Make setting and accessing class ivars lock-free

Now that class fields have been deletated to a T_IMEMO/class_fields
when we're in multi-ractor mode, we can read and write class instance
variable in an atomic way using Read-Copy-Update (RCU).

Note when in multi-ractor mode, we always use RCU. In theory
we don't need to, instead if we ensured the field is written
before the shape is updated it would be safe.

Benchmark:

```ruby
Warning[:experimental] = false

class Foo
  @foo = 1
  @bar = 2
  @baz = 3
  @egg = 4
  @spam = 5

  class << self
    attr_reader :foo, :bar, :baz, :egg, :spam
  end
end

ractors = 8.times.map do
  Ractor.new do
    1_000_000.times do
      Foo.bar + Foo.baz * Foo.egg - Foo.spam
    end
  end
end

if Ractor.method_defined?(:value)
  ractors.each(&:value)
else
  ractors.each(&:take)
end
```

This branch vs Ruby 3.4:

```bash
$ hyperfine -w 1 'ruby --disable-all ../test.rb' './miniruby ../test.rb'

Benchmark 1: ruby --disable-all ../test.rb
  Time (mean ± σ):      3.162 s ±  0.071 s    [User: 2.783 s, System: 10.809 s]
  Range (min … max):    3.093 s …  3.337 s    10 runs

Benchmark 2: ./miniruby ../test.rb
  Time (mean ± σ):     208.7 ms ±   4.6 ms    [User: 889.7 ms, System: 6.9 ms]
  Range (min … max):   202.8 ms … 222.0 ms    14 runs

Summary
  ./miniruby ../test.rb ran
   15.15 ± 0.47 times faster than ruby --disable-all ../test.rb
```

imemo.c

@@ -156,7 +156,6 @@ rb_imemo_class_fields_clone(VALUE fields_obj)
     if (rb_shape_too_complex_p(shape_id)) {
         clone = rb_imemo_class_fields_new_complex(CLASS_OF(fields_obj), 0);
         RBASIC_SET_SHAPE_ID(clone, shape_id);
-
         st_table *src_table = rb_imemo_class_fields_complex_tbl(fields_obj);
         st_replace(rb_imemo_class_fields_complex_tbl(clone), src_table);
     }

internal/class.h

@@ -531,13 +531,6 @@ RCLASS_WRITABLE_ENSURE_FIELDS_OBJ(VALUE obj)
     return ext->fields_obj;
 }
 
-static inline void
-RCLASSEXT_SET_FIELDS_OBJ(VALUE obj, rb_classext_t *ext, VALUE fields_obj)
-{
-    RUBY_ASSERT(RB_TYPE_P(obj, RUBY_T_CLASS) || RB_TYPE_P(obj, RUBY_T_MODULE));
-    RB_OBJ_WRITE(obj, &ext->fields_obj, fields_obj);
-}
-
 static inline VALUE
 RCLASS_WRITABLE_FIELDS_OBJ(VALUE obj)
 {
@@ -545,6 +538,16 @@ RCLASS_WRITABLE_FIELDS_OBJ(VALUE obj)
     return RCLASSEXT_FIELDS_OBJ(RCLASS_EXT_WRITABLE(obj));
 }
 
+static inline void
+RCLASSEXT_SET_FIELDS_OBJ(VALUE obj, rb_classext_t *ext, VALUE fields_obj)
+{
+    RUBY_ASSERT(RB_TYPE_P(obj, RUBY_T_CLASS) || RB_TYPE_P(obj, RUBY_T_MODULE));
+
+    VALUE old_fields_obj = ext->fields_obj;
+    RUBY_ATOMIC_VALUE_SET(ext->fields_obj, fields_obj);
+    RB_OBJ_WRITTEN(obj, old_fields_obj, fields_obj);
+}
+
 static inline void
 RCLASS_WRITABLE_SET_FIELDS_OBJ(VALUE obj, VALUE fields_obj)
 {

test/ruby/test_ractor.rb

@@ -79,6 +79,26 @@ class TestRactor < Test::Unit::TestCase
     end;
   end
 
+  def test_class_instance_variables
+    assert_ractor(<<~'RUBY')
+      # Once we're in multi-ractor mode, the codepaths
+      # for class instance variables are a bit different.
+      Ractor.new {}.value
+
+      class TestClass
+        @a = 1
+        @b = 2
+        @c = 3
+        @d = 4
+      end
+
+      assert_equal 4, TestClass.remove_instance_variable(:@d)
+      assert_nil TestClass.instance_variable_get(:@d)
+      assert_equal 4, TestClass.instance_variable_set(:@d, 4)
+      assert_equal 4, TestClass.instance_variable_get(:@d)
+    RUBY
+  end
+
   def test_require_raises_and_no_ractor_belonging_issue
     assert_ractor(<<~'RUBY')
       require "tempfile"

variable.c

@@ -1371,16 +1371,18 @@ rb_ivar_lookup(VALUE obj, ID id, VALUE undef)
 {
     if (SPECIAL_CONST_P(obj)) return undef;
 
-    if (BUILTIN_TYPE(obj) == T_CLASS || BUILTIN_TYPE(obj) == T_MODULE) {
+    shape_id_t shape_id;
-        VALUE val = undef;
+    VALUE *ivar_list;
-        RB_VM_LOCK_ENTER();
+
+    switch (BUILTIN_TYPE(obj)) {
+      case T_CLASS:
+      case T_MODULE:
         {
+            VALUE val = undef;
             VALUE fields_obj = RCLASS_WRITABLE_FIELDS_OBJ(obj);
             if (fields_obj) {
                 val = rb_ivar_lookup(fields_obj, id, undef);
             }
-        }
-        RB_VM_LOCK_LEAVE();
 
             if (val != undef &&
                     rb_is_instance_id(id) &&
@@ -1391,24 +1393,14 @@ rb_ivar_lookup(VALUE obj, ID id, VALUE undef)
             }
             return val;
         }
-
-    shape_id_t shape_id;
-    VALUE * ivar_list;
-    shape_id = RBASIC_SHAPE_ID(obj);
-
-    switch (BUILTIN_TYPE(obj)) {
-      case T_CLASS:
-      case T_MODULE:
-        {
-            rb_bug("Unreachable");
-        }
       case T_IMEMO:
         // Handled like T_OBJECT
         {
             RUBY_ASSERT(IMEMO_TYPE_P(obj, imemo_class_fields));
+            shape_id = RBASIC_SHAPE_ID(obj);
 
             if (rb_shape_too_complex_p(shape_id)) {
-                st_table * iv_table = rb_imemo_class_fields_complex_tbl(obj);
+                st_table *iv_table = rb_imemo_class_fields_complex_tbl(obj);
                 VALUE val;
                 if (rb_st_lookup(iv_table, (st_data_t)id, (st_data_t *)&val)) {
                     return val;
@@ -1424,8 +1416,9 @@ rb_ivar_lookup(VALUE obj, ID id, VALUE undef)
         }
       case T_OBJECT:
         {
+            shape_id = RBASIC_SHAPE_ID(obj);
             if (rb_shape_too_complex_p(shape_id)) {
-                st_table * iv_table = ROBJECT_FIELDS_HASH(obj);
+                st_table *iv_table = ROBJECT_FIELDS_HASH(obj);
                 VALUE val;
                 if (rb_st_lookup(iv_table, (st_data_t)id, (st_data_t *)&val)) {
                     return val;
@@ -1440,6 +1433,7 @@ rb_ivar_lookup(VALUE obj, ID id, VALUE undef)
             break;
         }
       default:
+        shape_id = RBASIC_SHAPE_ID(obj);
         if (FL_TEST_RAW(obj, FL_EXIVAR)) {
             struct gen_fields_tbl *fields_tbl;
             rb_gen_fields_tbl_get(obj, id, &fields_tbl);
@@ -1494,13 +1488,16 @@ rb_ivar_delete(VALUE obj, ID id, VALUE undef)
 
         VALUE fields_obj = RCLASS_WRITABLE_FIELDS_OBJ(obj);
         if (fields_obj) {
-            RB_VM_LOCK_ENTER();
+            if (rb_multi_ractor_p()) {
-            {
+                fields_obj = rb_imemo_class_fields_clone(fields_obj);
+                val = rb_ivar_delete(fields_obj, id, undef);
+                RCLASS_WRITABLE_SET_FIELDS_OBJ(obj, fields_obj);
+            }
+            else {
                 val = rb_ivar_delete(fields_obj, id, undef);
             }
-            RB_VM_LOCK_LEAVE();
-            return val;
         }
+        return val;
     }
 
     shape_id_t old_shape_id = rb_obj_shape_id(obj);
@@ -2127,8 +2124,6 @@ rb_ivar_set_internal(VALUE obj, ID id, VALUE val)
     ivar_set(obj, id, val);
 }
 
-static void class_field_set(VALUE obj, shape_id_t target_shape_id, VALUE val);
-
 void
 rb_obj_field_set(VALUE obj, shape_id_t target_shape_id, VALUE val)
 {
@@ -2138,8 +2133,8 @@ rb_obj_field_set(VALUE obj, shape_id_t target_shape_id, VALUE val)
         break;
       case T_CLASS:
       case T_MODULE:
-        ASSERT_vm_locking();
+        // The only field is object_id and T_CLASS handle it differently.
-        class_field_set(obj, target_shape_id, val);
+        rb_bug("Unreachable");
         break;
       default:
         generic_field_set(obj, target_shape_id, val);
@@ -2199,7 +2194,7 @@ rb_ivar_defined(VALUE obj, ID id)
     switch (BUILTIN_TYPE(obj)) {
       case T_CLASS:
       case T_MODULE:
-        RB_VM_LOCKING() {
+        {
             VALUE fields_obj = RCLASS_WRITABLE_FIELDS_OBJ(obj);
             if (fields_obj) {
                 defined = ivar_defined0(fields_obj, id);
@@ -2452,8 +2447,8 @@ rb_field_foreach(VALUE obj, rb_ivar_foreach_callback_func *func, st_data_t arg,
         break;
       case T_CLASS:
       case T_MODULE:
+        {
             IVAR_ACCESSOR_SHOULD_BE_MAIN_RACTOR(0);
-        RB_VM_LOCKING() {
             VALUE fields_obj = RCLASS_WRITABLE_FIELDS_OBJ(obj);
             if (fields_obj) {
                 class_fields_each(fields_obj, func, arg, ivar_only);
@@ -4701,15 +4696,16 @@ rb_iv_set(VALUE obj, const char *name, VALUE val)
     return rb_ivar_set(obj, id, val);
 }
 
-static int
+static bool
-class_ivar_set(VALUE obj, ID id, VALUE val)
+class_fields_ivar_set(VALUE klass, VALUE fields_obj, ID id, VALUE val, bool concurrent, VALUE *new_fields_obj)
 {
     bool existing = true;
-    const VALUE original_fields_obj = RCLASS_WRITABLE_FIELDS_OBJ(obj);
+    const VALUE original_fields_obj = fields_obj;
-    VALUE fields_obj = original_fields_obj ? original_fields_obj : rb_imemo_class_fields_new(obj, 1);
+    fields_obj = original_fields_obj ? original_fields_obj : rb_imemo_class_fields_new(klass, 1);
 
-    shape_id_t next_shape_id = 0;
     shape_id_t current_shape_id = RBASIC_SHAPE_ID(fields_obj);
+    shape_id_t next_shape_id = current_shape_id;
+
     if (UNLIKELY(rb_shape_too_complex_p(current_shape_id))) {
         goto too_complex;
     }
@@ -4726,7 +4722,7 @@ class_ivar_set(VALUE obj, ID id, VALUE val)
         next_shape_id = rb_shape_transition_add_ivar(fields_obj, id);
         if (UNLIKELY(rb_shape_too_complex_p(next_shape_id))) {
             attr_index_t current_len = RSHAPE_LEN(current_shape_id);
-            fields_obj = rb_imemo_class_fields_new_complex(obj, current_len + 1);
+            fields_obj = rb_imemo_class_fields_new_complex(klass, current_len + 1);
             if (current_len) {
                 rb_obj_copy_fields_to_hash_table(original_fields_obj, rb_imemo_class_fields_complex_tbl(fields_obj));
                 RBASIC_SET_SHAPE_ID(fields_obj, next_shape_id);
@@ -4737,10 +4733,12 @@ class_ivar_set(VALUE obj, ID id, VALUE val)
         attr_index_t next_capacity = RSHAPE_CAPACITY(next_shape_id);
         attr_index_t current_capacity = RSHAPE_CAPACITY(current_shape_id);
 
-        if (UNLIKELY(next_capacity != current_capacity)) {
+        if (concurrent || next_capacity != current_capacity) {
-            RUBY_ASSERT(next_capacity > current_capacity);
+            RUBY_ASSERT(concurrent || next_capacity > current_capacity);
-            // We allocate a new fields_obj so that we're embedded as long as possible
+
-            fields_obj = rb_imemo_class_fields_new(obj, next_capacity);
+            // We allocate a new fields_obj even when concurrency isn't a concern
+            // so that we're embedded as long as possible.
+            fields_obj = rb_imemo_class_fields_new(klass, next_capacity);
             if (original_fields_obj) {
                 MEMCPY(rb_imemo_class_fields_ptr(fields_obj), rb_imemo_class_fields_ptr(original_fields_obj), VALUE, RSHAPE_LEN(current_shape_id));
             }
@@ -4752,20 +4750,12 @@ class_ivar_set(VALUE obj, ID id, VALUE val)
 
     VALUE *fields = rb_imemo_class_fields_ptr(fields_obj);
     RB_OBJ_WRITE(fields_obj, &fields[index], val);
+
     if (!existing) {
         RBASIC_SET_SHAPE_ID(fields_obj, next_shape_id);
     }
 
-    if (fields_obj != original_fields_obj) {
+    *new_fields_obj = fields_obj;
-        RCLASS_WRITABLE_SET_FIELDS_OBJ(obj, fields_obj);
-        // TODO: What should we set as the T_CLASS shape_id?
-        // In most case we can replicate the single `fields_obj` shape
-        // but in namespaced case?
-        // Perhaps INVALID_SHAPE_ID?
-        RBASIC_SET_SHAPE_ID(obj, next_shape_id);
-    }
-
-    RB_GC_GUARD(fields_obj);
     return existing;
 
 too_complex:
@@ -4776,15 +4766,10 @@ too_complex:
 
         if (fields_obj != original_fields_obj) {
             RBASIC_SET_SHAPE_ID(fields_obj, next_shape_id);
-            RCLASS_WRITABLE_SET_FIELDS_OBJ(obj, fields_obj);
-            // TODO: What should we set as the T_CLASS shape_id?
-            // In most case we can replicate the single `fields_obj` shape
-            // but in namespaced case?
-            // Perhaps INVALID_SHAPE_ID?
-            RBASIC_SET_SHAPE_ID(obj, next_shape_id);
         }
     }
-    RB_GC_GUARD(fields_obj);
+
+    *new_fields_obj = fields_obj;
     return existing;
 }
 
@@ -4792,25 +4777,27 @@ int
 rb_class_ivar_set(VALUE obj, ID id, VALUE val)
 {
     RUBY_ASSERT(RB_TYPE_P(obj, T_CLASS) || RB_TYPE_P(obj, T_MODULE));
-    bool existing = false;
     rb_check_frozen(obj);
 
     rb_class_ensure_writable(obj);
 
-    RB_VM_LOCKING() {
+    const VALUE original_fields_obj = RCLASS_WRITABLE_FIELDS_OBJ(obj);
-        existing = class_ivar_set(obj, id, val);
+    VALUE new_fields_obj = 0;
+
+    bool existing = class_fields_ivar_set(obj, original_fields_obj, id, val, rb_multi_ractor_p(), &new_fields_obj);
+
+    if (new_fields_obj != original_fields_obj) {
+        RCLASS_WRITABLE_SET_FIELDS_OBJ(obj, new_fields_obj);
+
+        // TODO: What should we set as the T_CLASS shape_id?
+        // In most case we can replicate the single `fields_obj` shape
+        // but in namespaced case?
+        // Perhaps INVALID_SHAPE_ID?
+        RBASIC_SET_SHAPE_ID(obj, RBASIC_SHAPE_ID(new_fields_obj));
     }
-
     return existing;
 }
 
-static void
-class_field_set(VALUE obj, shape_id_t target_shape_id, VALUE val)
-{
-    RUBY_ASSERT(RB_TYPE_P(obj, T_CLASS) || RB_TYPE_P(obj, T_MODULE));
-    obj_field_set(RCLASS_WRITABLE_ENSURE_FIELDS_OBJ(obj), target_shape_id, val);
-}
-
 static int
 tbl_copy_i(ID key, VALUE val, st_data_t dest)
 {