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, 13 insertions, 3 deletions

diff --git a/Tools/clinic/libclinic/clanguage.py b/Tools/clinic/libclinic/clanguage.py
index 10efedd..73d4783 100644
--- a/Tools/clinic/libclinic/clanguage.py
+++ b/Tools/clinic/libclinic/clanguage.py
@@ -21,6 +21,16 @@ if TYPE_CHECKING:
     from libclinic.app import Clinic
 
 
+def c_id(name: str) -> str:
+    if len(name) == 1 and ord(name) < 256:
+        if name.isalnum():
+            return f"_Py_LATIN1_CHR('{name}')"
+        else:
+            return f'_Py_LATIN1_CHR({ord(name)})'
+    else:
+        return f'&_Py_ID({name})'
+
+
 class CLanguage(Language):
 
     body_prefix   = "#"
@@ -167,11 +177,11 @@ class CLanguage(Language):
                 if argname_fmt:
                     conditions.append(f"nargs < {i+1} && {argname_fmt % i}")
                 elif fastcall:
-                    conditions.append(f"nargs < {i+1} && PySequence_Contains(kwnames, &_Py_ID({p.name}))")
+                    conditions.append(f"nargs < {i+1} && PySequence_Contains(kwnames, {c_id(p.name)})")
                     containscheck = "PySequence_Contains"
                     codegen.add_include('pycore_runtime.h', '_Py_ID()')
                 else:
-                    conditions.append(f"nargs < {i+1} && PyDict_Contains(kwargs, &_Py_ID({p.name}))")
+                    conditions.append(f"nargs < {i+1} && PyDict_Contains(kwargs, {c_id(p.name)})")
                     containscheck = "PyDict_Contains"
                     codegen.add_include('pycore_runtime.h', '_Py_ID()')
             else:
@@ -459,7 +469,7 @@ class CLanguage(Language):
         template_dict['keywords_c'] = ' '.join('"' + k + '",'
                                                for k in data.keywords)
         keywords = [k for k in data.keywords if k]
-        template_dict['keywords_py'] = ' '.join('&_Py_ID(' + k + '),'
+        template_dict['keywords_py'] = ' '.join(c_id(k) + ','
                                                 for k in keywords)
         template_dict['format_units'] = ''.join(data.format_units)
         template_dict['parse_arguments'] = ', '.join(data.parse_arguments)