Disallow forking from non-main ractor

[Bug #17516]

`fork(2)` only leave the calling thread alive in the child.
Because of this forking from the non-main ractor can easily
leave the VM in a corrupted state.

It may be possible in the future to carefully allow forking from non-main
Ractor, but shot term it's preferable to add this restriction.

common.mk

@@ -14057,6 +14057,7 @@ process.$(OBJEXT): {$(VPATH)}onigmo.h
 process.$(OBJEXT): {$(VPATH)}oniguruma.h
 process.$(OBJEXT): {$(VPATH)}process.c
 process.$(OBJEXT): {$(VPATH)}ractor.h
+process.$(OBJEXT): {$(VPATH)}ractor_core.h
 process.$(OBJEXT): {$(VPATH)}ruby_assert.h
 process.$(OBJEXT): {$(VPATH)}ruby_atomic.h
 process.$(OBJEXT): {$(VPATH)}rubyparser.h

process.c

@@ -114,6 +114,7 @@ int initgroups(const char *, rb_gid_t);
 #include "ruby/st.h"
 #include "ruby/thread.h"
 #include "ruby/util.h"
+#include "ractor_core.h"
 #include "vm_core.h"
 #include "vm_sync.h"
 #include "ruby/ractor.h"
@@ -4120,6 +4121,10 @@ rb_fork_async_signal_safe(int *status,
 rb_pid_t
 rb_fork_ruby(int *status)
 {
+    if (UNLIKELY(!rb_ractor_main_p())) {
+        rb_raise(rb_eRactorIsolationError, "can not fork from non-main Ractors");
+    }
+
     struct rb_process_status child = {.status = 0};
     rb_pid_t pid;
     int try_gc = 1, err = 0;

test/ruby/test_ractor.rb

@@ -99,6 +99,17 @@ class TestRactor < Test::Unit::TestCase
     RUBY
   end
 
+  def test_fork_raise_isolation_error
+    assert_ractor(<<~'RUBY')
+      ractor = Ractor.new do
+        Process.fork
+      rescue Ractor::IsolationError => e
+        e
+      end
+      assert_equal Ractor::IsolationError, ractor.value.class
+    RUBY
+  end if Process.respond_to?(:fork)
+
   def test_require_raises_and_no_ractor_belonging_issue
     assert_ractor(<<~'RUBY')
       require "tempfile"