Read the object_id of a shareable T_DATA without the VM lock

object_id took the VM lock for every shareable object in multi-Ractor mode, even
when the object already had an id.

Kernel#hash reaches object_id through rb_obj_hash, so a shareable T_DATA such as
an Encoding, a Ractor or a Ractor::Port used as a Hash key serialised every
lookup across Ractors.

In this commit we make generic_field_set copy the fields of a shareable object
in multi-Ractor mode, and publish the new fields_obj of a T_DATA with an atomic
write.

```ruby
t = Process.clock_gettime(Process::CLOCK_MONOTONIC)
ractors = 4.times.map do
  Ractor.new do
    h = { Encoding::UTF_8 => 1 }
    1_000_000.times { h[Encoding::UTF_8] }
  end
end
ractors.each(&:join)
elapsed = Process.clock_gettime(Process::CLOCK_MONOTONIC) - t
puts "#{elapsed.round(2)}s"
```

I ran this 3 times before this patch and after and saw

    before: 0.58s 0.61s 0.64s
    after:  0.05s 0.05s 0.05s

gc.c

@@ -2402,16 +2402,16 @@ class_object_id(VALUE klass)
     return id;
 }
 
+static inline shape_id_t
+object_id_field_shape(shape_id_t shape_id)
+{
+    return rb_shape_complex_p(shape_id) ? ROOT_COMPLEX_WITH_OBJ_ID : rb_shape_object_id(shape_id);
+}
+
 static inline VALUE
 object_id_get(VALUE obj, shape_id_t shape_id)
 {
-    VALUE id;
-    if (rb_shape_complex_p(shape_id)) {
-        id = rb_obj_field_get(obj, ROOT_COMPLEX_WITH_OBJ_ID);
-    }
-    else {
-        id = rb_obj_field_get(obj, rb_shape_object_id(shape_id));
-    }
+    VALUE id = rb_obj_field_get(obj, object_id_field_shape(shape_id));
 
 #if RUBY_DEBUG
     if (!(FIXNUM_P(id) || RB_TYPE_P(id, T_BIGNUM))) {
@@ -2443,6 +2443,20 @@ object_id0(VALUE obj)
     return id;
 }
 
+// generic_field_set never mutates the published fields_obj of a shareable object,
+// and the fields_obj carries its owner's shape, so both are read from the one snapshot.
+static VALUE
+shareable_data_object_id(VALUE obj)
+{
+    VALUE fields_obj = RUBY_ATOMIC_VALUE_LOAD(RTYPEDDATA(obj)->fields_obj);
+    if (!fields_obj) return Qfalse;
+
+    shape_id_t shape_id = RBASIC_SHAPE_ID(fields_obj);
+    if (!rb_shape_has_object_id(shape_id)) return Qfalse;
+
+    return rb_imemo_fields_field_get(fields_obj, object_id_field_shape(shape_id));
+}
+
 static VALUE
 object_id(VALUE obj)
 {
@@ -2461,6 +2475,11 @@ object_id(VALUE obj)
     }
 
     if (UNLIKELY(rb_gc_multi_ractor_p() && rb_ractor_shareable_p(obj))) {
+        if (BUILTIN_TYPE(obj) == T_DATA) {
+            VALUE existing_id = shareable_data_object_id(obj);
+            if (existing_id) return existing_id;
+        }
+
         unsigned int lock_lev = RB_GC_VM_LOCK();
         VALUE id = object_id0(obj);
         RB_GC_VM_UNLOCK(lock_lev);

internal/variable.h

@@ -49,6 +49,7 @@ VALUE rb_mod_set_temporary_name(VALUE, VALUE);
 void rb_obj_replace_fields(VALUE obj, VALUE fields_obj);
 VALUE rb_obj_complex_fields_build(VALUE obj);
 VALUE rb_obj_field_get(VALUE obj, shape_id_t target_shape_id);
+VALUE rb_imemo_fields_field_get(VALUE fields_obj, shape_id_t target_shape_id);
 void rb_ivar_set_internal(VALUE obj, ID id, VALUE val);
 void rb_check_ivar_modifiable(VALUE obj);
 void rb_ivar_foreach_buffered(VALUE obj, int (*func)(ID name, VALUE val, st_data_t arg), st_data_t arg);

variable.c

@@ -1511,7 +1511,7 @@ rb_obj_set_fields(VALUE obj, VALUE fields_obj, ID field_name, VALUE original_fie
             ROBJECT_SET_EXTENDED(obj, fields_obj);
             break;
           case T_DATA:
-            RB_OBJ_WRITE(obj, &RTYPEDDATA(obj)->fields_obj, fields_obj);
+            RB_OBJ_ATOMIC_WRITE(obj, &RTYPEDDATA(obj)->fields_obj, fields_obj);
             break;
           case T_STRUCT:
             RSTRUCT_SET_FIELDS_OBJ(obj, fields_obj);
@@ -1577,13 +1577,10 @@ rb_obj_replace_fields(VALUE obj, VALUE fields_obj)
 }
 
 VALUE
-rb_obj_field_get(VALUE obj, shape_id_t target_shape_id)
+rb_imemo_fields_field_get(VALUE fields_obj, shape_id_t target_shape_id)
 {
-    RUBY_ASSERT(!SPECIAL_CONST_P(obj));
     RUBY_ASSERT(RSHAPE_TYPE_P(target_shape_id, SHAPE_IVAR) || RSHAPE_TYPE_P(target_shape_id, SHAPE_OBJ_ID));
 
-    VALUE fields_obj = rb_obj_fields(obj, RSHAPE_EDGE_NAME(target_shape_id));
-
     if (UNLIKELY(rb_shape_complex_p(target_shape_id))) {
         st_table *fields_hash = rb_imemo_fields_complex_tbl(fields_obj);
         VALUE value = Qundef;
@@ -1596,6 +1593,14 @@ rb_obj_field_get(VALUE obj, shape_id_t target_shape_id)
     return rb_imemo_fields_ptr(fields_obj)[index];
 }
 
+VALUE
+rb_obj_field_get(VALUE obj, shape_id_t target_shape_id)
+{
+    RUBY_ASSERT(!SPECIAL_CONST_P(obj));
+
+    return rb_imemo_fields_field_get(rb_obj_fields(obj, RSHAPE_EDGE_NAME(target_shape_id)), target_shape_id);
+}
+
 VALUE
 rb_ivar_lookup(VALUE obj, ID id, VALUE undef)
 {
@@ -1964,7 +1969,9 @@ generic_field_set(VALUE obj, shape_id_t target_shape_id, ID field_name, VALUE va
     }
 
     const VALUE original_fields_obj = rb_obj_fields(obj, field_name);
-    VALUE fields_obj = imemo_fields_set(obj, original_fields_obj, target_shape_id, field_name, val, false);
+    // Readers of a shareable object's fields take no lock, so never mutate a published fields_obj.
+    bool concurrent = rb_multi_ractor_p() && RB_OBJ_SHAREABLE_P(obj);
+    VALUE fields_obj = imemo_fields_set(obj, original_fields_obj, target_shape_id, field_name, val, concurrent);
 
     rb_obj_set_fields(obj, fields_obj, field_name, original_fields_obj);
     return rb_shape_complex_p(target_shape_id) ? ATTR_INDEX_NOT_SET : RSHAPE_INDEX(target_shape_id);