Stable abi and std::string_view

Using directorout and string_view is dangerous. Add in warning 473.

For the stable abi < Python 3.10, leak instead of undefined behaviour.
See #2727.

Update warning 473 message text to:
  Returning a reference, pointer or pointer wrapper in a director method is not recommended.

Doc/Manual/Warnings.html

@@ -492,7 +492,7 @@ example.i(4) : Syntax error in input(1).
 <li>470. Thread/reentrant unsafe wrapping, consider returning by value instead.
 <li>471. Unable to use return type <em>type</em> in director method
 <li>472. Overloaded method <em>method</em> with no explicit typecheck typemap for arg <em>number</em> of type '<em>type</em>'
-<li>473. Returning a pointer or reference in a director method is not recommended.
+<li>473. Returning a reference, pointer or pointer wrapper in a director method is not recommended.
 <li>474. Method <em>method</em> usage of the optimal attribute ignored in the out typemap as the following cannot be used to generate optimal code: <em>code</em>
 <li>475. Multiple calls to <em>method</em> might be generated due to optimal attribute usage in the out typemap.
 <li>476. Initialization using std::initializer_list.

Examples/test-suite/cpp17_director_string_view.i

@@ -41,11 +41,11 @@ struct A
   { return (int)s.size(); }
 
 
-  virtual void process_text(const char *text)
+  virtual void process_text(const char *text) const
   {
   }
 
-  void call_process_func() { process_text("hello"); }
+  void call_process_func() const { process_text("hello"); }
 
 private:
   std::vector<std::string> m_strings;

Examples/test-suite/python/cpp17_director_string_view_runme.py

@@ -7,7 +7,10 @@ class B(A):
         A.__init__(self, string)
 
     def get_first(self):
-        return A.get_first(self) + " world!"
+        # Given std::string_view is just a pointer into a string, the string
+        # cannot be a temporary in order to avoid undefined behaviour
+        self.cached_string = A.get_first(self) + " world!"
+        return self.cached_string
 
     def process_text(self, string):
         A.process_text(self, string)
@@ -20,6 +23,8 @@ b.get(0)
 if b.get_first() != "hello world!":
     raise RuntimeError("b.get_first(): {}".format(b.get_first()))
 
+if b.call_get_first() != "hello world!":
+    raise RuntimeError("b.call_get_first(): {}".format(b.call_get_first()))
 
 b.call_process_func()
 

Lib/python/std_string_view.i

@@ -77,7 +77,7 @@ namespace std {
         Py_XDECREF(bytes$argnum);
     %}
 
-    %typemap(directorout) string_view {
+    %typemap(directorout, warning=SWIGWARN_TYPEMAP_DIRECTOROUT_PTR_MSG) string_view {
         Py_ssize_t len;
 %#ifdef SWIG_PYTHON_STRICT_BYTE_CHAR
         const char *p = PyBytes_AsString($input);
@@ -86,9 +86,8 @@ namespace std {
         const char *p;
         PyObject *bytes = NULL;
         if (PyUnicode_Check($input)) {
-          /* Note: The UTF-8 data is cached in the PyObject so remains valid for
-           * the call to C/C++. */
-          p = PyUnicode_AsUTF8AndSize($input, &len);
+          p = SWIG_PyUnicode_AsUTF8AndSize($input, &len, &bytes);
+          // Py_XDECREF(bytes); // Avoid undefined behaviour ($input will be pointing to a temporary if bytes is not NULL), for now we just leak by not calling Py_XDECREF
         } else {
           p = PyBytes_AsString($input);
           if (p) len = PyBytes_Size($input);

Lib/swigwarnings.swg

@@ -54,7 +54,7 @@
 %define SWIGWARN_TYPEMAP_SWIGTYPELEAK_MSG     "454:Setting a pointer/reference variable may leak memory." %enddef
 %define SWIGWARN_TYPEMAP_WCHARLEAK_MSG        "455:Setting a const wchar_t * variable may leak memory." %enddef
 %define SWIGWARN_TYPEMAP_THREAD_UNSAFE_MSG    "470:Thread/reentrant unsafe wrapping, consider returning by value instead." %enddef
-%define SWIGWARN_TYPEMAP_DIRECTOROUT_PTR_MSG  "473:Returning a pointer or reference in a director method is not recommended." %enddef
+%define SWIGWARN_TYPEMAP_DIRECTOROUT_PTR_MSG  "473:Returning a reference, pointer or pointer wrapper in a director method is not recommended." %enddef
 %define SWIGWARN_TYPEMAP_INITIALIZER_LIST_MSG "476:Initialization using std::initializer_list." %enddef
 
 /* -----------------------------------------------------------------------------