gh-113993: 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.

* For free-threaded builds, intern `_Py_LATIN1_CHR` singletons at startup.

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

* Add lots of assertions

Co-Authored-By: Eric Snow <ericsnowcurrently@gmail.com>

1 files changed, 5 insertions, 1 deletions

diff --git a/Python/codecs.c b/Python/codecs.c
index bed2453..9c0a3fa 100644
--- a/Python/codecs.c
+++ b/Python/codecs.c
@@ -147,7 +147,9 @@ PyObject *_PyCodec_Lookup(const char *encoding)
     if (v == NULL) {
         return NULL;
     }
-    PyUnicode_InternInPlace(&v);
+
+    /* Intern the string. We'll make it immortal later if lookup succeeds. */
+    _PyUnicode_InternMortal(interp, &v);
 
     /* First, try to lookup the name in the registry dictionary */
     PyObject *result;
@@ -200,6 +202,8 @@ PyObject *_PyCodec_Lookup(const char *encoding)
         goto onError;
     }
 
+    _PyUnicode_InternImmortal(interp, &v);
+
     /* Cache and return the result */
     if (PyDict_SetItem(interp->codecs.search_cache, v, result) < 0) {
         Py_DECREF(result);