Kernel.ParallelCompiler do
   end
 
   defp write_module_binaries(result, {:compile, path}, timestamp) do
+    File.mkdir_p!(path)
+    Code.prepend_path(path)
+
     Enum.flat_map(result, fn
-      {{:module, module}, binary} when is_binary(binary) ->
+      {{:module, module}, {binary, _}} when is_binary(binary) ->
         full_path = Path.join(path, Atom.to_string(module) <> ".beam")
         File.write!(full_path, binary)
         if timestamp, do: File.touch!(full_path, timestamp)
@@ -351,7 +354,7 @@ defmodule Kernel.ParallelCompiler do
   end
 
   defp write_module_binaries(result, _output, _timestamp) do
-    for {{:module, module}, binary} when is_binary(binary) <- result, do: module
+    for {{:module, module}, {binary, _}} when is_binary(binary) <- result, do: module
   end
 
   ## Verification
@@ -366,7 +369,7 @@ defmodule Kernel.ParallelCompiler do
 
   defp maybe_check_modules(result, runtime_modules, state) do
     compiled_modules =
-      for {{:module, module}, binary} when is_binary(binary) <- result,
+      for {{:module, module}, {binary, _}} when is_binary(binary) <- result,
           do: module
 
     profile(
@@ -439,8 +442,8 @@ defmodule Kernel.ParallelCompiler do
 
         try do
           case output do
-            {:compile, path} -> compile_file(file, path, parent)
-            :compile -> compile_file(file, dest, parent)
+            {:compile, _} -> compile_file(file, dest, false, parent)
+            :compile -> compile_file(file, dest, true, parent)
             :require -> require_file(file, parent)
           end
         catch
@@ -546,9 +549,9 @@ defmodule Kernel.ParallelCompiler do
     wait_for_messages([], spawned, waiting, files, result, warnings, errors, state)
   end
 
-  defp compile_file(file, path, parent) do
+  defp compile_file(file, path, force_load?, parent) do
     :erlang.process_flag(:error_handler, Kernel.ErrorHandler)
-    :erlang.put(:elixir_compiler_dest, path)
+    :erlang.put(:elixir_compiler_dest, {path, force_load?})
     :elixir_compiler.file(file, &each_file(&1, &2, parent))
   end
 
@@ -582,7 +585,7 @@ defmodule Kernel.ParallelCompiler do
   end
 
   defp count_modules(result) do
-    Enum.count(result, &match?({{:module, _}, binary} when is_binary(binary), &1))
+    Enum.count(result, &match?({{:module, _}, {binary, _}} when is_binary(binary), &1))
   end
 
   defp each_cycle_return({kind, modules, warnings}), do: {kind, modules, warnings}
@@ -649,19 +652,37 @@ defmodule Kernel.ParallelCompiler do
           state
         )
 
-      {:module_available, child, ref, file, module, binary} ->
-        state.each_module.(file, module, binary)
+      {{:module_loaded, module}, _ref, _type, _pid, _reason} ->
+        result =
+          Map.update!(result, {:module, module}, fn {binary, _loader} -> {binary, true} end)
+
+        spawn_workers(queue, spawned, waiting, files, result, warnings, errors, state)
 
-        # Release the module loader which is waiting for an ack
+      {:module_available, child, ref, file, module, binary, loaded?} ->
+        state.each_module.(file, module, binary)
         send(child, {ref, :ack})
-        {available, result} = update_result(result, :module, module, binary)
+
+        {available, load_status} =
+          case Map.get(result, {:module, module}) do
+            [_ | _] = pids when loaded? ->
+              {Enum.map(pids, &{&1, :found}), loaded?}
+
+            # When compiling files to disk, we only load the module
+            # if other modules are waiting for it.
+            [_ | _] = pids ->
+              pid = load_module(module, binary, state.dest)
+              {Enum.map(pids, &{&1, {:loading, pid}}), pid}
+
+            _ ->
+              {[], loaded?}
+          end
 
         spawn_workers(
           available ++ queue,
           spawned,
           waiting,
           files,
-          result,
+          Map.put(result, {:module, module}, {binary, load_status}),
           warnings,
           errors,
           state
@@ -680,7 +701,9 @@ defmodule Kernel.ParallelCompiler do
 
         {waiting, files, result} =
           if not is_list(available_or_pending) or on in defining do
-            send(child_pid, {ref, :found})
+            # If what we are waiting on was defined but not loaded, we do it now.
+            {reply, result} = load_pending(kind, on, result, state)
+            send(child_pid, {ref, reply})
             {waiting, files, result}
           else
             waiting = Map.put(waiting, child_pid, {kind, ref, file_pid, on, defining, deadlock})
@@ -774,6 +797,55 @@ defmodule Kernel.ParallelCompiler do
     {{:error, Enum.reverse(errors, fun.()), info}, state}
   end
 
+  defp load_pending(kind, module, result, state) do
+    case result do
+      %{{:module, ^module} => {binary, load_status}}
+      when kind in [:module, :struct] and is_binary(binary) ->
+        case load_status do
+          true ->
+            {:found, result}
+
+          false ->
+            pid = load_module(module, binary, state.dest)
+            result = Map.put(result, {:module, module}, {binary, pid})
+            {{:loading, pid}, result}
+
+          pid when is_pid(pid) ->
+            {{:loading, pid}, result}
+        end
+
+      _ ->
+        {:found, result}
+    end
+  end
+
+  # We load modules in a separate process to avoid blocking
+  # the parallel compiler. We store the PID of this process and
+  # all entries monitor it to know once the module is loaded.
+  defp load_module(module, binary, dest) do
+    {pid, _ref} =
+      :erlang.spawn_opt(
+        fn ->
+          beam_location =
+            case dest do
+              nil ->
+                []
+
+              dest ->
+                :filename.join(
+                  :elixir_utils.characters_to_list(dest),
+                  Atom.to_charlist(module) ++ ~c".beam"
+                )
+            end
+
+          :code.load_binary(module, beam_location, binary)
+        end,
+        monitor: [tag: {:module_loaded, module}]
+      )
+
+    pid
+  end
+
   defp update_result(result, kind, module, value) do
     available =
       case Map.get(result, {kind, module}) do