Parallelize log dispatch on CaptureLog, closes #14288

Author: josevalim

lib/ex_unit/lib/ex_unit/capture_server.ex

@@ -231,18 +231,29 @@ defmodule ExUnit.CaptureServer do
   ## :logger handler callback.
 
   def log(event, _config) do
-    for {_ref, string_io, level, formatter_mod, formatter_config} <- :ets.tab2list(@ets),
-        :logger.compare_levels(event.level, level) in [:gt, :eq] do
-      chardata = formatter_mod.format(event, formatter_config)
-      # There is a race condition where the capture_log is removed
-      # but another process is attempting to log to string io device
-      # that no longer exists, so we wrap it in try/catch.
-      try do
-        IO.write(string_io, chardata)
-      rescue
-        _ -> :ok
+    :ets.tab2list(@ets)
+    |> Enum.filter(fn {_ref, _string_io, level, _formatter_mod, _formatter_config} ->
+      :logger.compare_levels(event.level, level) in [:gt, :eq]
+    end)
+    |> Enum.group_by(
+      fn {_ref, _string_io, _level, formatter_mod, formatter_config} ->
+        {formatter_mod, formatter_config}
+      end,
+      fn {_ref, string_io, _level, _formatter_mod, _formatter_config} ->
+        string_io
       end
-    end
+    )
+    |> Enum.map(fn {{formatter_mod, formatter_config}, string_ios} ->
+      Task.async(fn ->
+        chardata = formatter_mod.format(event, formatter_config)
+
+        # Simply send, do not wait for reply
+        for string_io <- string_ios do
+          send(string_io, {:io_request, self(), make_ref(), {:put_chars, :unicode, chardata}})
+        end
+      end)
+    end)
+    |> Task.await_many(:infinity)
 
     :ok
   end