bpo-40521: Disable Unicode caches in isolated subinterpreters (GH-19933)

When Python is built in the experimental isolated subinterpreters
mode, disable Unicode singletons and Unicode interned strings since
they are shared by all interpreters.

Temporary workaround until these caches are made per-interpreter.

Objects/typeobject.c

@@ -56,6 +56,11 @@ static size_t method_cache_misses = 0;
 static size_t method_cache_collisions = 0;
 #endif
 
+/* bpo-40521: Interned strings are shared by all subinterpreters */
+#ifndef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
+#  define INTERN_NAME_STRINGS
+#endif
+
 /* alphabetical order */
 _Py_IDENTIFIER(__abstractmethods__);
 _Py_IDENTIFIER(__class__);
@@ -3418,6 +3423,7 @@ type_setattro(PyTypeObject *type, PyObject *name, PyObject *value)
             if (name == NULL)
                 return -1;
         }
+#ifdef INTERN_NAME_STRINGS
         if (!PyUnicode_CHECK_INTERNED(name)) {
             PyUnicode_InternInPlace(&name);
             if (!PyUnicode_CHECK_INTERNED(name)) {
@@ -3427,6 +3433,7 @@ type_setattro(PyTypeObject *type, PyObject *name, PyObject *value)
                 return -1;
             }
         }
+#endif
     }
     else {
         /* Will fail in _PyObject_GenericSetAttrWithDict. */
@@ -7531,10 +7538,17 @@ _PyTypes_InitSlotDefs(void)
     for (slotdef *p = slotdefs; p->name; p++) {
         /* Slots must be ordered by their offset in the PyHeapTypeObject. */
         assert(!p[1].name || p->offset <= p[1].offset);
+#ifdef INTERN_NAME_STRINGS
         p->name_strobj = PyUnicode_InternFromString(p->name);
         if (!p->name_strobj || !PyUnicode_CHECK_INTERNED(p->name_strobj)) {
             return _PyStatus_NO_MEMORY();
         }
+#else
+        p->name_strobj = PyUnicode_FromString(p->name);
+        if (!p->name_strobj) {
+            return _PyStatus_NO_MEMORY();
+        }
+#endif
     }
     slotdefs_initialized = 1;
     return _PyStatus_OK();
@@ -7559,7 +7573,9 @@ update_slot(PyTypeObject *type, PyObject *name)
     int offset;
 
     assert(PyUnicode_CheckExact(name));
+#ifdef INTERN_NAME_STRINGS
     assert(PyUnicode_CHECK_INTERNED(name));
+#endif
 
     assert(slotdefs_initialized);
     pp = ptrs;

Objects/unicodeobject.c

@@ -198,6 +198,11 @@ extern "C" {
 #  define OVERALLOCATE_FACTOR 4
 #endif
 
+/* bpo-40521: Interned strings are shared by all interpreters. */
+#ifndef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
+#  define INTERNED_STRINGS
+#endif
+
 /* This dictionary holds all interned unicode strings.  Note that references
    to strings in this dictionary are *not* counted in the string's ob_refcnt.
    When the interned string reaches a refcnt of 0 the string deallocation
@@ -206,7 +211,9 @@ extern "C" {
    Another way to look at this is that to say that the actual reference
    count of a string is:  s->ob_refcnt + (s->state ? 2 : 0)
 */
+#ifdef INTERNED_STRINGS
 static PyObject *interned = NULL;
+#endif
 
 /* The empty Unicode object is shared to improve performance. */
 static PyObject *unicode_empty = NULL;
@@ -281,9 +288,16 @@ unicode_decode_utf8(const char *s, Py_ssize_t size,
 /* List of static strings. */
 static _Py_Identifier *static_strings = NULL;
 
+/* bpo-40521: Latin1 singletons are shared by all interpreters. */
+#ifndef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
+#  define LATIN1_SINGLETONS
+#endif
+
+#ifdef LATIN1_SINGLETONS
 /* Single character Unicode strings in the Latin-1 range are being
    shared as well. */
 static PyObject *unicode_latin1[256] = {NULL};
+#endif
 
 /* Fast detection of the most frequent whitespace characters */
 const unsigned char _Py_ascii_whitespace[] = {
@@ -662,6 +676,7 @@ unicode_result_ready(PyObject *unicode)
         return unicode_empty;
     }
 
+#ifdef LATIN1_SINGLETONS
     if (length == 1) {
         const void *data = PyUnicode_DATA(unicode);
         int kind = PyUnicode_KIND(unicode);
@@ -683,6 +698,7 @@ unicode_result_ready(PyObject *unicode)
             }
         }
     }
+#endif
 
     assert(_PyUnicode_CheckConsistency(unicode, 1));
     return unicode;
@@ -1913,10 +1929,12 @@ unicode_dealloc(PyObject *unicode)
     case SSTATE_INTERNED_MORTAL:
         /* revive dead object temporarily for DelItem */
         Py_SET_REFCNT(unicode, 3);
+#ifdef INTERNED_STRINGS
         if (PyDict_DelItem(interned, unicode) != 0) {
             _PyErr_WriteUnraisableMsg("deletion of interned string failed",
                                       NULL);
         }
+#endif
         break;
 
     case SSTATE_INTERNED_IMMORTAL:
@@ -1944,15 +1962,18 @@ unicode_dealloc(PyObject *unicode)
 static int
 unicode_is_singleton(PyObject *unicode)
 {
-    PyASCIIObject *ascii = (PyASCIIObject *)unicode;
+    if (unicode == unicode_empty) {
-    if (unicode == unicode_empty)
         return 1;
+    }
+#ifdef LATIN1_SINGLETONS
+    PyASCIIObject *ascii = (PyASCIIObject *)unicode;
     if (ascii->state.kind != PyUnicode_WCHAR_KIND && ascii->length == 1)
     {
         Py_UCS4 ch = PyUnicode_READ_CHAR(unicode, 0);
         if (ch < 256 && unicode_latin1[ch] == unicode)
             return 1;
     }
+#endif
     return 0;
 }
 #endif
@@ -2094,16 +2115,28 @@ unicode_write_cstr(PyObject *unicode, Py_ssize_t index,
 static PyObject*
 get_latin1_char(unsigned char ch)
 {
-    PyObject *unicode = unicode_latin1[ch];
+    PyObject *unicode;
-    if (!unicode) {
+
+#ifdef LATIN1_SINGLETONS
+    unicode = unicode_latin1[ch];
+    if (unicode) {
+        Py_INCREF(unicode);
+        return unicode;
+    }
+#endif
+
     unicode = PyUnicode_New(1, ch);
-        if (!unicode)
+    if (!unicode) {
         return NULL;
+    }
+
     PyUnicode_1BYTE_DATA(unicode)[0] = ch;
     assert(_PyUnicode_CheckConsistency(unicode, 1));
-        unicode_latin1[ch] = unicode;
+
-    }
+#ifdef LATIN1_SINGLETONS
     Py_INCREF(unicode);
+    unicode_latin1[ch] = unicode;
+#endif
     return unicode;
 }
 
@@ -11270,7 +11303,6 @@ int
 _PyUnicode_EqualToASCIIId(PyObject *left, _Py_Identifier *right)
 {
     PyObject *right_uni;
-    Py_hash_t hash;
 
     assert(_PyUnicode_CHECK(left));
     assert(right->string);
@@ -11302,10 +11334,12 @@ _PyUnicode_EqualToASCIIId(PyObject *left, _Py_Identifier *right)
     if (PyUnicode_CHECK_INTERNED(left))
         return 0;
 
+#ifdef INTERNED_STRINGS
     assert(_PyUnicode_HASH(right_uni) != -1);
-    hash = _PyUnicode_HASH(left);
+    Py_hash_t hash = _PyUnicode_HASH(left);
     if (hash != -1 && hash != _PyUnicode_HASH(right_uni))
         return 0;
+#endif
 
     return unicode_compare_eq(left, right_uni);
 }
@@ -15487,20 +15521,26 @@ void
 PyUnicode_InternInPlace(PyObject **p)
 {
     PyObject *s = *p;
-    PyObject *t;
 #ifdef Py_DEBUG
     assert(s != NULL);
     assert(_PyUnicode_CHECK(s));
 #else
-    if (s == NULL || !PyUnicode_Check(s))
+    if (s == NULL || !PyUnicode_Check(s)) {
         return;
+    }
 #endif
+
     /* If it's a subclass, we don't really know what putting
        it in the interned dict might do. */
-    if (!PyUnicode_CheckExact(s))
+    if (!PyUnicode_CheckExact(s)) {
         return;
-    if (PyUnicode_CHECK_INTERNED(s))
+    }
+
+    if (PyUnicode_CHECK_INTERNED(s)) {
         return;
+    }
+
+#ifdef INTERNED_STRINGS
     if (interned == NULL) {
         interned = PyDict_New();
         if (interned == NULL) {
@@ -15508,22 +15548,28 @@ PyUnicode_InternInPlace(PyObject **p)
             return;
         }
     }
+
+    PyObject *t;
     Py_ALLOW_RECURSION
     t = PyDict_SetDefault(interned, s, s);
     Py_END_ALLOW_RECURSION
+
     if (t == NULL) {
         PyErr_Clear();
         return;
     }
+
     if (t != s) {
         Py_INCREF(t);
         Py_SETREF(*p, t);
         return;
     }
+
     /* The two references in interned are not counted by refcnt.
        The deallocator will take care of this */
     Py_SET_REFCNT(s, Py_REFCNT(s) - 2);
     _PyUnicode_STATE(s).interned = SSTATE_INTERNED_MORTAL;
+#endif
 }
 
 void
@@ -16109,9 +16155,11 @@ _PyUnicode_Fini(PyThreadState *tstate)
 
         Py_CLEAR(unicode_empty);
 
+#ifdef LATIN1_SINGLETONS
         for (Py_ssize_t i = 0; i < 256; i++) {
             Py_CLEAR(unicode_latin1[i]);
         }
+#endif
         _PyUnicode_ClearStaticStrings();
     }