bpo-39465: Fix _PyUnicode_FromId() for subinterpreters (GH-20058)

Make _PyUnicode_FromId() function compatible with subinterpreters.
Each interpreter now has an array of identifier objects (interned
strings decoded from UTF-8).

* Add PyInterpreterState.unicode.identifiers: array of identifiers
  objects.
* Add _PyRuntimeState.unicode_ids used to allocate unique indexes
  to _Py_Identifier.
* Rewrite the _Py_Identifier structure.

Microbenchmark on _PyUnicode_FromId(&PyId_a) with _Py_IDENTIFIER(a):

[ref] 2.42 ns +- 0.00 ns -> [atomic] 3.39 ns +- 0.00 ns: 1.40x slower

This change adds 1 ns per _PyUnicode_FromId() call in average.

Objects/unicodeobject.c

@@ -41,6 +41,7 @@ OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 #define PY_SSIZE_T_CLEAN
 #include "Python.h"
 #include "pycore_abstract.h"      // _PyIndex_Check()
+#include "pycore_atomic_funcs.h"  // _Py_atomic_size_get()
 #include "pycore_bytes_methods.h" // _Py_bytes_lower()
 #include "pycore_format.h"        // F_LJUST
 #include "pycore_initconfig.h"    // _PyStatus_OK()
@@ -302,9 +303,6 @@ unicode_decode_utf8(const char *s, Py_ssize_t size,
                     _Py_error_handler error_handler, const char *errors,
                     Py_ssize_t *consumed);
 
-/* List of static strings. */
-static _Py_Identifier *static_strings = NULL;
-
 /* Fast detection of the most frequent whitespace characters */
 const unsigned char _Py_ascii_whitespace[] = {
     0, 0, 0, 0, 0, 0, 0, 0,
@@ -2312,42 +2310,85 @@ PyUnicode_FromString(const char *u)
     return PyUnicode_DecodeUTF8Stateful(u, (Py_ssize_t)size, NULL, NULL);
 }
 
+
 PyObject *
 _PyUnicode_FromId(_Py_Identifier *id)
 {
-    if (id->object) {
-        return id->object;
+    PyInterpreterState *interp = _PyInterpreterState_GET();
+    struct _Py_unicode_ids *ids = &interp->unicode.ids;
+
+    int index = _Py_atomic_size_get(&id->index);
+    if (index < 0) {
+        struct _Py_unicode_runtime_ids *rt_ids = &interp->runtime->unicode_ids;
+
+        PyThread_acquire_lock(rt_ids->lock, WAIT_LOCK);
+        // Check again to detect concurrent access. Another thread can have
+        // initialized the index while this thread waited for the lock.
+        index = _Py_atomic_size_get(&id->index);
+        if (index < 0) {
+            assert(rt_ids->next_index < PY_SSIZE_T_MAX);
+            index = rt_ids->next_index;
+            rt_ids->next_index++;
+            _Py_atomic_size_set(&id->index, index);
+        }
+        PyThread_release_lock(rt_ids->lock);
     }
+    assert(index >= 0);
 
     PyObject *obj;
-    obj = PyUnicode_DecodeUTF8Stateful(id->string,
-                                       strlen(id->string),
+    if (index < ids->size) {
+        obj = ids->array[index];
+        if (obj) {
+            // Return a borrowed reference
+            return obj;
+        }
+    }
+
+    obj = PyUnicode_DecodeUTF8Stateful(id->string, strlen(id->string),
                                        NULL, NULL);
     if (!obj) {
         return NULL;
     }
     PyUnicode_InternInPlace(&obj);
 
-    assert(!id->next);
-    id->object = obj;
-    id->next = static_strings;
-    static_strings = id;
-    return id->object;
+    if (index >= ids->size) {
+        // Overallocate to reduce the number of realloc
+        Py_ssize_t new_size = Py_MAX(index * 2, 16);
+        Py_ssize_t item_size = sizeof(ids->array[0]);
+        PyObject **new_array = PyMem_Realloc(ids->array, new_size * item_size);
+        if (new_array == NULL) {
+            PyErr_NoMemory();
+            return NULL;
+        }
+        memset(&new_array[ids->size], 0, (new_size - ids->size) * item_size);
+        ids->array = new_array;
+        ids->size = new_size;
+    }
+
+    // The array stores a strong reference
+    ids->array[index] = obj;
+
+    // Return a borrowed reference
+    return obj;
 }
 
+
 static void
-unicode_clear_static_strings(void)
+unicode_clear_identifiers(PyThreadState *tstate)
 {
-    _Py_Identifier *tmp, *s = static_strings;
-    while (s) {
-        Py_CLEAR(s->object);
-        tmp = s->next;
-        s->next = NULL;
-        s = tmp;
+    PyInterpreterState *interp = _PyInterpreterState_GET();
+    struct _Py_unicode_ids *ids = &interp->unicode.ids;
+    for (Py_ssize_t i=0; i < ids->size; i++) {
+        Py_XDECREF(ids->array[i]);
     }
-    static_strings = NULL;
+    ids->size = 0;
+    PyMem_Free(ids->array);
+    ids->array = NULL;
+    // Don't reset _PyRuntime next_index: _Py_Identifier.id remains valid
+    // after Py_Finalize().
 }
 
+
 /* Internal function, doesn't check maximum character */
 
 PyObject*
@@ -16238,9 +16279,7 @@ _PyUnicode_Fini(PyThreadState *tstate)
         Py_CLEAR(state->latin1[i]);
     }
 
-    if (_Py_IsMainInterpreter(tstate)) {
-        unicode_clear_static_strings();
-    }
+    unicode_clear_identifiers(tstate);
 
     _PyUnicode_FiniEncodings(&tstate->interp->unicode.fs_codec);
 }