chore(asm): performance improvement (#5233)

Avoid creating a new pyobject if the pyobject is not interned

## Checklist

- [x] Change(s) are motivated and described in the PR description.
- [x] Testing strategy is described if automated tests are not included
in the PR.
- [x] Risk is outlined (performance impact, potential for breakage,
maintainability, etc).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] [Library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/contributing.html#Release-Note-Guidelines)
are followed.
- [x] Documentation is included (in-code, generated user docs, [public
corp docs](https://github.com/DataDog/documentation/)).
- [x] Author is aware of the performance implications of this PR as
reported in the benchmarks PR comment.

## Reviewer Checklist

- [ ] Title is accurate.
- [ ] No unnecessary changes are introduced.
- [ ] Description motivates each change.
- [ ] Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes unless absolutely necessary.
- [ ] Testing strategy adequately addresses listed risk(s).
- [ ] Change is maintainable (easy to change, telemetry, documentation).
- [ ] Release note makes sense to a user of the library.
- [ ] Reviewer is aware of, and discussed the performance implications
of this PR as reported in the benchmarks PR comment.

ddtrace/appsec/iast/_taint_tracking/_taint_tracking.cpp

@@ -32,11 +32,21 @@ static PyObject *clear_taint_mapping(PyObject *Py_UNUSED(module),
   Py_RETURN_NONE;
 }
 
-static PyObject *new_pyobject_id(PyObject *tainted_object) {
+static PyObject *new_pyobject_id(PyObject *tainted_object,
+                                 Py_ssize_t object_length) {
   if (PyUnicode_Check(tainted_object)) {
+    if (PyUnicode_CHECK_INTERNED(tainted_object) == 0) { // SSTATE_NOT_INTERNED
+      Py_INCREF(tainted_object);
+      return tainted_object;
+    }
     return PyUnicode_Join(empty_unicode,
                           Py_BuildValue("(OO)", tainted_object, empty_unicode));
+  } else if (object_length > 1) {
+    // Bytes and bytearrays with length > 1 are not interned
+    Py_INCREF(tainted_object);
+    return tainted_object;
   } else if (PyBytes_Check(tainted_object)) {
+
     return PyObject_CallFunctionObjArgs(
         bytes_join, empty_bytes,
         Py_BuildValue("(OO)", tainted_object, empty_bytes), NULL);
@@ -52,15 +62,15 @@ static PyObject *taint_pyobject(PyObject *Py_UNUSED(module), PyObject *args) {
   T_input_info input_info;
   PyArg_ParseTuple(args, "OO", &tainted_object, &input_info);
   // DEV: could use PyUnicode_GET_LENGTH if we are only using unicode string
-  Py_ssize_t tainted_length = PyObject_Length(tainted_object);
-  if (tainted_length < 1) {
+  Py_ssize_t object_length = PyObject_Length(tainted_object);
+  if (object_length < 1) {
     Py_INCREF(tainted_object);
     return tainted_object;
   }
 
   Py_INCREF(input_info);
-  tainted_object = new_pyobject_id(tainted_object);
-  TaintMapping[tainted_object] = {{input_info, 0, tainted_length}};
+  tainted_object = new_pyobject_id(tainted_object, object_length);
+  TaintMapping[tainted_object] = {{input_info, 0, object_length}};
   return tainted_object;
 }
 
@@ -75,7 +85,8 @@ static PyObject *add_taint_pyobject(PyObject *Py_UNUSED(module),
     Py_INCREF(tainted_object);
     return tainted_object;
   }
-  tainted_object = new_pyobject_id(tainted_object);
+  tainted_object =
+      new_pyobject_id(tainted_object, PyObject_Length(tainted_object));
   if IS_TAINTED (op1)
     TaintMapping[tainted_object] = TaintMapping[op1];
   else

tests/appsec/iast/aspects/test_add_aspect.py

@@ -50,8 +50,10 @@ def test_add_aspect_type_error(obj1, obj2):
         (3.5, 3.3, False),
         (complex(2, 1), complex(3, 4), False),
         ("Hello ", "world", True),
-        (b"bye ", b"bye ", True),
-        ("🙀", "🙀", True),
+        (b"bye ", b"".join((b"bye", b" ")), True),
+        ("🙀", "".join(("🙀", "")), True),
+        ("a", "a", True),
+        (b"a", b"a", True),
         (b"Hi", b"", True),
         (b"Hi ", b" world", True),
         (["a"], ["b"], False),
@@ -72,18 +74,15 @@ def test_add_aspect_tainting_left_hand(obj1, obj2, should_be_tainted):
     clear_taint_mapping()
 
     if should_be_tainted:
-        new_obj1 = taint_pyobject(obj1, Input_info("test_add_aspect_tainting_left_hand", obj1, 0))
-        assert obj1 is not new_obj1
-    else:
-        new_obj1 = obj1
+        obj1 = taint_pyobject(obj1, Input_info("test_add_aspect_tainting_left_hand", obj1, 0))
 
-    result = ddtrace_aspects.add_aspect(new_obj1, obj2)
-    assert result == new_obj1 + obj2
+    result = ddtrace_aspects.add_aspect(obj1, obj2)
+    assert result == obj1 + obj2
     if isinstance(obj2, (bytes, str, bytearray)) and len(obj2):
-        assert result is not new_obj1 + obj2
+        assert result is not obj1 + obj2
     assert is_pyobject_tainted(result) == should_be_tainted
     if should_be_tainted:
-        assert get_tainted_ranges(result) == get_tainted_ranges(new_obj1)
+        assert get_tainted_ranges(result) == get_tainted_ranges(obj1)
 
 
 @pytest.mark.parametrize(