Add constant redefinition tracepoint.

Allow an interested party (like a JIT compiler) to be notified if/when a
constant is redefined.

The added tracepoint returns the class, variable, and new value of the
constant.

(cherry picked from commit 93bcbc3)

include/ruby/ruby.h

@@ -2065,6 +2065,7 @@ int ruby_native_thread_p(void);
 #define RUBY_EVENT_THREAD_END           0x0800
 #define RUBY_EVENT_FIBER_SWITCH         0x1000
 #define RUBY_EVENT_BASIC_OP_REDEFINED   0x2000
+#define RUBY_EVENT_CONSTANT_REDEFINED   0x4000
 #define RUBY_EVENT_TRACEPOINT_ALL       0xffff
 
 /* special events */

test/ruby/test_settracefunc.rb

@@ -1,6 +1,10 @@
 # frozen_string_literal: false
 require 'test/unit'
 
+module ModuleForConst 
+   P = 0
+end
+
 class TestSetTraceFunc < Test::Unit::TestCase
   def setup
     @original_compile_option = RubyVM::InstructionSequence.compile_option
@@ -1617,4 +1621,16 @@ def +(s)
      assert_equal events[0][1][:klass], String
      assert_equal events[0][1][:bop], :BOP_PLUS
   end
+
+  def test_constant_redefinition_tracepoint
+     events = []
+     TracePoint.new(:constant_redefined) { |tp|
+        next if !target_thread?
+        events << [tp.event, tp.constant_redefined ]
+     }.enable { 
+       ModuleForConst.const_set('P', 1)  
+     }
+     assert_equal events.length, 1, "Events #{events}"
+     assert_equal events[0][0], :constant_redefined
+  end
 end

variable.c

@@ -19,6 +19,7 @@
 #include "id.h"
 #include "ccan/list/list.h"
 #include "id_table.h"
+#include "vm_core.h"
 
 struct rb_id_table *rb_global_tbl;
 static ID autoload, classpath, tmp_classpath, classid;
@@ -2653,6 +2654,7 @@ const_tbl_update(struct autoload_const_set_args *args)
 		rb_compile_warn(RSTRING_PTR(ce->file), ce->line,
 				"previous definition of %"PRIsVALUE" was here", name);
 	    }
+            EXEC_EVENT_HOOK(GET_THREAD(), RUBY_EVENT_CONSTANT_REDEFINED, klass, 0, 0, ID2SYM(id), val);
 	}
 	rb_clear_constant_cache();
 	setup_const_entry(ce, klass, val, visibility);

vm_trace.c

@@ -599,6 +599,7 @@ get_event_id(rb_event_flag_t event)
 	C(fiber_switch, FIBER_SWITCH);
 	C(specified_line, SPECIFIED_LINE);
         C(basic_op_redefined, BASIC_OP_REDEFINED);
+        C(constant_redefined, CONSTANT_REDEFINED);
       case RUBY_EVENT_LINE | RUBY_EVENT_SPECIFIED_LINE: CONST_ID(id, "line"); return id;
 #undef C
       default:
@@ -708,6 +709,7 @@ symbol2event_flag(VALUE v)
     C(a_call, A_CALL);
     C(a_return, A_RETURN);
     C(basic_op_redefined, BASIC_OP_REDEFINED);
+    C(constant_redefined, CONSTANT_REDEFINED);
 #undef C
     rb_raise(rb_eArgError, "unknown event: %"PRIsVALUE, rb_sym2str(sym));
 }
@@ -909,6 +911,26 @@ rb_tracearg_basic_op_redefined(rb_trace_arg_t *trace_arg)
    return hash;
 }
 
+VALUE
+rb_tracearg_constant_redefined(rb_trace_arg_t *trace_arg)
+{
+   VALUE hash;
+   if (trace_arg->event & (RUBY_EVENT_CONSTANT_REDEFINED)) {
+      /* ok */
+   }
+   else {
+      rb_raise(rb_eRuntimeError, "not supported by this event");
+   }
+   if (trace_arg->data == Qundef) {
+      rb_bug("tp_basic_op_redefined_m: unreachable");
+   }
+
+   hash  = rb_hash_new();
+   rb_hash_aset(hash, ID2SYM(rb_intern("klass")),    trace_arg->self);
+   rb_hash_aset(hash, ID2SYM(rb_intern("variable")), trace_arg->klass);
+   rb_hash_aset(hash, ID2SYM(rb_intern("value")),    trace_arg->data);
+   return hash;
+}
 
 VALUE
 rb_tracearg_raised_exception(rb_trace_arg_t *trace_arg)
@@ -1075,6 +1097,16 @@ tracepoint_basic_operation_redefined(VALUE tpval)
    return rb_tracearg_basic_op_redefined(get_trace_arg());
 }
 
+/*
+ * klass and constant redefined on :basic_op_redefinition event.
+ */
+static VALUE
+tracepoint_constant_redefined(VALUE tpval)
+{
+   return rb_tracearg_constant_redefined(get_trace_arg());
+}
+
+
 static void
 tp_call_trace(VALUE tpval, rb_trace_arg_t *trace_arg)
 {
@@ -1569,6 +1601,7 @@ Init_vm_trace(void)
     rb_define_method(rb_cTracePoint, "return_value", tracepoint_attr_return_value, 0);
     rb_define_method(rb_cTracePoint, "raised_exception", tracepoint_attr_raised_exception, 0);
     rb_define_method(rb_cTracePoint, "basic_operation_redefined", tracepoint_basic_operation_redefined, 0);
+    rb_define_method(rb_cTracePoint, "constant_redefined", tracepoint_constant_redefined, 0);
 
     rb_define_singleton_method(rb_cTracePoint, "stat", tracepoint_stat_s, 0);