pystate.c

/* Thread and interpreter state structures and their interfaces */

#include "Python.h"
#include "pycore_ceval.h"
#include "pycore_code.h"          // stats
#include "pycore_critical_section.h"       // _PyCriticalSection_Resume()
#include "pycore_dtoa.h"          // _dtoa_state_INIT()
#include "pycore_emscripten_trampoline.h"  // _Py_EmscriptenTrampoline_Init()
#include "pycore_frame.h"
#include "pycore_initconfig.h"    // _PyStatus_OK()
#include "pycore_object.h"        // _PyType_InitCache()
#include "pycore_parking_lot.h"   // _PyParkingLot_AfterFork()
#include "pycore_pyerrors.h"      // _PyErr_Clear()
#include "pycore_pylifecycle.h"   // _PyAST_Fini()
#include "pycore_pymem.h"         // _PyMem_SetDefaultAllocator()
#include "pycore_pystate.h"
#include "pycore_runtime_init.h"  // _PyRuntimeState_INIT
#include "pycore_sysmodule.h"     // _PySys_Audit()

/* --------------------------------------------------------------------------
CAUTION

Always use PyMem_RawMalloc() and PyMem_RawFree() directly in this file.  A
number of these functions are advertised as safe to call when the GIL isn't
held, and in a debug build Python redirects (e.g.) PyMem_NEW (etc) to Python's
debugging obmalloc functions.  Those aren't thread-safe (they rely on the GIL
to avoid the expense of doing their own locking).
-------------------------------------------------------------------------- */

#ifdef HAVE_DLOPEN
#  ifdef HAVE_DLFCN_H
#    include <dlfcn.h>
#  endif
#  if !HAVE_DECL_RTLD_LAZY
#    define RTLD_LAZY 1
#  endif
#endif


/****************************************/
/* helpers for the current thread state */
/****************************************/

// API for the current thread state is further down.

/* "current" means one of:
   - bound to the current OS thread
   - holds the GIL
 */

//-------------------------------------------------
// a highly efficient lookup for the current thread
//-------------------------------------------------

/*
   The stored thread state is set by PyThreadState_Swap().

   For each of these functions, the GIL must be held by the current thread.
 */


#ifdef HAVE_THREAD_LOCAL
_Py_thread_local PyThreadState *_Py_tss_tstate = NULL;
#endif

static inline PyThreadState *
current_fast_get(_PyRuntimeState *Py_UNUSED(runtime))
{
#ifdef HAVE_THREAD_LOCAL
    return _Py_tss_tstate;
#else
    // XXX Fall back to the PyThread_tss_*() API.
#  error "no supported thread-local variable storage classifier"
#endif
}

static inline void
current_fast_set(_PyRuntimeState *Py_UNUSED(runtime), PyThreadState *tstate)
{
    assert(tstate != NULL);
#ifdef HAVE_THREAD_LOCAL
    _Py_tss_tstate = tstate;
#else
    // XXX Fall back to the PyThread_tss_*() API.
#  error "no supported thread-local variable storage classifier"
#endif
}

static inline void
current_fast_clear(_PyRuntimeState *Py_UNUSED(runtime))
{
#ifdef HAVE_THREAD_LOCAL
    _Py_tss_tstate = NULL;
#else
    // XXX Fall back to the PyThread_tss_*() API.
#  error "no supported thread-local variable storage classifier"
#endif
}

#define tstate_verify_not_active(tstate) \
    if (tstate == current_fast_get((tstate)->interp->runtime)) { \
        _Py_FatalErrorFormat(__func__, "tstate %p is still current", tstate); \
    }

PyThreadState *
_PyThreadState_GetCurrent(void)
{
    return current_fast_get(&_PyRuntime);
}


//------------------------------------------------
// the thread state bound to the current OS thread
//------------------------------------------------

static inline int
tstate_tss_initialized(Py_tss_t *key)
{
    return PyThread_tss_is_created(key);
}

static inline int
tstate_tss_init(Py_tss_t *key)
{
    assert(!tstate_tss_initialized(key));
    return PyThread_tss_create(key);
}

static inline void
tstate_tss_fini(Py_tss_t *key)
{
    assert(tstate_tss_initialized(key));
    PyThread_tss_delete(key);
}

static inline PyThreadState *
tstate_tss_get(Py_tss_t *key)
{
    assert(tstate_tss_initialized(key));
    return (PyThreadState *)PyThread_tss_get(key);
}

static inline int
tstate_tss_set(Py_tss_t *key, PyThreadState *tstate)
{
    assert(tstate != NULL);
    assert(tstate_tss_initialized(key));
    return PyThread_tss_set(key, (void *)tstate);
}

static inline int
tstate_tss_clear(Py_tss_t *key)
{
    assert(tstate_tss_initialized(key));
    return PyThread_tss_set(key, (void *)NULL);
}

#ifdef HAVE_FORK
/* Reset the TSS key - called by PyOS_AfterFork_Child().
 * This should not be necessary, but some - buggy - pthread implementations
 * don't reset TSS upon fork(), see issue #10517.
 */
static PyStatus
tstate_tss_reinit(Py_tss_t *key)
{
    if (!tstate_tss_initialized(key)) {
        return _PyStatus_OK();
    }
    PyThreadState *tstate = tstate_tss_get(key);

    tstate_tss_fini(key);
    if (tstate_tss_init(key) != 0) {
        return _PyStatus_NO_MEMORY();
    }

    /* If the thread had an associated auto thread state, reassociate it with
     * the new key. */
    if (tstate && tstate_tss_set(key, tstate) != 0) {
        return _PyStatus_ERR("failed to re-set autoTSSkey");
    }
    return _PyStatus_OK();
}
#endif


/*
   The stored thread state is set by bind_tstate() (AKA PyThreadState_Bind().

   The GIL does no need to be held for these.
  */

#define gilstate_tss_initialized(runtime) \
    tstate_tss_initialized(&(runtime)->autoTSSkey)
#define gilstate_tss_init(runtime) \
    tstate_tss_init(&(runtime)->autoTSSkey)
#define gilstate_tss_fini(runtime) \
    tstate_tss_fini(&(runtime)->autoTSSkey)
#define gilstate_tss_get(runtime) \
    tstate_tss_get(&(runtime)->autoTSSkey)
#define _gilstate_tss_set(runtime, tstate) \
    tstate_tss_set(&(runtime)->autoTSSkey, tstate)
#define _gilstate_tss_clear(runtime) \
    tstate_tss_clear(&(runtime)->autoTSSkey)
#define gilstate_tss_reinit(runtime) \
    tstate_tss_reinit(&(runtime)->autoTSSkey)

static inline void
gilstate_tss_set(_PyRuntimeState *runtime, PyThreadState *tstate)
{
    assert(tstate != NULL && tstate->interp->runtime == runtime);
    if (_gilstate_tss_set(runtime, tstate) != 0) {
        Py_FatalError("failed to set current tstate (TSS)");
    }
}

static inline void
gilstate_tss_clear(_PyRuntimeState *runtime)
{
    if (_gilstate_tss_clear(runtime) != 0) {
        Py_FatalError("failed to clear current tstate (TSS)");
    }
}


#ifndef NDEBUG
static inline int tstate_is_alive(PyThreadState *tstate);

static inline int
tstate_is_bound(PyThreadState *tstate)
{
    return tstate->_status.bound && !tstate->_status.unbound;
}
#endif  // !NDEBUG

static void bind_gilstate_tstate(PyThreadState *);
static void unbind_gilstate_tstate(PyThreadState *);

static void tstate_mimalloc_bind(PyThreadState *);

static void
bind_tstate(PyThreadState *tstate)
{
    assert(tstate != NULL);
    assert(tstate_is_alive(tstate) && !tstate->_status.bound);
    assert(!tstate->_status.unbound);  // just in case
    assert(!tstate->_status.bound_gilstate);
    assert(tstate != gilstate_tss_get(tstate->interp->runtime));
    assert(!tstate->_status.active);
    assert(tstate->thread_id == 0);
    assert(tstate->native_thread_id == 0);

    // Currently we don't necessarily store the thread state
    // in thread-local storage (e.g. per-interpreter).

    tstate->thread_id = PyThread_get_thread_ident();
#ifdef PY_HAVE_THREAD_NATIVE_ID
    tstate->native_thread_id = PyThread_get_thread_native_id();
#endif

    // mimalloc state needs to be initialized from the active thread.
    tstate_mimalloc_bind(tstate);

    tstate->_status.bound = 1;
}

static void
unbind_tstate(PyThreadState *tstate)
{
    assert(tstate != NULL);
    assert(tstate_is_bound(tstate));
#ifndef HAVE_PTHREAD_STUBS
    assert(tstate->thread_id > 0);
#endif
#ifdef PY_HAVE_THREAD_NATIVE_ID
    assert(tstate->native_thread_id > 0);
#endif

    // We leave thread_id and native_thread_id alone
    // since they can be useful for debugging.
    // Check the `_status` field to know if these values
    // are still valid.

    // We leave tstate->_status.bound set to 1
    // to indicate it was previously bound.
    tstate->_status.unbound = 1;
}


/* Stick the thread state for this thread in thread specific storage.

   When a thread state is created for a thread by some mechanism
   other than PyGILState_Ensure(), it's important that the GILState
   machinery knows about it so it doesn't try to create another
   thread state for the thread.
   (This is a better fix for SF bug #1010677 than the first one attempted.)

   The only situation where you can legitimately have more than one
   thread state for an OS level thread is when there are multiple
   interpreters.

   Before 3.12, the PyGILState_*() APIs didn't work with multiple
   interpreters (see bpo-10915 and bpo-15751), so this function used
   to set TSS only once.  Thus, the first thread state created for that
   given OS level thread would "win", which seemed reasonable behaviour.
*/

static void
bind_gilstate_tstate(PyThreadState *tstate)
{
    assert(tstate != NULL);
    assert(tstate_is_alive(tstate));
    assert(tstate_is_bound(tstate));
    // XXX assert(!tstate->_status.active);
    assert(!tstate->_status.bound_gilstate);

    _PyRuntimeState *runtime = tstate->interp->runtime;
    PyThreadState *tcur = gilstate_tss_get(runtime);
    assert(tstate != tcur);

    if (tcur != NULL) {
        tcur->_status.bound_gilstate = 0;
    }
    gilstate_tss_set(runtime, tstate);
    tstate->_status.bound_gilstate = 1;
}

static void
unbind_gilstate_tstate(PyThreadState *tstate)
{
    assert(tstate != NULL);
    // XXX assert(tstate_is_alive(tstate));
    assert(tstate_is_bound(tstate));
    // XXX assert(!tstate->_status.active);
    assert(tstate->_status.bound_gilstate);
    assert(tstate == gilstate_tss_get(tstate->interp->runtime));

    gilstate_tss_clear(tstate->interp->runtime);
    tstate->_status.bound_gilstate = 0;
}


//----------------------------------------------
// the thread state that currently holds the GIL
//----------------------------------------------

/* This is not exported, as it is not reliable!  It can only
   ever be compared to the state for the *current* thread.
   * If not equal, then it doesn't matter that the actual
     value may change immediately after comparison, as it can't
     possibly change to the current thread's state.
   * If equal, then the current thread holds the lock, so the value can't
     change until we yield the lock.
*/
static int
holds_gil(PyThreadState *tstate)
{
    // XXX Fall back to tstate->interp->runtime->ceval.gil.last_holder
    // (and tstate->interp->runtime->ceval.gil.locked).
    assert(tstate != NULL);
    _PyRuntimeState *runtime = tstate->interp->runtime;
    /* Must be the tstate for this thread */
    assert(tstate == gilstate_tss_get(runtime));
    return tstate == current_fast_get(runtime);
}


/****************************/
/* the global runtime state */
/****************************/

//----------
// lifecycle
//----------

/* Suppress deprecation warning for PyBytesObject.ob_shash */
_Py_COMP_DIAG_PUSH
_Py_COMP_DIAG_IGNORE_DEPR_DECLS
/* We use "initial" if the runtime gets re-used
   (e.g. Py_Finalize() followed by Py_Initialize().
   Note that we initialize "initial" relative to _PyRuntime,
   to ensure pre-initialized pointers point to the active
   runtime state (and not "initial"). */
static const _PyRuntimeState initial = _PyRuntimeState_INIT(_PyRuntime);
_Py_COMP_DIAG_POP

#define LOCKS_INIT(runtime) \
    { \
        &(runtime)->interpreters.mutex, \
        &(runtime)->xi.registry.mutex, \
        &(runtime)->unicode_state.ids.mutex, \
        &(runtime)->imports.extensions.mutex, \
        &(runtime)->ceval.pending_mainthread.mutex, \
        &(runtime)->atexit.mutex, \
        &(runtime)->audit_hooks.mutex, \
        &(runtime)->allocators.mutex, \
    }

static void
init_runtime(_PyRuntimeState *runtime,
             void *open_code_hook, void *open_code_userdata,
             _Py_AuditHookEntry *audit_hook_head,
             Py_ssize_t unicode_next_index)
{
    assert(!runtime->preinitializing);
    assert(!runtime->preinitialized);
    assert(!runtime->core_initialized);
    assert(!runtime->initialized);
    assert(!runtime->_initialized);

    runtime->open_code_hook = open_code_hook;
    runtime->open_code_userdata = open_code_userdata;
    runtime->audit_hooks.head = audit_hook_head;

    PyPreConfig_InitPythonConfig(&runtime->preconfig);

    // Set it to the ID of the main thread of the main interpreter.
    runtime->main_thread = PyThread_get_thread_ident();

    runtime->unicode_state.ids.next_index = unicode_next_index;

#if defined(__EMSCRIPTEN__) && defined(PY_CALL_TRAMPOLINE)
    _Py_EmscriptenTrampoline_Init(runtime);
#endif

    runtime->_initialized = 1;
}

PyStatus
_PyRuntimeState_Init(_PyRuntimeState *runtime)
{
    /* We preserve the hook across init, because there is
       currently no public API to set it between runtime
       initialization and interpreter initialization. */
    void *open_code_hook = runtime->open_code_hook;
    void *open_code_userdata = runtime->open_code_userdata;
    _Py_AuditHookEntry *audit_hook_head = runtime->audit_hooks.head;
    // bpo-42882: Preserve next_index value if Py_Initialize()/Py_Finalize()
    // is called multiple times.
    Py_ssize_t unicode_next_index = runtime->unicode_state.ids.next_index;

    if (runtime->_initialized) {
        // Py_Initialize() must be running again.
        // Reset to _PyRuntimeState_INIT.
        memcpy(runtime, &initial, sizeof(*runtime));
        assert(!runtime->_initialized);
    }

    if (gilstate_tss_init(runtime) != 0) {
        _PyRuntimeState_Fini(runtime);
        return _PyStatus_NO_MEMORY();
    }

    if (PyThread_tss_create(&runtime->trashTSSkey) != 0) {
        _PyRuntimeState_Fini(runtime);
        return _PyStatus_NO_MEMORY();
    }

    init_runtime(runtime, open_code_hook, open_code_userdata, audit_hook_head,
                 unicode_next_index);

    return _PyStatus_OK();
}

void
_PyRuntimeState_Fini(_PyRuntimeState *runtime)
{
#ifdef Py_REF_DEBUG
    /* The count is cleared by _Py_FinalizeRefTotal(). */
    assert(runtime->object_state.interpreter_leaks == 0);
#endif

    if (gilstate_tss_initialized(runtime)) {
        gilstate_tss_fini(runtime);
    }

    if (PyThread_tss_is_created(&runtime->trashTSSkey)) {
        PyThread_tss_delete(&runtime->trashTSSkey);
    }
}

#ifdef HAVE_FORK
/* This function is called from PyOS_AfterFork_Child to ensure that
   newly created child processes do not share locks with the parent. */
PyStatus
_PyRuntimeState_ReInitThreads(_PyRuntimeState *runtime)
{
    // This was initially set in _PyRuntimeState_Init().
    runtime->main_thread = PyThread_get_thread_ident();

    // Clears the parking lot. Any waiting threads are dead. This must be
    // called before releasing any locks that use the parking lot.
    _PyParkingLot_AfterFork();

    // Re-initialize global locks
    PyMutex *locks[] = LOCKS_INIT(runtime);
    for (size_t i = 0; i < Py_ARRAY_LENGTH(locks); i++) {
        _PyMutex_at_fork_reinit(locks[i]);
    }

    /* bpo-42540: id_mutex is freed by _PyInterpreterState_Delete, which does
     * not force the default allocator. */
    if (_PyThread_at_fork_reinit(&runtime->interpreters.main->id_mutex) < 0) {
        return _PyStatus_ERR("Failed to reinitialize runtime locks");
    }

    PyStatus status = gilstate_tss_reinit(runtime);
    if (_PyStatus_EXCEPTION(status)) {
        return status;
    }

    if (PyThread_tss_is_created(&runtime->trashTSSkey)) {
        PyThread_tss_delete(&runtime->trashTSSkey);
    }
    if (PyThread_tss_create(&runtime->trashTSSkey) != 0) {
        return _PyStatus_NO_MEMORY();
    }

    return _PyStatus_OK();
}
#endif


/*************************************/
/* the per-interpreter runtime state */
/*************************************/

//----------
// lifecycle
//----------

/* Calling this indicates that the runtime is ready to create interpreters. */

PyStatus
_PyInterpreterState_Enable(_PyRuntimeState *runtime)
{
    struct pyinterpreters *interpreters = &runtime->interpreters;
    interpreters->next_id = 0;
    return _PyStatus_OK();
}


static PyInterpreterState *
alloc_interpreter(void)
{
    return PyMem_RawCalloc(1, sizeof(PyInterpreterState));
}

static void
free_interpreter(PyInterpreterState *interp)
{
    // The main interpreter is statically allocated so
    // should not be freed.
    if (interp != &_PyRuntime._main_interpreter) {
        PyMem_RawFree(interp);
    }
}

/* Get the interpreter state to a minimal consistent state.
   Further init happens in pylifecycle.c before it can be used.
   All fields not initialized here are expected to be zeroed out,
   e.g. by PyMem_RawCalloc() or memset(), or otherwise pre-initialized.
   The runtime state is not manipulated.  Instead it is assumed that
   the interpreter is getting added to the runtime.

   Note that the main interpreter was statically initialized as part
   of the runtime and most state is already set properly.  That leaves
   a small number of fields to initialize dynamically, as well as some
   that are initialized lazily.

   For subinterpreters we memcpy() the main interpreter in
   PyInterpreterState_New(), leaving it in the same mostly-initialized
   state.  The only difference is that the interpreter has some
   self-referential state that is statically initializexd to the
   main interpreter.  We fix those fields here, in addition
   to the other dynamically initialized fields.
  */
static PyStatus
init_interpreter(PyInterpreterState *interp,
                 _PyRuntimeState *runtime, int64_t id,
                 PyInterpreterState *next)
{
    if (interp->_initialized) {
        return _PyStatus_ERR("interpreter already initialized");
    }

    assert(runtime != NULL);
    interp->runtime = runtime;

    assert(id > 0 || (id == 0 && interp == runtime->interpreters.main));
    interp->id = id;

    assert(runtime->interpreters.head == interp);
    assert(next != NULL || (interp == runtime->interpreters.main));
    interp->next = next;

    /* Initialize obmalloc, but only for subinterpreters,
       since the main interpreter is initialized statically. */
    if (interp != &runtime->_main_interpreter) {
        poolp temp[OBMALLOC_USED_POOLS_SIZE] = \
                _obmalloc_pools_INIT(interp->obmalloc.pools);
        memcpy(&interp->obmalloc.pools.used, temp, sizeof(temp));
    }

    PyStatus status = _PyObject_InitState(interp);
    if (_PyStatus_EXCEPTION(status)) {
        return status;
    }

    _PyEval_InitState(interp);
    _PyGC_InitState(&interp->gc);
    PyConfig_InitPythonConfig(&interp->config);
    _PyType_InitCache(interp);
    for (int i = 0; i < _PY_MONITORING_UNGROUPED_EVENTS; i++) {
        interp->monitors.tools[i] = 0;
    }
    for (int t = 0; t < PY_MONITORING_TOOL_IDS; t++) {
        for (int e = 0; e < _PY_MONITORING_EVENTS; e++) {
            interp->monitoring_callables[t][e] = NULL;

        }
    }
    interp->sys_profile_initialized = false;
    interp->sys_trace_initialized = false;
    interp->optimizer = &_PyOptimizer_Default;
    interp->optimizer_backedge_threshold = _PyOptimizer_Default.backedge_threshold;
    interp->optimizer_resume_threshold = _PyOptimizer_Default.backedge_threshold;
    interp->next_func_version = 1;
    interp->executor_list_head = NULL;
    if (interp != &runtime->_main_interpreter) {
        /* Fix the self-referential, statically initialized fields. */
        interp->dtoa = (struct _dtoa_state)_dtoa_state_INIT(interp);
    }

    interp->_initialized = 1;
    return _PyStatus_OK();
}


PyStatus
_PyInterpreterState_New(PyThreadState *tstate, PyInterpreterState **pinterp)
{
    *pinterp = NULL;

    // Don't get runtime from tstate since tstate can be NULL
    _PyRuntimeState *runtime = &_PyRuntime;

    // tstate is NULL when pycore_create_interpreter() calls
    // _PyInterpreterState_New() to create the main interpreter.
    if (tstate != NULL) {
        if (_PySys_Audit(tstate, "cpython.PyInterpreterState_New", NULL) < 0) {
            return _PyStatus_ERR("sys.audit failed");
        }
    }

    /* We completely serialize creation of multiple interpreters, since
       it simplifies things here and blocking concurrent calls isn't a problem.
       Regardless, we must fully block subinterpreter creation until
       after the main interpreter is created. */
    HEAD_LOCK(runtime);

    struct pyinterpreters *interpreters = &runtime->interpreters;
    int64_t id = interpreters->next_id;
    interpreters->next_id += 1;

    // Allocate the interpreter and add it to the runtime state.
    PyInterpreterState *interp;
    PyStatus status;
    PyInterpreterState *old_head = interpreters->head;
    if (old_head == NULL) {
        // We are creating the main interpreter.
        assert(interpreters->main == NULL);
        assert(id == 0);

        interp = &runtime->_main_interpreter;
        assert(interp->id == 0);
        assert(interp->next == NULL);

        interpreters->main = interp;
    }
    else {
        assert(interpreters->main != NULL);
        assert(id != 0);

        interp = alloc_interpreter();
        if (interp == NULL) {
            status = _PyStatus_NO_MEMORY();
            goto error;
        }
        // Set to _PyInterpreterState_INIT.
        memcpy(interp, &initial._main_interpreter, sizeof(*interp));

        if (id < 0) {
            /* overflow or Py_Initialize() not called yet! */
            status = _PyStatus_ERR("failed to get an interpreter ID");
            goto error;
        }
    }
    interpreters->head = interp;

    status = init_interpreter(interp, runtime,
                              id, old_head);
    if (_PyStatus_EXCEPTION(status)) {
        goto error;
    }

    HEAD_UNLOCK(runtime);

    assert(interp != NULL);
    *pinterp = interp;
    return _PyStatus_OK();

error:
    HEAD_UNLOCK(runtime);

    if (interp != NULL) {
        free_interpreter(interp);
    }
    return status;
}


PyInterpreterState *
PyInterpreterState_New(void)
{
    // tstate can be NULL
    PyThreadState *tstate = current_fast_get(&_PyRuntime);

    PyInterpreterState *interp;
    PyStatus status = _PyInterpreterState_New(tstate, &interp);
    if (_PyStatus_EXCEPTION(status)) {
        Py_ExitStatusException(status);
    }
    assert(interp != NULL);
    return interp;
}


static void
interpreter_clear(PyInterpreterState *interp, PyThreadState *tstate)
{
    assert(interp != NULL);
    assert(tstate != NULL);
    _PyRuntimeState *runtime = interp->runtime;

    /* XXX Conditions we need to enforce:

       * the GIL must be held by the current thread
       * tstate must be the "current" thread state (current_fast_get())
       * tstate->interp must be interp
       * for the main interpreter, tstate must be the main thread
     */
    // XXX Ideally, we would not rely on any thread state in this function
    // (and we would drop the "tstate" argument).

    if (_PySys_Audit(tstate, "cpython.PyInterpreterState_Clear", NULL) < 0) {
        _PyErr_Clear(tstate);
    }

    // Clear the current/main thread state last.
    HEAD_LOCK(runtime);
    PyThreadState *p = interp->threads.head;
    HEAD_UNLOCK(runtime);
    while (p != NULL) {
        // See https://github.com/python/cpython/issues/102126
        // Must be called without HEAD_LOCK held as it can deadlock
        // if any finalizer tries to acquire that lock.
        PyThreadState_Clear(p);
        HEAD_LOCK(runtime);
        p = p->next;
        HEAD_UNLOCK(runtime);
    }
    if (tstate->interp == interp) {
        /* We fix tstate->_status below when we for sure aren't using it
           (e.g. no longer need the GIL). */
        // XXX Eliminate the need to do this.
        tstate->_status.cleared = 0;
    }

    Py_CLEAR(interp->optimizer);
    interp->optimizer = &_PyOptimizer_Default;
    interp->optimizer_backedge_threshold = _PyOptimizer_Default.backedge_threshold;
    interp->optimizer_resume_threshold = _PyOptimizer_Default.backedge_threshold;

    /* It is possible that any of the objects below have a finalizer
       that runs Python code or otherwise relies on a thread state
       or even the interpreter state.  For now we trust that isn't
       a problem.
     */
    // XXX Make sure we properly deal with problematic finalizers.

    Py_CLEAR(interp->audit_hooks);

    // At this time, all the threads should be cleared so we don't need
    // atomic operations for eval_breaker
    interp->ceval.eval_breaker = 0;

    for (int i = 0; i < _PY_MONITORING_UNGROUPED_EVENTS; i++) {
        interp->monitors.tools[i] = 0;
    }
    for (int t = 0; t < PY_MONITORING_TOOL_IDS; t++) {
        for (int e = 0; e < _PY_MONITORING_EVENTS; e++) {
            Py_CLEAR(interp->monitoring_callables[t][e]);
        }
    }
    interp->sys_profile_initialized = false;
    interp->sys_trace_initialized = false;
    for (int t = 0; t < PY_MONITORING_TOOL_IDS; t++) {
        Py_CLEAR(interp->monitoring_tool_names[t]);
    }

    PyConfig_Clear(&interp->config);
    Py_CLEAR(interp->codec_search_path);
    Py_CLEAR(interp->codec_search_cache);
    Py_CLEAR(interp->codec_error_registry);

    assert(interp->imports.modules == NULL);
    assert(interp->imports.modules_by_index == NULL);
    assert(interp->imports.importlib == NULL);
    assert(interp->imports.import_func == NULL);

    Py_CLEAR(interp->sysdict_copy);
    Py_CLEAR(interp->builtins_copy);
    Py_CLEAR(interp->dict);
#ifdef HAVE_FORK
    Py_CLEAR(interp->before_forkers);
    Py_CLEAR(interp->after_forkers_parent);
    Py_CLEAR(interp->after_forkers_child);
#endif

    _PyAST_Fini(interp);
    _PyWarnings_Fini(interp);
    _PyAtExit_Fini(interp);

    // All Python types must be destroyed before the last GC collection. Python
    // types create a reference cycle to themselves in their in their
    // PyTypeObject.tp_mro member (the tuple contains the type).

    /* Last garbage collection on this interpreter */
    _PyGC_CollectNoFail(tstate);
    _PyGC_Fini(interp);

    /* We don't clear sysdict and builtins until the end of this function.
       Because clearing other attributes can execute arbitrary Python code
       which requires sysdict and builtins. */
    PyDict_Clear(interp->sysdict);
    PyDict_Clear(interp->builtins);
    Py_CLEAR(interp->sysdict);
    Py_CLEAR(interp->builtins);

    if (tstate->interp == interp) {
        /* We are now safe to fix tstate->_status.cleared. */
        // XXX Do this (much) earlier?
        tstate->_status.cleared = 1;
    }

    for (int i=0; i < DICT_MAX_WATCHERS; i++) {
        interp->dict_state.watchers[i] = NULL;
    }

    for (int i=0; i < TYPE_MAX_WATCHERS; i++) {
        interp->type_watchers[i] = NULL;
    }

    for (int i=0; i < FUNC_MAX_WATCHERS; i++) {
        interp->func_watchers[i] = NULL;
    }
    interp->active_func_watchers = 0;

    for (int i=0; i < CODE_MAX_WATCHERS; i++) {
        interp->code_watchers[i] = NULL;
    }
    interp->active_code_watchers = 0;
    // XXX Once we have one allocator per interpreter (i.e.
    // per-interpreter GC) we must ensure that all of the interpreter's
    // objects have been cleaned up at the point.
}


void
PyInterpreterState_Clear(PyInterpreterState *interp)
{
    // Use the current Python thread state to call audit hooks and to collect
    // garbage. It can be different than the current Python thread state
    // of 'interp'.
    PyThreadState *current_tstate = current_fast_get(interp->runtime);
    _PyImport_ClearCore(interp);
    interpreter_clear(interp, current_tstate);
}


void
_PyInterpreterState_Clear(PyThreadState *tstate)
{
    _PyImport_ClearCore(tstate->interp);
    interpreter_clear(tstate->interp, tstate);
}


static inline void tstate_deactivate(PyThreadState *tstate);
static void tstate_set_detached(PyThreadState *tstate);
static void zapthreads(PyInterpreterState *interp);

void
PyInterpreterState_Delete(PyInterpreterState *interp)
{
    _PyRuntimeState *runtime = interp->runtime;
    struct pyinterpreters *interpreters = &runtime->interpreters;

    // XXX Clearing the "current" thread state should happen before
    // we start finalizing the interpreter (or the current thread state).
    PyThreadState *tcur = current_fast_get(runtime);
    if (tcur != NULL && interp == tcur->interp) {
        /* Unset current thread.  After this, many C API calls become crashy. */
        _PyThreadState_Detach(tcur);
    }

    zapthreads(interp);

    // XXX These two calls should be done at the end of clear_interpreter(),
    // but currently some objects get decref'ed after that.
#ifdef Py_REF_DEBUG
    _PyInterpreterState_FinalizeRefTotal(interp);
#endif
    _PyInterpreterState_FinalizeAllocatedBlocks(interp);

    HEAD_LOCK(runtime);
    PyInterpreterState **p;
    for (p = &interpreters->head; ; p = &(*p)->next) {
        if (*p == NULL) {
            Py_FatalError("NULL interpreter");
        }
        if (*p == interp) {
            break;
        }
    }
    if (interp->threads.head != NULL) {
        Py_FatalError("remaining threads");
    }
    *p = interp->next;

    if (interpreters->main == interp) {
        interpreters->main = NULL;
        if (interpreters->head != NULL) {
            Py_FatalError("remaining subinterpreters");
        }
    }
    HEAD_UNLOCK(runtime);

    if (interp->id_mutex != NULL) {
        PyThread_free_lock(interp->id_mutex);
    }

    _PyObject_FiniState(interp);

    free_interpreter(interp);
}


#ifdef HAVE_FORK
/*
 * Delete all interpreter states except the main interpreter.  If there
 * is a current interpreter state, it *must* be the main interpreter.
 */
PyStatus
_PyInterpreterState_DeleteExceptMain(_PyRuntimeState *runtime)
{
    struct pyinterpreters *interpreters = &runtime->interpreters;

    PyThreadState *tstate = _PyThreadState_Swap(runtime, NULL);
    if (tstate != NULL && tstate->interp != interpreters->main) {
        return _PyStatus_ERR("not main interpreter");
    }

    HEAD_LOCK(runtime);
    PyInterpreterState *interp = interpreters->head;
    interpreters->head = NULL;
    while (interp != NULL) {
        if (interp == interpreters->main) {
            interpreters->main->next = NULL;
            interpreters->head = interp;
            interp = interp->next;
            continue;
        }

        // XXX Won't this fail since PyInterpreterState_Clear() requires
        // the "current" tstate to be set?
        PyInterpreterState_Clear(interp);  // XXX must activate?
        zapthreads(interp);
        if (interp->id_mutex != NULL) {
            PyThread_free_lock(interp->id_mutex);
        }
        PyInterpreterState *prev_interp = interp;
        interp = interp->next;
        free_interpreter(prev_interp);
    }
    HEAD_UNLOCK(runtime);

    if (interpreters->head == NULL) {
        return _PyStatus_ERR("missing main interpreter");