[3.12] gh-113993: Make interned strings mortal (GH-120520, GH-121364, GH-121903, GH-122303) (#123065)

This backports several PRs for gh-113993, making interned strings mortal so they can be garbage-collected when no longer needed.

* Allow interned strings to be mortal, and fix related issues (GH-120520)

  * Add an InternalDocs file describing how interning should work and how to use it.

  * Add internal functions to *explicitly* request what kind of interning is done:
    - `_PyUnicode_InternMortal`
    - `_PyUnicode_InternImmortal`
    - `_PyUnicode_InternStatic`

  * Switch uses of `PyUnicode_InternInPlace` to those.

  * Disallow using `_Py_SetImmortal` on strings directly.
    You should use `_PyUnicode_InternImmortal` instead:
    - Strings should be interned before immortalization, otherwise you're possibly
      interning a immortalizing copy.
    - `_Py_SetImmortal` doesn't handle the `SSTATE_INTERNED_MORTAL` to
      `SSTATE_INTERNED_IMMORTAL` update, and those flags can't be changed in
      backports, as they are now part of public API and version-specific ABI.

  * Add private `_only_immortal` argument for `sys.getunicodeinternedsize`, used in refleak test machinery.

   Make sure the statically allocated string singletons are unique. This means these sets are now disjoint:
    - `_Py_ID`
    - `_Py_STR` (including the empty string)
    - one-character latin-1 singletons

    Now, when you intern a singleton, that exact singleton will be interned.

  * Add a `_Py_LATIN1_CHR` macro, use it instead of `_Py_ID`/`_Py_STR` for one-character latin-1 singletons everywhere (including Clinic).

  * Intern `_Py_STR` singletons at startup.

  * Beef up the tests. Cover internal details (marked with `@cpython_only`).

  * Add lots of assertions

* Don't immortalize in PyUnicode_InternInPlace; keep immortalizing in other API (GH-121364)

  * Switch PyUnicode_InternInPlace to _PyUnicode_InternMortal, clarify docs

  * Document immortality in some functions that take `const char *`

  This is PyUnicode_InternFromString;
  PyDict_SetItemString, PyObject_SetAttrString;
  PyObject_DelAttrString; PyUnicode_InternFromString;
  and the PyModule_Add convenience functions.

  Always point out a non-immortalizing alternative.

  * Don't immortalize user-provided attr names in _ctypes

* Immortalize names in code objects to avoid crash (GH-121903)

* Intern latin-1 one-byte strings at startup (GH-122303)

There are some 3.12-specific changes, mainly to allow statically allocated strings in deepfreeze. (In 3.13, deepfreeze switched to the general `_Py_ID`/`_Py_STR`.)

Co-authored-by: Eric Snow <ericsnowcurrently@gmail.com>

Modules/_operator.c

@@ -2,6 +2,9 @@
 #include "pycore_moduleobject.h"  // _PyModule_GetState()
 #include "structmember.h"         // PyMemberDef
 #include "pycore_runtime.h"       // _Py_ID()
+#include "pycore_pystate.h"       // _PyInterpreterState_GET()
+
+
 #include "clinic/_operator.c.h"
 
 typedef struct {
@@ -1224,6 +1227,7 @@ attrgetter_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
         return NULL;
 
     /* prepare attr while checking args */
+    PyInterpreterState *interp = _PyInterpreterState_GET();
     for (idx = 0; idx < nattrs; ++idx) {
         PyObject *item = PyTuple_GET_ITEM(args, idx);
         int dot_count;
@@ -1251,7 +1255,7 @@ attrgetter_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
 
         if (dot_count == 0) {
             Py_INCREF(item);
-            PyUnicode_InternInPlace(&item);
+            _PyUnicode_InternMortal(interp, &item);
             PyTuple_SET_ITEM(attr, idx, item);
         } else { /* make it a tuple of non-dotted attrnames */
             PyObject *attr_chain = PyTuple_New(dot_count + 1);
@@ -1277,7 +1281,7 @@ attrgetter_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
                     Py_DECREF(attr);
                     return NULL;
                 }
-                PyUnicode_InternInPlace(&attr_chain_item);
+                _PyUnicode_InternMortal(interp, &attr_chain_item);
                 PyTuple_SET_ITEM(attr_chain, attr_chain_idx, attr_chain_item);
                 ++attr_chain_idx;
                 unibuff_till = unibuff_from = unibuff_till + 1;
@@ -1291,7 +1295,7 @@ attrgetter_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
                 Py_DECREF(attr);
                 return NULL;
             }
-            PyUnicode_InternInPlace(&attr_chain_item);
+            _PyUnicode_InternMortal(interp, &attr_chain_item);
             PyTuple_SET_ITEM(attr_chain, attr_chain_idx, attr_chain_item);
 
             PyTuple_SET_ITEM(attr, idx, attr_chain);
@@ -1587,7 +1591,8 @@ methodcaller_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
 
     name = PyTuple_GET_ITEM(args, 0);
     Py_INCREF(name);
-    PyUnicode_InternInPlace(&name);
+    PyInterpreterState *interp = _PyInterpreterState_GET();
+    _PyUnicode_InternMortal(interp, &name);
     mc->name = name;
 
     mc->kwds = Py_XNewRef(kwds);