add after_fork{} hook

process.c

@@ -3687,6 +3687,28 @@ rb_f_abort(int argc, VALUE *argv)
     UNREACHABLE;
 }
 
+
+/*
+ *  call-seq:
+ *     after_fork{ puts "Forked #{Process.pid} from #{Process.ppid}" } -> proc
+ *
+ *  Run block in child process after the VM forks.
+ */
+
+VALUE
+rb_f_after_fork(VALUE self)
+{
+    VALUE proc;
+    VALUE *hooks = &(GET_THREAD()->vm->after_fork);
+
+    rb_need_block();
+    proc = rb_block_proc();
+    if (!RTEST(*hooks)) *hooks = rb_ary_new();
+    rb_ary_push(*hooks, proc);
+    return proc;
+}
+
+
 void
 rb_syswait(rb_pid_t pid)
 {
@@ -7302,6 +7324,7 @@ Init_process(void)
     rb_define_global_function("sleep", rb_f_sleep, -1);
     rb_define_global_function("exit", rb_f_exit, -1);
     rb_define_global_function("abort", rb_f_abort, -1);
+    rb_define_global_function("after_fork", rb_f_after_fork, 0);
 
     rb_mProcess = rb_define_module("Process");
 

test/ruby/test_process.rb

@@ -686,6 +686,17 @@ def test_popen_fork
   rescue NotImplementedError
   end
 
+  def test_after_fork
+    assert_separately([], <<-END)
+    r, w = IO.pipe
+    after_fork{ w.write "Forked \#{Process.pid} from \#{Process.ppid}" }
+    Process.wait(child = fork{ exit })
+    w.close
+
+    assert_equal "Forked \#{child} from \#{Process.pid}", r.read
+    END
+  end
+
   def test_fd_inheritance
     skip "inheritance of fd other than stdin,stdout and stderr is not supported" if windows?
     with_pipe {|r, w|

thread.c

@@ -3915,11 +3915,18 @@ terminate_atfork_i(st_data_t key, st_data_t val, st_data_t current_th)
 void
 rb_thread_atfork(void)
 {
+    VALUE hooks = GET_THREAD()->vm->after_fork;
+    long i;
+
     rb_thread_atfork_internal(terminate_atfork_i);
     GET_THREAD()->join_list = NULL;
 
     /* We don't want reproduce CVE-2003-0900. */
     rb_reset_random_seed();
+
+    if (RTEST(hooks))
+	for (i=0; i<RARRAY_LEN(hooks); i++)
+	    rb_proc_call(RARRAY_AREF(hooks, i), rb_ary_new());
 }
 
 static int

vm.c

@@ -1607,6 +1607,7 @@ rb_vm_mark(void *ptr)
 	RUBY_MARK_UNLESS_NULL(vm->top_self);
 	RUBY_MARK_UNLESS_NULL(vm->coverages);
 	RUBY_MARK_UNLESS_NULL(vm->defined_module_hash);
+	RUBY_MARK_UNLESS_NULL(vm->after_fork);
 	rb_gc_mark_locations(vm->special_exceptions, vm->special_exceptions + ruby_special_error_count);
 
 	if (vm->loading_table) {

vm_core.h

@@ -412,7 +412,7 @@ typedef struct rb_vm_struct {
      * objects so do *NOT* mark this when you GC.
      */
     struct RArray at_exit;
-
+    VALUE after_fork;
     VALUE *defined_strings;
 
     /* params */