`intern_static` is not thread-safe with multiple interpreters

[colesbury]

Bug report

Most static strings are interned during Python initialization in _PyUnicode_InitStaticStrings. However, the _Py_LATIN1_CHR characters (code points 0-255) are static, but not interned. They may be interned later while the Python is running. This can happen for various reasons, including calls to sys.intern.

This isn't thread-safe: it modifies the hashtable _PyRuntime.cached_objects.interned_strings, which is shared across threads and interpreters, without any synchronization.

It also can break the interning identity invariant. You can have a non-static, interned 1-characters string later shadowed by the global interning of the static 1-character string.

Suggestions

• The _PyRuntime.cached_objects.interned_strings should be immutable. We should not modify it after Py_Initialize() until shutdown (i.e., _PyUnicode_ClearInterned called from finalize_interp_types())
• The 1-character latin1 strings should be interned. This can either be by explicitly interning them during startup, or by handling 1-character strings specially in intern_common.

cc @encukou @ericsnowcurrently

Linked PRs

• gh-122291: Intern latin-1 one-byte strings at startup #122303
• [3.13] gh-122291: Intern latin-1 one-byte strings at startup (GH-122303) #122347

[encukou]

Interning them makes sense. I'll send a PR.

[encukou]

FWIW:

This isn't thread-safe: it modifies the hashtable _PyRuntime.cached_objects.interned_strings, which is shared across threads and interpreters, without any synchronization.

Yup, that's a bug, so the fix should be backported.

It also can break the interning identity invariant. You can have a non-static, interned 1-characters string later shadowed by the global interning of the static 1-character string.

Hmm, interning a short string should give you the static singleton.
But the fix will definitely make this easier to follow, and harder to hide any bugs.

[encukou]

interning a short string should give you the static singleton.

Before the PR for this issue we had:

cpython/Objects/unicodeobject.c

Lines 15480 to 15493 in c086962

	/* if it's a short string, get the singleton -- and intern it */
	if (PyUnicode_GET_LENGTH(s) == 1 &&
	PyUnicode_KIND(s) == PyUnicode_1BYTE_KIND) {
	PyObject *r = LATIN1(*(unsigned char*)PyUnicode_DATA(s));
	if (!PyUnicode_CHECK_INTERNED(r)) {
	r = intern_static(interp, r);
	}
	Py_DECREF(s);
	return r;
	}
	#ifdef Py_DEBUG
	assert(!unicode_is_singleton(s));
	#endif

[colesbury]

Looks like this has been fixed for a while.