Let on_dir_autoloaded be reentrant

lib/zeitwerk/loader.rb

@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 
+require "monitor"
 require "set"
 
 module Zeitwerk
@@ -91,9 +92,9 @@ class Loader
     attr_reader :mutex
     private :mutex
 
-    # @sig Mutex
-    attr_reader :mutex2
-    private :mutex2
+    # @sig Monitor
+    attr_reader :dirs_autoload_monitor
+    private :dirs_autoload_monitor
 
     def initialize
       super
@@ -103,11 +104,12 @@ def initialize
       @to_unload       = {}
       @namespace_dirs  = Hash.new { |h, cpath| h[cpath] = [] }
       @shadowed_files  = Set.new
-      @mutex           = Mutex.new
-      @mutex2          = Mutex.new
       @setup           = false
       @eager_loaded    = false
 
+      @mutex = Mutex.new
+      @dirs_autoload_monitor = Monitor.new
+
       Registry.register_loader(self)
     end
 

lib/zeitwerk/loader/callbacks.rb

@@ -44,7 +44,7 @@ def on_dir_autoloaded(dir)
     # That not only would reassign the constant (undesirable per se) but, worse,
     # the module object created by t2 wouldn't have any of the autoloads for its
     # children, since t1 would have correctly deleted its namespace_dirs entry.
-    mutex2.synchronize do
+    dirs_autoload_monitor.synchronize do
       if cref = autoloads.delete(dir)
         autovivified_module = cref[0].const_set(cref[1], Module.new)
         cpath = autovivified_module.name

test/lib/zeitwerk/test_on_load.rb

@@ -191,4 +191,15 @@ class TestOnLoad < LoaderTest
       assert loader.send(:on_load_callbacks).empty?
     end
   end
+
+  test "on_load is reentrant for implicit namespaces" do
+    files = [["m/x.rb", "M::X = true"], ["n/x.rb", "N::X = true"]]
+    with_setup(files) do
+      loaded = false
+      loader.on_load("M") { loaded = N }
+
+      assert M
+      assert loaded
+    end
+  end
 end