Fix live object count for multi-Ractor forking

Since we do not run a Ractor barrier before forking, it's possible that
another other Ractor is halfway through allocating an object during forking.
This may lead to allocated_objects_count being off by one.

For example, the following script reproduces the bug:

    100.times do |i|
      Ractor.new(i) do |j|
        10000.times do |i|
          "#{j}-#{i}"
        end
        Ractor.receive
      end
      pid = fork { GC.verify_internal_consistency }
      _, status = Process.waitpid2 pid
      raise unless status.success?
    end

We need to run with `taskset -c 1` to force it to use a single CPU core
to more consistenly reproduce the bug:

    heap_pages_final_slots: 1, total_freed_objects: 16628
    test.rb:8: [BUG] inconsistent live slot number: expect 19589, but 19588.
    ruby 4.0.0dev (2025-11-25T03:06:55Z master 55892f5994) +PRISM [x86_64-linux]

    -- Control frame information -----------------------------------------------
    c:0007 p:---- s:0029 e:000028 l:y b:---- CFUNC  :verify_internal_consistency
    c:0006 p:0004 s:0025 e:000024 l:n b:---- BLOCK  test.rb:8 [FINISH]
    c:0005 p:---- s:0022 e:000021 l:y b:---- CFUNC  :fork
    c:0004 p:0012 s:0018 E:0014c0 l:n b:---- BLOCK  test.rb:8
    c:0003 p:0024 s:0011 e:000010 l:y b:0001 METHOD <internal:numeric>:257
    c:0002 p:0005 s:0006 E:001730 l:n b:---- EVAL   test.rb:1 [FINISH]
    c:0001 p:0000 s:0003 E:001d20 l:y b:---- DUMMY  [FINISH]

    -- Ruby level backtrace information ----------------------------------------
    test.rb:1:in '<main>'
    <internal:numeric>:257:in 'times'
    test.rb:8:in 'block in <main>'
    test.rb:8:in 'fork'
    test.rb:8:in 'block (2 levels) in <main>'
    test.rb:8:in 'verify_internal_consistency'

    -- Threading information ---------------------------------------------------
    Total ractor count: 1
    Ruby thread count for this ractor: 1

    -- C level backtrace information -------------------------------------------
    ruby(rb_print_backtrace+0x14) [0x61b67ac48b60] vm_dump.c:1105
    ruby(rb_vm_bugreport) vm_dump.c:1450
    ruby(rb_bug_without_die_internal+0x5f) [0x61b67a818a28] error.c:1098
    ruby(rb_bug) error.c:1116
    ruby(gc_verify_internal_consistency_+0xbdd) [0x61b67a83d8ed] gc/default/default.c:5186
    ruby(gc_verify_internal_consistency+0x2d) [0x61b67a83d960] gc/default/default.c:5241
    ruby(rb_gc_verify_internal_consistency) gc/default/default.c:8950
    ruby(gc_verify_internal_consistency_m) gc/default/default.c:8966
    ruby(vm_call_cfunc_with_frame_+0x10d) [0x61b67a9e50fd] vm_insnhelper.c:3902
    ruby(vm_sendish+0x111) [0x61b67a9eeaf1] vm_insnhelper.c:6124
    ruby(vm_exec_core+0x84) [0x61b67aa07434] insns.def:903
    ruby(vm_exec_loop+0xa) [0x61b67a9f8155] vm.c:2811
    ruby(rb_vm_exec) vm.c:2787
    ruby(vm_yield_with_cref+0x90) [0x61b67a9fd2ea] vm.c:1865
    ruby(vm_yield) vm.c:1873
    ruby(rb_yield) vm_eval.c:1362
    ruby(rb_protect+0xef) [0x61b67a81fe6f] eval.c:1154
    ruby(rb_f_fork+0x16) [0x61b67a8e98ab] process.c:4293
    ruby(rb_f_fork) process.c:4284

2 files changed, 32 insertions, 1 deletions

diff --git a/bootstraptest/test_ractor.rb b/bootstraptest/test_ractor.rb
index ed80dd0862..6c3a45148c 100644
--- a/bootstraptest/test_ractor.rb
+++ b/bootstraptest/test_ractor.rb
@@ -2370,3 +2370,24 @@ assert_equal 'ok', <<~'RUBY'
     :ok
   end
 RUBY
+
+assert_equal 'ok', <<~'RUBY'
+  begin
+    100.times do |i|
+      Ractor.new(i) do |j|
+        1000.times do |i|
+          "#{j}-#{i}"
+        end
+      end
+      pid = fork do
+        GC.verify_internal_consistency
+      end
+      _, status = Process.waitpid2 pid
+      raise unless status.success?
+    end
+
+    :ok
+  rescue NotImplementedError
+    :ok
+  end
+RUBY
diff --git a/gc/default/default.c b/gc/default/default.c
index b6a9dfa1f5..9a2efab562 100644
--- a/gc/default/default.c
+++ b/gc/default/default.c
@@ -622,6 +622,8 @@ typedef struct rb_objspace {
     rb_postponed_job_handle_t finalize_deferred_pjob;
 
     unsigned long live_ractor_cache_count;
+
+    int fork_vm_lock_lev;
 } rb_objspace_t;
 
 #ifndef HEAP_PAGE_ALIGN_LOG
@@ -9324,12 +9326,20 @@ gc_malloc_allocations(VALUE self)
 void
 rb_gc_impl_before_fork(void *objspace_ptr)
 {
-    /* no-op */
+    rb_objspace_t *objspace = objspace_ptr;
+
+    objspace->fork_vm_lock_lev = RB_GC_VM_LOCK();
+    rb_gc_vm_barrier();
 }
 
 void
 rb_gc_impl_after_fork(void *objspace_ptr, rb_pid_t pid)
 {
+    rb_objspace_t *objspace = objspace_ptr;
+
+    RB_GC_VM_UNLOCK(objspace->fork_vm_lock_lev);
+    objspace->fork_vm_lock_lev = 0;
+
     if (pid == 0) { /* child process */
         rb_gc_ractor_newobj_cache_foreach(gc_ractor_newobj_cache_clear, NULL);
     }