`TracePoint.allow_reentry`

In general, while TracePoint callback is running,
other registerred callbacks are not called to avoid
confusion by reentrace.
This method allow the reentrace. This method should be
used carefully, otherwize the callback can be easily called
infinitely.

[Feature #15912]

Co-authored-by: Jean Boussier <jean.boussier@gmail.com>

test/ruby/test_settracefunc.rb

@@ -2575,4 +2575,28 @@ CODE
     }
     assert_equal [__LINE__ - 5, __LINE__ - 4, __LINE__ - 3], lines, 'Bug #17868'
   end
+
+  def test_allow_reentry
+    event_lines = []
+    _l1 = _l2 = _l3 = _l4 = nil
+    TracePoint.new(:line) do |tp|
+      next unless target_thread?
+
+      event_lines << tp.lineno
+      next if (__LINE__ + 2 .. __LINE__ + 4).cover?(tp.lineno)
+      TracePoint.allow_reentry do
+        _a = 1; _l3 = __LINE__
+        _b = 2; _l4 = __LINE__
+      end
+    end.enable do
+      _c = 3; _l1 = __LINE__
+      _d = 4; _l2 = __LINE__
+    end
+
+    assert_equal [_l1, _l3, _l4, _l2, _l3, _l4], event_lines
+
+    assert_raise RuntimeError do
+      TracePoint.allow_reentry{}
+    end
+  end
 end

trace_point.rb

@@ -135,6 +135,22 @@ class TracePoint
     Primitive.tracepoint_trace_s(events)
   end
 
+  # call-seq:
+  #   TracePoint.allow_reentry
+  #
+  # In general, while a TracePoint callback is running,
+  # other registered callbacks are not called to avoid
+  # confusion by reentrance.
+  # This method allows the reentrance in a given block.
+  # This method should be used carefully, otherwise the callback
+  # can be easily called infinitely.
+  #
+  # If this method is called when the reentrance is already allowed,
+  # it raises a RuntimeError.
+  def self.allow_reentry
+    Primitive.tracepoint_allow_reentry
+  end
+
   # call-seq:
   #    trace.enable(target: nil, target_line: nil, target_thread: nil)    -> true or false
   #    trace.enable(target: nil, target_line: nil, target_thread: nil) { block }  -> obj

vm_trace.c

@@ -1517,6 +1517,25 @@ tracepoint_stat_s(rb_execution_context_t *ec, VALUE self)
     return stat;
 }
 
+static VALUE
+disallow_reentry(VALUE val)
+{
+    rb_trace_arg_t *arg = (rb_trace_arg_t *)val;
+    rb_execution_context_t *ec = GET_EC();
+    if (ec->trace_arg != NULL) rb_bug("should be NULL, but %p", (void *)ec->trace_arg);
+    ec->trace_arg = arg;
+    return Qnil;
+}
+
+static VALUE
+tracepoint_allow_reentry(rb_execution_context_t *ec, VALUE self)
+{
+    const rb_trace_arg_t *arg = ec->trace_arg;
+    if (arg == NULL) rb_raise(rb_eRuntimeError, "No need to allow reentrance.");
+    ec->trace_arg = NULL;
+    return rb_ensure(rb_yield, Qnil, disallow_reentry, (VALUE)arg);
+}
+
 #include "trace_point.rbinc"
 
 /* This function is called from inits.c */