Expose need_major_gc via GC.latest_gc_info (#6791)

gc.c

@@ -10954,7 +10954,7 @@ gc_count(rb_execution_context_t *ec, VALUE self)
 static VALUE
 gc_info_decode(rb_objspace_t *objspace, const VALUE hash_or_key, const unsigned int orig_flags)
 {
-    static VALUE sym_major_by = Qnil, sym_gc_by, sym_immediate_sweep, sym_have_finalizer, sym_state;
+    static VALUE sym_major_by = Qnil, sym_gc_by, sym_immediate_sweep, sym_have_finalizer, sym_state, sym_need_major_by;
     static VALUE sym_nofree, sym_oldgen, sym_shady, sym_force, sym_stress;
 #if RGENGC_ESTIMATE_OLDMALLOC
     static VALUE sym_oldmalloc;
@@ -10962,7 +10962,7 @@ gc_info_decode(rb_objspace_t *objspace, const VALUE hash_or_key, const unsigned
     static VALUE sym_newobj, sym_malloc, sym_method, sym_capi;
     static VALUE sym_none, sym_marking, sym_sweeping;
     VALUE hash = Qnil, key = Qnil;
-    VALUE major_by;
+    VALUE major_by, need_major_by;
     unsigned int flags = orig_flags ? orig_flags : objspace->profile.latest_gc_info;
 
     if (SYMBOL_P(hash_or_key)) {
@@ -10982,6 +10982,7 @@ gc_info_decode(rb_objspace_t *objspace, const VALUE hash_or_key, const unsigned
         S(immediate_sweep);
         S(have_finalizer);
         S(state);
+        S(need_major_by);
 
         S(stress);
         S(nofree);
@@ -11019,6 +11020,20 @@ gc_info_decode(rb_objspace_t *objspace, const VALUE hash_or_key, const unsigned
       Qnil;
     SET(major_by, major_by);
 
+    if (orig_flags == 0) { /* set need_major_by only if flags not set explicitly */
+        unsigned int need_major_flags = objspace->rgengc.need_major_gc;
+        need_major_by =
+            (need_major_flags & GPR_FLAG_MAJOR_BY_NOFREE) ? sym_nofree :
+            (need_major_flags & GPR_FLAG_MAJOR_BY_OLDGEN) ? sym_oldgen :
+            (need_major_flags & GPR_FLAG_MAJOR_BY_SHADY)  ? sym_shady :
+            (need_major_flags & GPR_FLAG_MAJOR_BY_FORCE)  ? sym_force :
+#if RGENGC_ESTIMATE_OLDMALLOC
+            (need_major_flags & GPR_FLAG_MAJOR_BY_OLDMALLOC) ? sym_oldmalloc :
+#endif
+            Qnil;
+        SET(need_major_by, need_major_by);
+    }
+
     SET(gc_by,
         (flags & GPR_FLAG_NEWOBJ) ? sym_newobj :
         (flags & GPR_FLAG_MALLOC) ? sym_malloc :

test/ruby/test_gc.rb

@@ -229,6 +229,7 @@ class TestGc < Test::Unit::TestCase
     assert_equal :force,  h[:major_by] if use_rgengc?
     assert_equal :method, h[:gc_by]
     assert_equal true,    h[:immediate_sweep]
+    assert_equal true,    h.key?(:need_major_by)
 
     GC.stress = true
     assert_equal :force, GC.latest_gc_info[:major_by]
@@ -246,6 +247,25 @@ class TestGc < Test::Unit::TestCase
     assert_raise_with_message(ArgumentError, /\u{30eb 30d3 30fc}/) {GC.latest_gc_info(:"\u{30eb 30d3 30fc}")}
   end
 
+  def test_latest_gc_info_need_major_by
+    return unless use_rgengc?
+    omit 'stress' if GC.stress
+
+    3.times { GC.start }
+    assert_nil GC.latest_gc_info(:need_major_by)
+
+    # allocate objects until need_major_by is set or major GC happens
+    major_count = GC.stat(:major_gc_count)
+    objects = []
+    while GC.stat(:major_gc_count) == major_count && GC.latest_gc_info(:need_major_by).nil?
+      objects.append(100.times.map { '*' })
+    end
+
+    assert_not_nil GC.latest_gc_info(:need_major_by)
+    GC.start(full_mark: false) # should be upgraded to major
+    assert_not_nil GC.latest_gc_info(:major_by)
+  end
+
   def test_stress_compile_send
     assert_in_out_err(%w[--disable-gems], <<-EOS, [], [], "")
       GC.stress = true