Fix sending backtraces across ractors

[Bug #21818]

Currently exceptions can be sent across ractors,
but because of a limitation in the TypedData API,
the exception backtrace is duped as an empty backtrace.

The problem is that backtraces are embedded objects,
hence the classic `rb_class_alloc(klass)` API is insufficient
because we need to know the size of the Backtrace object we're
duping to instantiate the copy.

This is worked around by changing Ractors to call `#clone` on objects
rather than use `rb_obj_clone`, and to implement `Thread::Backtrace#clone`
to properly clone the variable size object.

Author: byroot

bootstraptest/test_ractor.rb

@@ -507,7 +507,7 @@ def test n
 }
 
 # To copy the object, now Marshal#dump is used
-assert_match /can not copy unshareable object/, %q{
+assert_match /can't clone unshareable instance of Thread/, %q{
   obj = Thread.new{}
   begin
     r = Ractor.new obj do |msg|

defs/id.def

@@ -28,6 +28,7 @@ firstline, predefined = __LINE__+1, %[\
   send
   __send__
   __recursive_key__
+  clone
   initialize
   initialize_copy
   initialize_clone

depend

@@ -19980,6 +19980,7 @@ vm_backtrace.$(OBJEXT): $(top_srcdir)/internal/compilers.h
 vm_backtrace.$(OBJEXT): $(top_srcdir)/internal/error.h
 vm_backtrace.$(OBJEXT): $(top_srcdir)/internal/gc.h
 vm_backtrace.$(OBJEXT): $(top_srcdir)/internal/imemo.h
+vm_backtrace.$(OBJEXT): $(top_srcdir)/internal/object.h
 vm_backtrace.$(OBJEXT): $(top_srcdir)/internal/sanitizers.h
 vm_backtrace.$(OBJEXT): $(top_srcdir)/internal/serial.h
 vm_backtrace.$(OBJEXT): $(top_srcdir)/internal/set_table.h

ractor.c

@@ -2066,6 +2066,31 @@ ractor_move(VALUE obj)
     }
 }
 
+static VALUE
+ractor_call_clone_try(VALUE obj)
+{
+    return rb_funcall(obj, idClone, 0);
+}
+
+static VALUE
+ractor_call_clone_rescue(VALUE obj, VALUE exc)
+{
+    rb_raise(rb_eRactorError, "can't clone unshareable instance of %"PRIsVALUE, rb_class_of(obj));
+    UNREACHABLE_RETURN(Qnil);
+}
+
+static VALUE
+ractor_obj_clone(VALUE obj)
+{
+    VALUE clone = rb_rescue(ractor_call_clone_try, obj, ractor_call_clone_rescue, obj);
+
+    if (obj == clone) {
+        rb_raise(rb_eRactorError, "#clone returned self");
+    }
+
+    return clone;
+}
+
 static enum obj_traverse_iterator_result
 copy_enter(VALUE obj, struct obj_traverse_replace_data *data)
 {
@@ -2074,10 +2099,7 @@ copy_enter(VALUE obj, struct obj_traverse_replace_data *data)
         return traverse_skip;
     }
     else {
-        if (!rb_get_alloc_func(rb_obj_class(obj))) {
-            rb_raise(rb_eRactorError, "can not copy unshareable object %+"PRIsVALUE, obj);
-        }
-        data->replacement = rb_obj_clone(obj);
+        data->replacement = ractor_obj_clone(obj);
         return traverse_cont;
     }
 }

template/id.c.tmpl

@@ -1,5 +1,5 @@
 %#  -*- c -*-
-/* DO NOT EDIT THIS FILE DIRECTLY */
+/* DO NOT EDIT THIS FILE DIRECTLY: source is at template/id.c.tmpl */
 /**********************************************************************
 
   id.c -

test/ruby/test_ractor.rb

@@ -50,6 +50,57 @@ def x.to_s
     assert_unshareable(x, "can not make shareable object for #<Method: String#to_s()> because it refers unshareable objects", exception: Ractor::Error)
   end
 
+  def test_sending_exception_with_backtrace
+    assert_ractor(<<~'RUBY')
+      def build_error
+        raise "Test"
+      rescue => error
+        error
+      end
+
+      error = build_error
+      refute_empty error.backtrace
+      refute_empty error.backtrace_locations
+
+      backtrace, backtrace_locations = Ractor.new(error) do |error2|
+        [error2.backtrace, error2.backtrace_locations]
+      end.value
+
+      assert_equal error.backtrace, backtrace
+      refute_empty backtrace_locations
+    RUBY
+  end
+
+  def test_sending_exception_with_array_backtrace
+    assert_ractor(<<~'RUBY')
+      error = StandardError.new
+      error.set_backtrace(["foo", "bar"])
+      refute_empty error.backtrace
+      assert_nil error.backtrace_locations
+
+      backtrace, backtrace_locations = Ractor.new(error) do |error2|
+        [error2.backtrace, error2.backtrace_locations]
+      end.value
+
+      assert_equal error.backtrace, backtrace
+      assert_nil backtrace_locations
+    RUBY
+  end
+
+  def test_sending_object_with_broken_clone
+    assert_ractor(<<~'RUBY')
+      o = Object.new
+      def o.clone
+        self
+      end
+      ractor = Ractor.new { Ractor.receive }
+      error = assert_raise Ractor::Error do
+        ractor.send(o)
+      end
+      assert_match "#clone returned self", error.message
+    RUBY
+  end
+
   def test_default_thread_group
     assert_separately([], "#{<<~"begin;"}\n#{<<~'end;'}")
     begin;
@@ -219,7 +270,7 @@ def test_copy_unshareable_object_error_message
       err = assert_raise(Ractor::Error) do
         Ractor.new(pr) {}.join
       end
-      assert_match(/can not copy unshareable object/, err.message)
+      assert_match(/can not copy Proc object/, err.message)
     RUBY
   end
 

vm_backtrace.c

@@ -13,6 +13,7 @@
 #include "internal.h"
 #include "internal/class.h"
 #include "internal/error.h"
+#include "internal/object.h"
 #include "internal/vm.h"
 #include "iseq.h"
 #include "ruby/debug.h"
@@ -576,14 +577,6 @@ rb_backtrace_p(VALUE obj)
     return rb_typeddata_is_kind_of(obj, &backtrace_data_type);
 }
 
-static VALUE
-backtrace_alloc(VALUE klass)
-{
-    rb_backtrace_t *bt;
-    VALUE obj = TypedData_Make_Struct(klass, rb_backtrace_t, &backtrace_data_type, bt);
-    return obj;
-}
-
 static VALUE
 backtrace_alloc_capa(long num_frames, rb_backtrace_t **backtrace)
 {
@@ -962,6 +955,46 @@ backtrace_limit(VALUE self)
     return LONG2NUM(rb_backtrace_length_limit);
 }
 
+/* :nodoc: */
+static VALUE
+backtrace_clone(VALUE self)
+{
+    rb_backtrace_t *bt;
+    TypedData_Get_Struct(self, rb_backtrace_t, &backtrace_data_type, bt);
+
+    rb_backtrace_t *other_bt;
+    VALUE clone = backtrace_alloc_capa(bt->backtrace_size, &other_bt);
+
+    rb_obj_clone_setup(self, clone, Qfalse);
+
+    return clone;
+}
+
+/* :nodoc: */
+static VALUE
+backtrace_dup(VALUE self)
+{
+    rb_notimplement();
+
+    UNREACHABLE_RETURN(Qnil);
+}
+
+/* :nodoc: */
+static VALUE
+backtrace_initialize_copy(VALUE self, VALUE original)
+{
+    rb_backtrace_t *bt;
+    TypedData_Get_Struct(self, rb_backtrace_t, &backtrace_data_type, bt);
+
+    rb_backtrace_t *original_bt;
+    TypedData_Get_Struct(original, rb_backtrace_t, &backtrace_data_type, original_bt);
+
+    bt->backtrace_size = original_bt->backtrace_size;
+    MEMCPY(bt->backtrace, original_bt->backtrace, rb_backtrace_location_t, original_bt->backtrace_size);
+
+    return Qnil;
+}
+
 VALUE
 rb_ec_backtrace_str_ary(const rb_execution_context_t *ec, long lev, long n)
 {
@@ -1406,6 +1439,14 @@ each_caller_location(int argc, VALUE *argv, VALUE _)
     return Qnil;
 }
 
+static VALUE
+backtrace_no_allocator(VALUE klass)
+{
+    rb_notimplement();
+
+    UNREACHABLE_RETURN(Qnil);
+}
+
 /* called from Init_vm() in vm.c */
 void
 Init_vm_backtrace(void)
@@ -1416,11 +1457,18 @@ Init_vm_backtrace(void)
      *  settings of the current session.
      */
     rb_cBacktrace = rb_define_class_under(rb_cThread, "Backtrace", rb_cObject);
-    rb_define_alloc_func(rb_cBacktrace, backtrace_alloc);
-    rb_undef_method(CLASS_OF(rb_cBacktrace), "new");
+
+    // Can't undefine the allocator, as it's needed as a key by Marshal
+    rb_define_alloc_func(rb_cBacktrace, backtrace_no_allocator);
     rb_marshal_define_compat(rb_cBacktrace, rb_cArray, backtrace_dump_data, backtrace_load_data);
+
+    rb_undef_method(CLASS_OF(rb_cBacktrace), "new");
     rb_define_singleton_method(rb_cBacktrace, "limit", backtrace_limit, 0);
 
+    rb_define_method(rb_cBacktrace, "clone", backtrace_clone, 0);
+    rb_define_method(rb_cBacktrace, "dup", backtrace_dup, 0);
+    rb_define_method(rb_cBacktrace, "initialize_copy", backtrace_initialize_copy, 1);
+
     /*
      *	An object representation of a stack frame, initialized by
      *	Kernel#caller_locations.