Make ForkTracker.check! a noop on Ruby 3.1+

Fix: rails#47418

We should trust the `Process._fork` callback, as it can't really
be bypassed except throught `Process.daemonize` but I don't think
it's a problem.

We initially kept the PID check even if the callback was present,
but it turns out that on modern GLIBC the getpid() syscall isn't
cached and it causes a significant overhead.

Author: byroot

activesupport/lib/active_support/fork_tracker.rb

@@ -6,7 +6,7 @@ module ModernCoreExt
       def _fork
         pid = super
         if pid == 0
-          ForkTracker.check!
+          ForkTracker.after_fork_callback
         end
         pid
       end
@@ -37,14 +37,22 @@ module CoreExtPrivate
     @callbacks = []
 
     class << self
-      def check!
+      def after_fork_callback
         new_pid = Process.pid
         if @pid != new_pid
           @callbacks.each(&:call)
           @pid = new_pid
         end
       end
 
+      if Process.respond_to?(:_fork) # Ruby 3.1+
+        def check!
+          # We trust the `_fork` callback
+        end
+      else
+        alias_method :check!, :after_fork_callback
+      end
+
       def hook!
         if Process.respond_to?(:_fork) # Ruby 3.1+
           ::Process.singleton_class.prepend(ModernCoreExt)

activesupport/test/fork_tracker_test.rb

@@ -160,15 +160,27 @@ def test_check
       3.times { ActiveSupport::ForkTracker.check! }
     end
 
-    Process.stub(:pid, Process.pid + 1) do
+    if Process.respond_to?(:_fork)
+      Process.stub(:pid, Process.pid + 1) do
+        assert_no_difference -> { count } do
+          3.times { ActiveSupport::ForkTracker.check! }
+        end
+
+        assert_no_difference -> { count } do
+          3.times { ActiveSupport::ForkTracker.check! }
+        end
+      end
+    else
+      Process.stub(:pid, Process.pid + 1) do
+        assert_difference -> { count }, +1 do
+          3.times { ActiveSupport::ForkTracker.check! }
+        end
+      end
+
       assert_difference -> { count }, +1 do
         3.times { ActiveSupport::ForkTracker.check! }
       end
     end
-
-    assert_difference -> { count }, +1 do
-      3.times { ActiveSupport::ForkTracker.check! }
-    end
   ensure
     ActiveSupport::ForkTracker.unregister(handler)
   end