Ruby::Box fix stale cached values for exception-related global variables ($! and $@)

Ruby::Box fix stale cached values for exception-related global variables ($! and $@)

The exception-related virtual variables $! (current exception) and
$@ (its backtrace) are stored on the execution context (ec->errinfo
and the rescue/ensure frame's local slot accessed via errinfo_place),
not in box->gvar_tbl. Caching their values in box->gvar_tbl makes the
second read return a stale value from the previous raise/rescue:

```
begin; raise "first";  rescue; p $!; end
begin; raise "second"; rescue; p $!; end
# before: #<RuntimeError: first> / #<RuntimeError: first>
# after:  #<RuntimeError: first> / #<RuntimeError: second>
```

```
begin; raise "first";  rescue; p $@.first; end
begin; raise "second"; rescue; p $@.first; end
# before: same backtrace returned for both
# after:  distinct backtrace per raise
```

Fixes [Bug #21991](https://bugs.ruby-lang.org/issues/21991)
Related PR: https://github.com/ruby/ruby/pull/16303

2 files changed, 123 insertions, 0 deletions

diff --git a/eval.c b/eval.c
index e3630c246a..2b613ffff3 100644
--- a/eval.c
+++ b/eval.c
@@ -2224,6 +2224,9 @@ Init_eval(void)
     rb_gvar_ractor_local("$@");
     rb_gvar_ractor_local("$!");
 
+    rb_gvar_box_dynamic("$@");
+    rb_gvar_box_dynamic("$!");
+
     rb_define_global_function("raise", f_raise, -1);
     rb_define_global_function("fail", f_raise, -1);
 
diff --git a/test/ruby/test_box.rb b/test/ruby/test_box.rb
index 8644b84bcd..9d60941526 100644
--- a/test/ruby/test_box.rb
+++ b/test/ruby/test_box.rb
@@ -626,6 +626,126 @@ class TestBox < Test::Unit::TestCase
     end;
   end
 
+  def test_errinfo_not_cached_in_box
+    assert_separately([ENV_ENABLE_BOX], __FILE__, __LINE__, "#{<<~"begin;"}\n#{<<~'end;'}", ignore_stderr: true)
+    begin;
+      first, second = Ruby::Box.new.eval(<<~'CODE')
+        a = begin; raise "first"; rescue RuntimeError; $!.message; end
+        b = begin; raise "second"; rescue RuntimeError; $!.message; end
+        [a, b]
+      CODE
+      assert_equal "first", first
+      assert_equal "second", second
+    end;
+  end
+
+  def test_errinfo_not_cached_in_nested_boxes
+    assert_separately([ENV_ENABLE_BOX], __FILE__, __LINE__, "#{<<~"begin;"}\n#{<<~'end;'}", ignore_stderr: true)
+    begin;
+      inner_msg, outer_msg = Ruby::Box.new.eval(<<~'CODE')
+        outer_a = begin; raise "outer1"; rescue RuntimeError; $!.message; end
+
+        inner_msg = Ruby::Box.new.eval(<<~'INNER')
+          begin; raise "inner1"; rescue RuntimeError; $!; end
+          begin; raise "inner2"; rescue RuntimeError; $!.message; end
+        INNER
+
+        outer_b = begin; raise "outer2"; rescue RuntimeError; $!.message; end
+        [inner_msg, outer_b]
+      CODE
+      assert_equal "inner2", inner_msg
+      assert_equal "outer2", outer_msg
+    end;
+  end
+
+  def test_backtrace_not_cached_in_box
+    assert_separately([ENV_ENABLE_BOX], __FILE__, __LINE__, "#{<<~"begin;"}\n#{<<~'end;'}", ignore_stderr: true)
+    begin;
+      a_actual, b_actual = Ruby::Box.new.eval(<<~'CODE')
+        a_actual = begin; raise "first"; rescue RuntimeError; $@.first[/:(\d+):/, 1].to_i; end
+        b_actual = begin; raise "second"; rescue RuntimeError; $@.first[/:(\d+):/, 1].to_i; end
+        [a_actual, b_actual]
+      CODE
+      assert_equal 1, a_actual
+      assert_equal 2, b_actual
+    end;
+  end
+
+  def test_backtrace_not_cached_in_nested_boxes
+    assert_separately([ENV_ENABLE_BOX], __FILE__, __LINE__, "#{<<~"begin;"}\n#{<<~'end;'}", ignore_stderr: true)
+    begin;
+      inner_actual, outer_actual = Ruby::Box.new.eval(<<~'CODE')
+        begin; raise "outer1"; rescue RuntimeError; $@; end
+        inner_actual = Ruby::Box.new.eval(<<~'INNER')
+          begin; raise "inner1"; rescue RuntimeError; $@; end
+          begin; raise "inner2"; rescue RuntimeError; $@.first[/:(\d+):/, 1].to_i; end
+        INNER
+        outer_actual = begin; raise "outer2"; rescue RuntimeError; $@.first[/:(\d+):/, 1].to_i; end
+        [inner_actual, outer_actual]
+      CODE
+      assert_equal 2, inner_actual
+      assert_equal 6, outer_actual
+    end;
+  end
+
+  def test_errinfo_isolated_between_boxes
+    assert_separately([ENV_ENABLE_BOX], __FILE__, __LINE__, "#{<<~"begin;"}\n#{<<~'end;'}", ignore_stderr: true)
+    begin;
+      box_a = Ruby::Box.new
+      box_b = Ruby::Box.new
+
+      a = box_a.eval('begin; raise "a"; rescue; $!.message; end')
+      b = box_b.eval('begin; raise "b"; rescue; $!.message; end')
+
+      assert_equal "a", a
+      assert_equal "b", b
+    end;
+  end
+
+  def test_backtrace_isolated_between_boxes
+    assert_separately([ENV_ENABLE_BOX], __FILE__, __LINE__, "#{<<~"begin;"}\n#{<<~'end;'}", ignore_stderr: true)
+    begin;
+      box_a = Ruby::Box.new
+      box_b = Ruby::Box.new
+
+      a_line = box_a.eval("\nbegin; raise; rescue; $@.first[/:(\\d+):/, 1].to_i; end")
+      b_line = box_b.eval('begin; raise; rescue; $@.first[/:(\d+):/, 1].to_i; end')
+
+      assert_equal 2, a_line
+      assert_equal 1, b_line
+    end;
+  end
+
+  def test_inner_box_rescue_does_not_disturb_outer_box_errinfo
+    assert_separately([ENV_ENABLE_BOX], __FILE__, __LINE__, "#{<<~"begin;"}\n#{<<~'end;'}", ignore_stderr: true)
+    begin;
+      box_a = Ruby::Box.new
+      errinfo_in_inner_rescue, errinfo_after_inner_rescue, errinfo_back_in_outer_rescue = box_a.eval(<<~'A')
+        errinfo_in_inner_rescue = errinfo_after_inner_rescue = errinfo_back_in_outer_rescue = nil
+        begin
+          raise "outer"
+        rescue
+          errinfo_in_inner_rescue, errinfo_after_inner_rescue = Ruby::Box.new.eval(<<~'B')
+            in_rescue = after_rescue = nil
+            begin
+              raise "inner"
+            rescue
+              in_rescue = $! && $!.message
+            end
+            after_rescue = $! && $!.message
+            [in_rescue, after_rescue]
+          B
+          errinfo_back_in_outer_rescue = $! && $!.message
+        end
+        [errinfo_in_inner_rescue, errinfo_after_inner_rescue, errinfo_back_in_outer_rescue]
+      A
+
+      assert_equal "inner", errinfo_in_inner_rescue
+      assert_equal "outer", errinfo_after_inner_rescue
+      assert_equal "outer", errinfo_back_in_outer_rescue
+    end;
+  end
+
   def test_load_path_and_loaded_features
     setup_box