frameobject.c

/* Frame object implementation */

#include "Python.h"
#include "pycore_ceval.h"         // _PyEval_BuiltinsFromGlobals()
#include "pycore_code.h"          // CO_FAST_LOCAL, etc.
#include "pycore_function.h"      // _PyFunction_FromConstructor()
#include "pycore_moduleobject.h"  // _PyModule_GetDict()
#include "pycore_object.h"        // _PyObject_GC_UNTRACK()
#include "pycore_opcode.h"        // _PyOpcode_Caches

#include "frameobject.h"          // PyFrameObject
#include "pycore_frame.h"
#include "opcode.h"               // EXTENDED_ARG
#include "structmember.h"         // PyMemberDef

#define OFF(x) offsetof(PyFrameObject, x)

static PyMemberDef frame_memberlist[] = {
    {"f_trace_lines",   T_BOOL,         OFF(f_trace_lines), 0},
    {NULL}      /* Sentinel */
};


static PyObject *
frame_getlocals(PyFrameObject *f, void *closure)
{
    if (PyFrame_FastToLocalsWithError(f) < 0)
        return NULL;
    PyObject *locals = f->f_frame->f_locals;
    return Py_NewRef(locals);
}

int
PyFrame_GetLineNumber(PyFrameObject *f)
{
    assert(f != NULL);
    if (f->f_lineno != 0) {
        return f->f_lineno;
    }
    else {
        return PyUnstable_InterpreterFrame_GetLine(f->f_frame);
    }
}

static PyObject *
frame_getlineno(PyFrameObject *f, void *closure)
{
    int lineno = PyFrame_GetLineNumber(f);
    if (lineno < 0) {
        Py_RETURN_NONE;
    }
    else {
        return PyLong_FromLong(lineno);
    }
}

static PyObject *
frame_getlasti(PyFrameObject *f, void *closure)
{
    int lasti = _PyInterpreterFrame_LASTI(f->f_frame);
    if (lasti < 0) {
        return PyLong_FromLong(-1);
    }
    return PyLong_FromLong(lasti * sizeof(_Py_CODEUNIT));
}

static PyObject *
frame_getglobals(PyFrameObject *f, void *closure)
{
    PyObject *globals = f->f_frame->f_globals;
    if (globals == NULL) {
        globals = Py_None;
    }
    return Py_NewRef(globals);
}

static PyObject *
frame_getbuiltins(PyFrameObject *f, void *closure)
{
    PyObject *builtins = f->f_frame->f_builtins;
    if (builtins == NULL) {
        builtins = Py_None;
    }
    return Py_NewRef(builtins);
}

static PyObject *
frame_getcode(PyFrameObject *f, void *closure)
{
    if (PySys_Audit("object.__getattr__", "Os", f, "f_code") < 0) {
        return NULL;
    }
    return (PyObject *)PyFrame_GetCode(f);
}

static PyObject *
frame_getback(PyFrameObject *f, void *closure)
{
    PyObject *res = (PyObject *)PyFrame_GetBack(f);
    if (res == NULL) {
        Py_RETURN_NONE;
    }
    return res;
}

static PyObject *
frame_gettrace_opcodes(PyFrameObject *f, void *closure)
{
    PyObject *result = f->f_trace_opcodes ? Py_True : Py_False;
    return Py_NewRef(result);
}

static int
frame_settrace_opcodes(PyFrameObject *f, PyObject* value, void *Py_UNUSED(ignored))
{
    if (!PyBool_Check(value)) {
        PyErr_SetString(PyExc_TypeError,
                        "attribute value type must be bool");
        return -1;
    }
    if (value == Py_True) {
        f->f_trace_opcodes = 1;
        _PyInterpreterState_GET()->f_opcode_trace_set = true;
    }
    else {
        f->f_trace_opcodes = 0;
    }
    return 0;
}

/* Model the evaluation stack, to determine which jumps
 * are safe and how many values needs to be popped.
 * The stack is modelled by a 64 integer, treating any
 * stack that can't fit into 64 bits as "overflowed".
 */

typedef enum kind {
    Iterator = 1,
    Except = 2,
    Object = 3,
    Null = 4,
    Lasti = 5,
} Kind;

static int
compatible_kind(Kind from, Kind to) {
    if (to == 0) {
        return 0;
    }
    if (to == Object) {
        return from != Null;
    }
    if (to == Null) {
        return 1;
    }
    return from == to;
}

#define BITS_PER_BLOCK 3

#define UNINITIALIZED -2
#define OVERFLOWED -1

#define MAX_STACK_ENTRIES (63/BITS_PER_BLOCK)
#define WILL_OVERFLOW (1ULL<<((MAX_STACK_ENTRIES-1)*BITS_PER_BLOCK))

#define EMPTY_STACK 0

static inline int64_t
push_value(int64_t stack, Kind kind)
{
    if (((uint64_t)stack) >= WILL_OVERFLOW) {
        return OVERFLOWED;
    }
    else {
        return (stack << BITS_PER_BLOCK) | kind;
    }
}

static inline int64_t
pop_value(int64_t stack)
{
    return Py_ARITHMETIC_RIGHT_SHIFT(int64_t, stack, BITS_PER_BLOCK);
}

#define MASK ((1<<BITS_PER_BLOCK)-1)

static inline Kind
top_of_stack(int64_t stack)
{
    return stack & MASK;
}

static inline Kind
peek(int64_t stack, int n)
{
    assert(n >= 1);
    return (stack>>(BITS_PER_BLOCK*(n-1))) & MASK;
}

static Kind
stack_swap(int64_t stack, int n)
{
    assert(n >= 1);
    Kind to_swap = peek(stack, n);
    Kind top = top_of_stack(stack);
    int shift = BITS_PER_BLOCK*(n-1);
    int64_t replaced_low = (stack & ~(MASK << shift)) | (top << shift);
    int64_t replaced_top = (replaced_low & ~MASK) | to_swap;
    return replaced_top;
}

static int64_t
pop_to_level(int64_t stack, int level) {
    if (level == 0) {
        return EMPTY_STACK;
    }
    int64_t max_item = (1<<BITS_PER_BLOCK) - 1;
    int64_t level_max_stack = max_item << ((level-1) * BITS_PER_BLOCK);
    while (stack > level_max_stack) {
        stack = pop_value(stack);
    }
    return stack;
}

#if 0
/* These functions are useful for debugging the stack marking code */

static char
tos_char(int64_t stack) {
    switch(top_of_stack(stack)) {
        case Iterator:
            return 'I';
        case Except:
            return 'E';
        case Object:
            return 'O';
        case Lasti:
            return 'L';
        case Null:
            return 'N';
    }
    return '?';
}

static void
print_stack(int64_t stack) {
    if (stack < 0) {
        if (stack == UNINITIALIZED) {
            printf("---");
        }
        else if (stack == OVERFLOWED) {
            printf("OVERFLOWED");
        }
        else {
            printf("??");
        }
        return;
    }
    while (stack) {
        printf("%c", tos_char(stack));
        stack = pop_value(stack);
    }
}

static void
print_stacks(int64_t *stacks, int n) {
    for (int i = 0; i < n; i++) {
        printf("%d: ", i);
        print_stack(stacks[i]);
        printf("\n");
    }
}

#endif

static int64_t *
mark_stacks(PyCodeObject *code_obj, int len)
{
    PyObject *co_code = _PyCode_GetCode(code_obj);
    if (co_code == NULL) {
        return NULL;
    }
    _Py_CODEUNIT *code = (_Py_CODEUNIT *)PyBytes_AS_STRING(co_code);
    int64_t *stacks = PyMem_New(int64_t, len+1);
    int i, j, opcode;

    if (stacks == NULL) {
        PyErr_NoMemory();
        Py_DECREF(co_code);
        return NULL;
    }
    for (int i = 1; i <= len; i++) {
        stacks[i] = UNINITIALIZED;
    }
    stacks[0] = EMPTY_STACK;
    if (code_obj->co_flags & (CO_GENERATOR | CO_COROUTINE | CO_ASYNC_GENERATOR))
    {
        // Generators get sent None while starting:
        stacks[0] = push_value(stacks[0], Object);
    }
    int todo = 1;
    while (todo) {
        todo = 0;
        /* Scan instructions */
        for (i = 0; i < len;) {
            int64_t next_stack = stacks[i];
            opcode = _Py_GetBaseOpcode(code_obj, i);
            int oparg = 0;
            while (opcode == EXTENDED_ARG) {
                oparg = (oparg << 8) | code[i].op.arg;
                i++;
                opcode = _Py_GetBaseOpcode(code_obj, i);
                stacks[i] = next_stack;
            }
            int next_i = i + _PyOpcode_Caches[opcode] + 1;
            if (next_stack == UNINITIALIZED) {
                i = next_i;
                continue;
            }
            oparg = (oparg << 8) | code[i].op.arg;
            switch (opcode) {
                case POP_JUMP_IF_FALSE:
                case POP_JUMP_IF_TRUE:
                {
                    int64_t target_stack;
                    int j = next_i + oparg;
                    assert(j < len);
                    next_stack = pop_value(next_stack);
                    target_stack = next_stack;
                    assert(stacks[j] == UNINITIALIZED || stacks[j] == target_stack);
                    stacks[j] = target_stack;
                    stacks[next_i] = next_stack;
                    break;
                }
                case SEND:
                    j = oparg + i + INLINE_CACHE_ENTRIES_SEND + 1;
                    assert(j < len);
                    assert(stacks[j] == UNINITIALIZED || stacks[j] == next_stack);
                    stacks[j] = next_stack;
                    stacks[next_i] = next_stack;
                    break;
                case JUMP_FORWARD:
                    j = oparg + i + 1;
                    assert(j < len);
                    assert(stacks[j] == UNINITIALIZED || stacks[j] == next_stack);
                    stacks[j] = next_stack;
                    break;
                case JUMP_BACKWARD:
                case JUMP_BACKWARD_NO_INTERRUPT:
                    j = next_i - oparg;
                    assert(j >= 0);
                    assert(j < len);
                    if (stacks[j] == UNINITIALIZED && j < i) {
                        todo = 1;
                    }
                    assert(stacks[j] == UNINITIALIZED || stacks[j] == next_stack);
                    stacks[j] = next_stack;
                    break;
                case GET_ITER:
                case GET_AITER:
                    next_stack = push_value(pop_value(next_stack), Iterator);
                    stacks[next_i] = next_stack;
                    break;
                case FOR_ITER:
                {
                    int64_t target_stack = push_value(next_stack, Object);
                    stacks[next_i] = target_stack;
                    j = oparg + 1 + INLINE_CACHE_ENTRIES_FOR_ITER + i;
                    assert(j < len);
                    assert(stacks[j] == UNINITIALIZED || stacks[j] == target_stack);
                    stacks[j] = target_stack;
                    break;
                }
                case END_ASYNC_FOR:
                    next_stack = pop_value(pop_value(next_stack));
                    stacks[next_i] = next_stack;
                    break;
                case PUSH_EXC_INFO:
                    next_stack = push_value(next_stack, Except);
                    stacks[next_i] = next_stack;
                    break;
                case POP_EXCEPT:
                    assert(top_of_stack(next_stack) == Except);
                    next_stack = pop_value(next_stack);
                    stacks[next_i] = next_stack;
                    break;
                case RETURN_VALUE:
                    assert(pop_value(next_stack) == EMPTY_STACK);
                    assert(top_of_stack(next_stack) == Object);
                    break;
                case RETURN_CONST:
                    break;
                case RAISE_VARARGS:
                    break;
                case RERAISE:
                    assert(top_of_stack(next_stack) == Except);
                    /* End of block */
                    break;
                case PUSH_NULL:
                    next_stack = push_value(next_stack, Null);
                    stacks[next_i] = next_stack;
                    break;
                case LOAD_GLOBAL:
                {
                    int j = oparg;
                    if (j & 1) {
                        next_stack = push_value(next_stack, Null);
                    }
                    next_stack = push_value(next_stack, Object);
                    stacks[next_i] = next_stack;
                    break;
                }
                case LOAD_ATTR:
                {
                    assert(top_of_stack(next_stack) == Object);
                    int j = oparg;
                    if (j & 1) {
                        next_stack = pop_value(next_stack);
                        next_stack = push_value(next_stack, Null);
                        next_stack = push_value(next_stack, Object);
                    }
                    stacks[next_i] = next_stack;
                    break;
                }
                case CALL:
                {
                    int args = oparg;
                    for (int j = 0; j < args+2; j++) {
                        next_stack = pop_value(next_stack);
                    }
                    next_stack = push_value(next_stack, Object);
                    stacks[next_i] = next_stack;
                    break;
                }
                case SWAP:
                {
                    int n = oparg;
                    next_stack = stack_swap(next_stack, n);
                    stacks[next_i] = next_stack;
                    break;
                }
                case COPY:
                {
                    int n = oparg;
                    next_stack = push_value(next_stack, peek(next_stack, n));
                    stacks[next_i] = next_stack;
                    break;
                }
                case CACHE:
                case RESERVED:
                {
                    assert(0);
                }
                default:
                {
                    int delta = PyCompile_OpcodeStackEffect(opcode, oparg);
                    assert(delta != PY_INVALID_STACK_EFFECT);
                    while (delta < 0) {
                        next_stack = pop_value(next_stack);
                        delta++;
                    }
                    while (delta > 0) {
                        next_stack = push_value(next_stack, Object);
                        delta--;
                    }
                    stacks[next_i] = next_stack;
                }
            }
            i = next_i;
        }
        /* Scan exception table */
        unsigned char *start = (unsigned char *)PyBytes_AS_STRING(code_obj->co_exceptiontable);
        unsigned char *end = start + PyBytes_GET_SIZE(code_obj->co_exceptiontable);
        unsigned char *scan = start;
        while (scan < end) {
            int start_offset, size, handler;
            scan = parse_varint(scan, &start_offset);
            assert(start_offset >= 0 && start_offset < len);
            scan = parse_varint(scan, &size);
            assert(size >= 0 && start_offset+size <= len);
            scan = parse_varint(scan, &handler);
            assert(handler >= 0 && handler < len);
            int depth_and_lasti;
            scan = parse_varint(scan, &depth_and_lasti);
            int level = depth_and_lasti >> 1;
            int lasti = depth_and_lasti & 1;
            if (stacks[start_offset] != UNINITIALIZED) {
                if (stacks[handler] == UNINITIALIZED) {
                    todo = 1;
                    uint64_t target_stack = pop_to_level(stacks[start_offset], level);
                    if (lasti) {
                        target_stack = push_value(target_stack, Lasti);
                    }
                    target_stack = push_value(target_stack, Except);
                    stacks[handler] = target_stack;
                }
            }
        }
    }
    Py_DECREF(co_code);
    return stacks;
}

static int
compatible_stack(int64_t from_stack, int64_t to_stack)
{
    if (from_stack < 0 || to_stack < 0) {
        return 0;
    }
    while(from_stack > to_stack) {
        from_stack = pop_value(from_stack);
    }
    while(from_stack) {
        Kind from_top = top_of_stack(from_stack);
        Kind to_top = top_of_stack(to_stack);
        if (!compatible_kind(from_top, to_top)) {
            return 0;
        }
        from_stack = pop_value(from_stack);
        to_stack = pop_value(to_stack);
    }
    return to_stack == 0;
}

static const char *
explain_incompatible_stack(int64_t to_stack)
{
    assert(to_stack != 0);
    if (to_stack == OVERFLOWED) {
        return "stack is too deep to analyze";
    }
    if (to_stack == UNINITIALIZED) {
        return "can't jump into an exception handler, or code may be unreachable";
    }
    Kind target_kind = top_of_stack(to_stack);
    switch(target_kind) {
        case Except:
            return "can't jump into an 'except' block as there's no exception";
        case Lasti:
            return "can't jump into a re-raising block as there's no location";
        case Object:
        case Null:
            return "incompatible stacks";
        case Iterator:
            return "can't jump into the body of a for loop";
        default:
            Py_UNREACHABLE();
    }
}

static int *
marklines(PyCodeObject *code, int len)
{
    PyCodeAddressRange bounds;
    _PyCode_InitAddressRange(code, &bounds);
    assert (bounds.ar_end == 0);
    int last_line = -1;

    int *linestarts = PyMem_New(int, len);
    if (linestarts == NULL) {
        return NULL;
    }
    for (int i = 0; i < len; i++) {
        linestarts[i] = -1;
    }

    while (_PyLineTable_NextAddressRange(&bounds)) {
        assert(bounds.ar_start / (int)sizeof(_Py_CODEUNIT) < len);
        if (bounds.ar_line != last_line && bounds.ar_line != -1) {
            linestarts[bounds.ar_start / sizeof(_Py_CODEUNIT)] = bounds.ar_line;
            last_line = bounds.ar_line;
        }
    }
    return linestarts;
}

static int
first_line_not_before(int *lines, int len, int line)
{
    int result = INT_MAX;
    for (int i = 0; i < len; i++) {
        if (lines[i] < result && lines[i] >= line) {
            result = lines[i];
        }
    }
    if (result == INT_MAX) {
        return -1;
    }
    return result;
}

static PyFrameState
_PyFrame_GetState(PyFrameObject *frame)
{
    assert(!_PyFrame_IsIncomplete(frame->f_frame));
    if (frame->f_frame->stacktop == 0) {
        return FRAME_CLEARED;
    }
    switch(frame->f_frame->owner) {
        case FRAME_OWNED_BY_GENERATOR:
        {
            PyGenObject *gen = _PyFrame_GetGenerator(frame->f_frame);
            return gen->gi_frame_state;
        }
        case FRAME_OWNED_BY_THREAD:
        {
            if (_PyInterpreterFrame_LASTI(frame->f_frame) < 0) {
                return FRAME_CREATED;
            }
            switch (frame->f_frame->prev_instr->op.code)
            {
                case COPY_FREE_VARS:
                case MAKE_CELL:
                case RETURN_GENERATOR:
                    /* Frame not fully initialized */
                    return FRAME_CREATED;
                default:
                    return FRAME_EXECUTING;
            }
        }
        case FRAME_OWNED_BY_FRAME_OBJECT:
            return FRAME_COMPLETED;
    }
    Py_UNREACHABLE();
}

/* Setter for f_lineno - you can set f_lineno from within a trace function in
 * order to jump to a given line of code, subject to some restrictions.  Most
 * lines are OK to jump to because they don't make any assumptions about the
 * state of the stack (obvious because you could remove the line and the code
 * would still work without any stack errors), but there are some constructs
 * that limit jumping:
 *
 *  o Any exception handlers.
 *  o 'for' and 'async for' loops can't be jumped into because the
 *    iterator needs to be on the stack.
 *  o Jumps cannot be made from within a trace function invoked with a
 *    'return' or 'exception' event since the eval loop has been exited at
 *    that time.
 */
static int
frame_setlineno(PyFrameObject *f, PyObject* p_new_lineno, void *Py_UNUSED(ignored))
{
    if (p_new_lineno == NULL) {
        PyErr_SetString(PyExc_AttributeError, "cannot delete attribute");
        return -1;
    }
    /* f_lineno must be an integer. */
    if (!PyLong_CheckExact(p_new_lineno)) {
        PyErr_SetString(PyExc_ValueError,
                        "lineno must be an integer");
        return -1;
    }

    PyFrameState state = _PyFrame_GetState(f);
    /*
     * This code preserves the historical restrictions on
     * setting the line number of a frame.
     * Jumps are forbidden on a 'return' trace event (except after a yield).
     * Jumps from 'call' trace events are also forbidden.
     * In addition, jumps are forbidden when not tracing,
     * as this is a debugging feature.
     */
    int what_event = PyThreadState_GET()->what_event;
    if (what_event < 0) {
        PyErr_Format(PyExc_ValueError,
                    "f_lineno can only be set in a trace function");
        return -1;
    }
    switch (what_event) {
        case PY_MONITORING_EVENT_PY_RESUME:
        case PY_MONITORING_EVENT_JUMP:
        case PY_MONITORING_EVENT_BRANCH:
        case PY_MONITORING_EVENT_LINE:
        case PY_MONITORING_EVENT_PY_YIELD:
            /* Setting f_lineno is allowed for the above events */
            break;
        case PY_MONITORING_EVENT_PY_START:
            PyErr_Format(PyExc_ValueError,
                     "can't jump from the 'call' trace event of a new frame");
            return -1;
        case PY_MONITORING_EVENT_CALL:
        case PY_MONITORING_EVENT_C_RETURN:
            PyErr_SetString(PyExc_ValueError,
                "can't jump during a call");
            return -1;
        case PY_MONITORING_EVENT_PY_RETURN:
        case PY_MONITORING_EVENT_PY_UNWIND:
        case PY_MONITORING_EVENT_PY_THROW:
        case PY_MONITORING_EVENT_RAISE:
        case PY_MONITORING_EVENT_C_RAISE:
        case PY_MONITORING_EVENT_INSTRUCTION:
        case PY_MONITORING_EVENT_EXCEPTION_HANDLED:
            PyErr_Format(PyExc_ValueError,
                "can only jump from a 'line' trace event");
            return -1;
        default:
            PyErr_SetString(PyExc_SystemError,
                "unexpected event type");
            return -1;
    }

    int new_lineno;

    /* Fail if the line falls outside the code block and
        select first line with actual code. */
    int overflow;
    long l_new_lineno = PyLong_AsLongAndOverflow(p_new_lineno, &overflow);
    if (overflow
#if SIZEOF_LONG > SIZEOF_INT
        || l_new_lineno > INT_MAX
        || l_new_lineno < INT_MIN
#endif
    ) {
        PyErr_SetString(PyExc_ValueError,
                        "lineno out of range");
        return -1;
    }
    new_lineno = (int)l_new_lineno;

    if (new_lineno < f->f_frame->f_code->co_firstlineno) {
        PyErr_Format(PyExc_ValueError,
                    "line %d comes before the current code block",
                    new_lineno);
        return -1;
    }

    /* PyCode_NewWithPosOnlyArgs limits co_code to be under INT_MAX so this
     * should never overflow. */
    int len = (int)Py_SIZE(f->f_frame->f_code);
    int *lines = marklines(f->f_frame->f_code, len);
    if (lines == NULL) {
        return -1;
    }

    new_lineno = first_line_not_before(lines, len, new_lineno);
    if (new_lineno < 0) {
        PyErr_Format(PyExc_ValueError,
                    "line %d comes after the current code block",
                    (int)l_new_lineno);
        PyMem_Free(lines);
        return -1;
    }

    int64_t *stacks = mark_stacks(f->f_frame->f_code, len);
    if (stacks == NULL) {
        PyMem_Free(lines);
        return -1;
    }

    int64_t best_stack = OVERFLOWED;
    int best_addr = -1;
    int64_t start_stack = stacks[_PyInterpreterFrame_LASTI(f->f_frame)];
    int err = -1;
    const char *msg = "cannot find bytecode for specified line";
    for (int i = 0; i < len; i++) {
        if (lines[i] == new_lineno) {
            int64_t target_stack = stacks[i];
            if (compatible_stack(start_stack, target_stack)) {
                err = 0;
                if (target_stack > best_stack) {
                    best_stack = target_stack;
                    best_addr = i;
                }
            }
            else if (err < 0) {
                if (start_stack == OVERFLOWED) {
                    msg = "stack to deep to analyze";
                }
                else if (start_stack == UNINITIALIZED) {
                    msg = "can't jump from unreachable code";
                }
                else {
                    msg = explain_incompatible_stack(target_stack);
                    err = 1;
                }
            }
        }
    }
    PyMem_Free(stacks);
    PyMem_Free(lines);
    if (err) {
        PyErr_SetString(PyExc_ValueError, msg);
        return -1;
    }
    // Populate any NULL locals that the compiler might have "proven" to exist
    // in the new location. Rather than crashing or changing co_code, just bind
    // None instead:
    int unbound = 0;
    for (int i = 0; i < f->f_frame->f_code->co_nlocalsplus; i++) {
        // Counting every unbound local is overly-cautious, but a full flow
        // analysis (like we do in the compiler) is probably too expensive:
        unbound += f->f_frame->localsplus[i] == NULL;
    }
    if (unbound) {
        const char *e = "assigning None to %d unbound local%s";
        const char *s = (unbound == 1) ? "" : "s";
        if (PyErr_WarnFormat(PyExc_RuntimeWarning, 0, e, unbound, s)) {
            return -1;
        }
        // Do this in a second pass to avoid writing a bunch of Nones when
        // warnings are being treated as errors and the previous bit raises:
        for (int i = 0; i < f->f_frame->f_code->co_nlocalsplus; i++) {
            if (f->f_frame->localsplus[i] == NULL) {
                f->f_frame->localsplus[i] = Py_NewRef(Py_None);
                unbound--;
            }
        }
        assert(unbound == 0);
    }
    if (state == FRAME_SUSPENDED) {
        /* Account for value popped by yield */
        start_stack = pop_value(start_stack);
    }
    while (start_stack > best_stack) {
        if (top_of_stack(start_stack) == Except) {
            /* Pop exception stack as well as the evaluation stack */
            PyThreadState *tstate = _PyThreadState_GET();
            _PyErr_StackItem *exc_info = tstate->exc_info;
            PyObject *value = exc_info->exc_value;
            PyObject *exc = _PyFrame_StackPop(f->f_frame);
            assert(PyExceptionInstance_Check(exc) || exc == Py_None);
            exc_info->exc_value = exc;
            Py_XDECREF(value);
        }
        else {
            PyObject *v = _PyFrame_StackPop(f->f_frame);
            Py_XDECREF(v);
        }
        start_stack = pop_value(start_stack);
    }
    /* Finally set the new lasti and return OK. */
    f->f_lineno = 0;
    f->f_frame->prev_instr = _PyCode_CODE(f->f_frame->f_code) + best_addr;
    return 0;
}

static PyObject *
frame_gettrace(PyFrameObject *f, void *closure)
{
    PyObject* trace = f->f_trace;
    if (trace == NULL)
        trace = Py_None;
    return Py_NewRef(trace);
}

static int
frame_settrace(PyFrameObject *f, PyObject* v, void *closure)
{
    if (v == Py_None) {
        v = NULL;
    }
    if (v != f->f_trace) {
        Py_XSETREF(f->f_trace, Py_XNewRef(v));
    }
    return 0;
}


static PyGetSetDef frame_getsetlist[] = {
    {"f_back",          (getter)frame_getback, NULL, NULL},
    {"f_locals",        (getter)frame_getlocals, NULL, NULL},
    {"f_lineno",        (getter)frame_getlineno,
                    (setter)frame_setlineno, NULL},
    {"f_trace",         (getter)frame_gettrace, (setter)frame_settrace, NULL},
    {"f_lasti",         (getter)frame_getlasti, NULL, NULL},
    {"f_globals",       (getter)frame_getglobals, NULL, NULL},
    {"f_builtins",      (getter)frame_getbuiltins, NULL, NULL},
    {"f_code",          (getter)frame_getcode, NULL, NULL},
    {"f_trace_opcodes", (getter)frame_gettrace_opcodes, (setter)frame_settrace_opcodes, NULL},
    {0}
};

static void
frame_dealloc(PyFrameObject *f)
{
    /* It is the responsibility of the owning generator/coroutine
     * to have cleared the generator pointer */

    assert(f->f_frame->owner != FRAME_OWNED_BY_GENERATOR ||
        _PyFrame_GetGenerator(f->f_frame)->gi_frame_state == FRAME_CLEARED);

    if (_PyObject_GC_IS_TRACKED(f)) {
        _PyObject_GC_UNTRACK(f);
    }

    Py_TRASHCAN_BEGIN(f, frame_dealloc);
    PyCodeObject *co = NULL;

    /* Kill all local variables including specials, if we own them */
    if (f->f_frame->owner == FRAME_OWNED_BY_FRAME_OBJECT) {
        assert(f->f_frame == (_PyInterpreterFrame *)f->_f_frame_data);
        _PyInterpreterFrame *frame = (_PyInterpreterFrame *)f->_f_frame_data;
        /* Don't clear code object until the end */
        co = frame->f_code;
        frame->f_code = NULL;
        Py_CLEAR(frame->f_funcobj);
        Py_CLEAR(frame->f_locals);
        PyObject **locals = _PyFrame_GetLocalsArray(frame);
        for (int i = 0; i < frame->stacktop; i++) {
            Py_CLEAR(locals[i]);
        }
    }
    Py_CLEAR(f->f_back);
    Py_CLEAR(f->f_trace);
    PyObject_GC_Del(f);
    Py_XDECREF(co);
    Py_TRASHCAN_END;
}

static int
frame_traverse(PyFrameObject *f, visitproc visit, void *arg)
{
    Py_VISIT(f->f_back);
    Py_VISIT(f->f_trace);
    if (f->f_frame->owner != FRAME_OWNED_BY_FRAME_OBJECT) {
        return 0;
    }
    assert(f->f_frame->frame_obj == NULL);
    return _PyFrame_Traverse(f->f_frame, visit, arg);
}

static int
frame_tp_clear(PyFrameObject *f)
{
    Py_CLEAR(f->f_trace);

    /* locals and stack */
    PyObject **locals = _PyFrame_GetLocalsArray(f->f_frame);
    assert(f->f_frame->stacktop >= 0);
    for (int i = 0; i < f->f_frame->stacktop; i++) {
        Py_CLEAR(locals[i]);
    }
    f->f_frame->stacktop = 0;
    return 0;
}

static PyObject *
frame_clear(PyFrameObject *f, PyObject *Py_UNUSED(ignored))
{
    if (f->f_frame->owner == FRAME_OWNED_BY_GENERATOR) {
        PyGenObject *gen = _PyFrame_GetGenerator(f->f_frame);
        if (gen->gi_frame_state == FRAME_EXECUTING) {
            goto running;
        }
        _PyGen_Finalize((PyObject *)gen);
    }
    else if (f->f_frame->owner == FRAME_OWNED_BY_THREAD) {
        goto running;
    }
    else {
        assert(f->f_frame->owner == FRAME_OWNED_BY_FRAME_OBJECT);
        (void)frame_tp_clear(f);
    }
    Py_RETURN_NONE;
running:
    PyErr_SetString(PyExc_RuntimeError,
                    "cannot clear an executing frame");
    return NULL;
}

PyDoc_STRVAR(clear__doc__,
"F.clear(): clear most references held by the frame");

static PyObject *
frame_sizeof(PyFrameObject *f, PyObject *Py_UNUSED(ignored))
{
    Py_ssize_t res;
    res = offsetof(PyFrameObject, _f_frame_data) + offsetof(_PyInterpreterFrame, localsplus);
    PyCodeObject *code = f->f_frame->f_code;
    res += _PyFrame_NumSlotsForCodeObject(code) * sizeof(PyObject *);
    return PyLong_FromSsize_t(res);
}

PyDoc_STRVAR(sizeof__doc__,
"F.__sizeof__() -> size of F in memory, in bytes");

static PyObject *
frame_repr(PyFrameObject *f)
{
    int lineno = PyFrame_GetLineNumber(f);
    PyCodeObject *code = f->f_frame->f_code;
    return PyUnicode_FromFormat(
        "<frame at %p, file %R, line %d, code %S>",
        f, code->co_filename, lineno, code->co_name);
}

static PyMethodDef frame_methods[] = {
    {"clear",           (PyCFunction)frame_clear,       METH_NOARGS,
     clear__doc__},
    {"__sizeof__",      (PyCFunction)frame_sizeof,      METH_NOARGS,
     sizeof__doc__},
    {NULL,              NULL}   /* sentinel */
};

PyTypeObject PyFrame_Type = {
    PyVarObject_HEAD_INIT(&PyType_Type, 0)
    "frame",
    offsetof(PyFrameObject, _f_frame_data) +
    offsetof(_PyInterpreterFrame, localsplus),
    sizeof(PyObject *),
    (destructor)frame_dealloc,                  /* tp_dealloc */
    0,                                          /* tp_vectorcall_offset */
    0,                                          /* tp_getattr */
    0,                                          /* tp_setattr */
    0,                                          /* tp_as_async */
    (reprfunc)frame_repr,                       /* tp_repr */
    0,                                          /* tp_as_number */
    0,                                          /* tp_as_sequence */
    0,                                          /* tp_as_mapping */
    0,                                          /* tp_hash */
    0,                                          /* tp_call */
    0,                                          /* tp_str */
    PyObject_GenericGetAttr,                    /* tp_getattro */
    PyObject_GenericSetAttr,                    /* tp_setattro */
    0,                                          /* tp_as_buffer */
    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,/* tp_flags */
    0,                                          /* tp_doc */
    (traverseproc)frame_traverse,               /* tp_traverse */
    (inquiry)frame_tp_clear,                    /* tp_clear */
    0,                                          /* tp_richcompare */
    0,                                          /* tp_weaklistoffset */
    0,                                          /* tp_iter */
    0,                                          /* tp_iternext */
    frame_methods,                              /* tp_methods */
    frame_memberlist,                           /* tp_members */
    frame_getsetlist,                           /* tp_getset */
    0,                                          /* tp_base */
    0,                                          /* tp_dict */
};

static void
init_frame(_PyInterpreterFrame *frame, PyFunctionObject *func, PyObject *locals)
{
    PyCodeObject *code = (PyCodeObject *)func->func_code;
    _PyFrame_Initialize(frame, (PyFunctionObject*)Py_NewRef(func),
                        Py_XNewRef(locals), code, 0);
    frame->previous = NULL;
}

PyFrameObject*
_PyFrame_New_NoTrack(PyCodeObject *code)
{
    CALL_STAT_INC(frame_objects_created);
    int slots = code->co_nlocalsplus + code->co_stacksize;
    PyFrameObject *f = PyObject_GC_NewVar(PyFrameObject, &PyFrame_Type, slots);
    if (f == NULL) {
        return NULL;
    }
    f->f_back = NULL;
    f->f_trace = NULL;
    f->f_trace_lines = 1;
    f->f_trace_opcodes = 0;
    f->f_fast_as_locals = 0;
    f->f_lineno = 0;
    return f;
}

/* Legacy API */
PyFrameObject*
PyFrame_New(PyThreadState *tstate, PyCodeObject *code,
            PyObject *globals, PyObject *locals)
{
    PyObject *builtins = _PyEval_BuiltinsFromGlobals(tstate, globals); // borrowed ref
    if (builtins == NULL) {
        return NULL;
    }
    PyFrameConstructor desc = {
        .fc_globals = globals,
        .fc_builtins = builtins,
        .fc_name = code->co_name,
        .fc_qualname = code->co_name,
        .fc_code = (PyObject *)code,
        .fc_defaults = NULL,
        .fc_kwdefaults = NULL,
        .fc_closure = NULL
    };
    PyFunctionObject *func = _PyFunction_FromConstructor(&desc);
    if (func == NULL) {
        return NULL;
    }
    PyFrameObject *f = _PyFrame_New_NoTrack(code);
    if (f == NULL) {
        Py_DECREF(func);
        return NULL;
    }
    init_frame((_PyInterpreterFrame *)f->_f_frame_data, func, locals);
    f->f_frame = (_PyInterpreterFrame *)f->_f_frame_data;
    f->f_frame->owner = FRAME_OWNED_BY_FRAME_OBJECT;
    // This frame needs to be "complete", so pretend that the first RESUME ran:
    f->f_frame->prev_instr = _PyCode_CODE(code) + code->_co_firsttraceable;
    assert(!_PyFrame_IsIncomplete(f->f_frame));
    Py_DECREF(func);
    _PyObject_GC_TRACK(f);
    return f;
}

static int
_PyFrame_OpAlreadyRan(_PyInterpreterFrame *frame, int opcode, int oparg)
{
    // This only works when opcode is a non-quickened form:
    assert(_PyOpcode_Deopt[opcode] == opcode);
    int check_oparg = 0;
    for (_Py_CODEUNIT *instruction = _PyCode_CODE(frame->f_code);
         instruction < frame->prev_instr; instruction++)
    {
        int check_opcode = _PyOpcode_Deopt[instruction->op.code];
        check_oparg |= instruction->op.arg;
        if (check_opcode == opcode && check_oparg == oparg) {
            return 1;
        }
        if (check_opcode == EXTENDED_ARG) {
            check_oparg <<= 8;
        }
        else {
            check_oparg = 0;
        }
        instruction += _PyOpcode_Caches[check_opcode];
    }
    return 0;
}


// Initialize frame free variables if needed
static void
frame_init_get_vars(_PyInterpreterFrame *frame)
{
    // COPY_FREE_VARS has no quickened forms, so no need to use _PyOpcode_Deopt
    // here:
    PyCodeObject *co = frame->f_code;
    int lasti = _PyInterpreterFrame_LASTI(frame);
    if (!(lasti < 0 && _PyCode_CODE(co)->op.code == COPY_FREE_VARS
          && PyFunction_Check(frame->f_funcobj)))
    {
        /* Free vars are initialized */
        return;
    }

    /* Free vars have not been initialized -- Do that */
    PyObject *closure = ((PyFunctionObject *)frame->f_funcobj)->func_closure;
    int offset = PyCode_GetFirstFree(co);
    for (int i = 0; i < co->co_nfreevars; ++i) {
        PyObject *o = PyTuple_GET_ITEM(closure, i);
        frame->localsplus[offset + i] = Py_NewRef(o);
    }
    // COPY_FREE_VARS doesn't have inline CACHEs, either:
    frame->prev_instr = _PyCode_CODE(frame->f_code);
}


static int
frame_get_var(_PyInterpreterFrame *frame, PyCodeObject *co, int i,
              PyObject **pvalue)
{
    _PyLocals_Kind kind = _PyLocals_GetKind(co->co_localspluskinds, i);

    /* If the namespace is unoptimized, then one of the
       following cases applies:
       1. It does not contain free variables, because it
          uses import * or is a top-level namespace.
       2. It is a class namespace.
       We don't want to accidentally copy free variables
       into the locals dict used by the class.
    */
    if (kind & CO_FAST_FREE && !(co->co_flags & CO_OPTIMIZED)) {
        return 0;
    }

    PyObject *value = frame->localsplus[i];
    if (frame->stacktop) {
        if (kind & CO_FAST_FREE) {
            // The cell was set by COPY_FREE_VARS.
            assert(value != NULL && PyCell_Check(value));
            value = PyCell_GET(value);
        }
        else if (kind & CO_FAST_CELL) {
            // Note that no *_DEREF ops can happen before MAKE_CELL
            // executes.  So there's no need to duplicate the work
            // that MAKE_CELL would otherwise do later, if it hasn't
            // run yet.
            if (value != NULL) {
                if (PyCell_Check(value) &&
                        _PyFrame_OpAlreadyRan(frame, MAKE_CELL, i)) {
                    // (likely) MAKE_CELL must have executed already.
                    value = PyCell_GET(value);
                }
                // (likely) Otherwise it it is an arg (kind & CO_FAST_LOCAL),
                // with the initial value set when the frame was created...
                // (unlikely) ...or it was set to some initial value by
                // an earlier call to PyFrame_LocalsToFast().
            }
        }
    }
    else {
        assert(value == NULL);
    }
    *pvalue = value;
    return 1;
}

int
_PyFrame_FastToLocalsWithError(_PyInterpreterFrame *frame)
{
    /* Merge fast locals into f->f_locals */
    PyObject *locals = frame->f_locals;
    if (locals == NULL) {
        locals = frame->f_locals = PyDict_New();
        if (locals == NULL) {
            return -1;
        }
    }

    frame_init_get_vars(frame);

    PyCodeObject *co = frame->f_code;
    for (int i = 0; i < co->co_nlocalsplus; i++) {
        PyObject *value;  // borrowed reference
        if (!frame_get_var(frame, co, i, &value)) {
            continue;
        }

        PyObject *name = PyTuple_GET_ITEM(co->co_localsplusnames, i);
        _PyLocals_Kind kind = _PyLocals_GetKind(co->co_localspluskinds, i);
        if (kind & CO_FAST_HIDDEN) {
            continue;
        }
        if (value == NULL) {
            if (PyObject_DelItem(locals, name) != 0) {
                if (PyErr_ExceptionMatches(PyExc_KeyError)) {
                    PyErr_Clear();
                }
                else {
                    return -1;
                }
            }
        }
        else {
            if (PyObject_SetItem(locals, name, value) != 0) {
                return -1;
            }
        }
    }
    return 0;
}


PyObject *
PyFrame_GetVar(PyFrameObject *frame_obj, PyObject *name)
{
    if (!PyUnicode_Check(name)) {
        PyErr_Format(PyExc_TypeError, "name must be str, not %s",
                     Py_TYPE(name)->tp_name);
        return NULL;
    }

    _PyInterpreterFrame *frame = frame_obj->f_frame;
    frame_init_get_vars(frame);

    PyCodeObject *co = frame->f_code;
    for (int i = 0; i < co->co_nlocalsplus; i++) {
        PyObject *var_name = PyTuple_GET_ITEM(co->co_localsplusnames, i);
        if (!_PyUnicode_Equal(var_name, name)) {
            continue;
        }

        PyObject *value;  // borrowed reference
        if (!frame_get_var(frame, co, i, &value)) {
            break;
        }
        if (value == NULL) {
            break;
        }
        return Py_NewRef(value);
    }

    PyErr_Format(PyExc_NameError, "variable %R does not exist", name);
    return NULL;
}


PyObject *
PyFrame_GetVarString(PyFrameObject *frame, const char *name)
{
    PyObject *name_obj = PyUnicode_FromString(name);
    if (name_obj == NULL) {
        return NULL;
    }
    PyObject *value = PyFrame_GetVar(frame, name_obj);
    Py_DECREF(name_obj);
    return value;
}


int
PyFrame_FastToLocalsWithError(PyFrameObject *f)
{
    assert(!_PyFrame_IsIncomplete(f->f_frame));
    if (f == NULL) {
        PyErr_BadInternalCall();
        return -1;
    }
    int err = _PyFrame_FastToLocalsWithError(f->f_frame);
    if (err == 0) {
        f->f_fast_as_locals = 1;
    }
    return err;
}

void
PyFrame_FastToLocals(PyFrameObject *f)
{
    int res;
    assert(!_PyFrame_IsIncomplete(f->f_frame));
    assert(!PyErr_Occurred());

    res = PyFrame_FastToLocalsWithError(f);
    if (res < 0)
        PyErr_Clear();
}

void
_PyFrame_LocalsToFast(_PyInterpreterFrame *frame, int clear)
{
    /* Merge locals into fast locals */
    PyObject *locals;
    PyObject **fast;
    PyCodeObject *co;
    locals = frame->f_locals;
    if (locals == NULL) {
        return;
    }
    fast = _PyFrame_GetLocalsArray(frame);
    co = frame->f_code;

    PyObject *exc = PyErr_GetRaisedException();
    for (int i = 0; i < co->co_nlocalsplus; i++) {
        _PyLocals_Kind kind = _PyLocals_GetKind(co->co_localspluskinds, i);

        /* Same test as in PyFrame_FastToLocals() above. */
        if (kind & CO_FAST_FREE && !(co->co_flags & CO_OPTIMIZED)) {
            continue;
        }
        PyObject *name = PyTuple_GET_ITEM(co->co_localsplusnames, i);
        PyObject *value = PyObject_GetItem(locals, name);
        /* We only care about NULLs if clear is true. */
        if (value == NULL) {
            PyErr_Clear();
            if (!clear) {
                continue;
            }
        }
        PyObject *oldvalue = fast[i];
        PyObject *cell = NULL;
        if (kind == CO_FAST_FREE) {
            // The cell was set when the frame was created from
            // the function's closure.
            assert(oldvalue != NULL && PyCell_Check(oldvalue));
            cell = oldvalue;
        }
        else if (kind & CO_FAST_CELL && oldvalue != NULL) {
            /* Same test as in PyFrame_FastToLocals() above. */
            if (PyCell_Check(oldvalue) &&
                    _PyFrame_OpAlreadyRan(frame, MAKE_CELL, i)) {
                // (likely) MAKE_CELL must have executed already.
                cell = oldvalue;
            }
            // (unlikely) Otherwise, it must have been set to some
            // initial value by an earlier call to PyFrame_LocalsToFast().
        }
        if (cell != NULL) {
            oldvalue = PyCell_GET(cell);
            if (value != oldvalue) {
                PyCell_SET(cell, Py_XNewRef(value));
                Py_XDECREF(oldvalue);
            }
        }
        else if (value != oldvalue) {
            if (value == NULL) {
                // Probably can't delete this, since the compiler's flow
                // analysis may have already "proven" that it exists here:
                const char *e = "assigning None to unbound local %R";
                if (PyErr_WarnFormat(PyExc_RuntimeWarning, 0, e, name)) {
                    // It's okay if frame_obj is NULL, just try anyways:
                    PyErr_WriteUnraisable((PyObject *)frame->frame_obj);
                }
                value = Py_NewRef(Py_None);
            }
            Py_XSETREF(fast[i], Py_NewRef(value));
        }
        Py_XDECREF(value);
    }
    PyErr_SetRaisedException(exc);
}

void
PyFrame_LocalsToFast(PyFrameObject *f, int clear)
{
    assert(!_PyFrame_IsIncomplete(f->f_frame));
    if (f && f->f_fast_as_locals && _PyFrame_GetState(f) != FRAME_CLEARED) {
        _PyFrame_LocalsToFast(f->f_frame, clear);
        f->f_fast_as_locals = 0;
    }
}

int
_PyFrame_IsEntryFrame(PyFrameObject *frame)
{
    assert(frame != NULL);
    _PyInterpreterFrame *f = frame->f_frame;
    assert(!_PyFrame_IsIncomplete(f));
    return f->previous && f->previous->owner == FRAME_OWNED_BY_CSTACK;
}

PyCodeObject *
PyFrame_GetCode(PyFrameObject *frame)
{
    assert(frame != NULL);
    assert(!_PyFrame_IsIncomplete(frame->f_frame));
    PyCodeObject *code = frame->f_frame->f_code;
    assert(code != NULL);
    return (PyCodeObject*)Py_NewRef(code);
}


PyFrameObject*
PyFrame_GetBack(PyFrameObject *frame)
{
    assert(frame != NULL);
    assert(!_PyFrame_IsIncomplete(frame->f_frame));
    PyFrameObject *back = frame->f_back;
    if (back == NULL) {
        _PyInterpreterFrame *prev = frame->f_frame->previous;
        prev = _PyFrame_GetFirstComplete(prev);
        if (prev) {
            back = _PyFrame_GetFrameObject(prev);
        }
    }
    return (PyFrameObject*)Py_XNewRef(back);
}

PyObject*
PyFrame_GetLocals(PyFrameObject *frame)
{
    assert(!_PyFrame_IsIncomplete(frame->f_frame));
    return frame_getlocals(frame, NULL);
}

PyObject*
PyFrame_GetGlobals(PyFrameObject *frame)
{
    assert(!_PyFrame_IsIncomplete(frame->f_frame));
    return frame_getglobals(frame, NULL);
}

PyObject*
PyFrame_GetBuiltins(PyFrameObject *frame)
{
    assert(!_PyFrame_IsIncomplete(frame->f_frame));
    return frame_getbuiltins(frame, NULL);
}

int
PyFrame_GetLasti(PyFrameObject *frame)
{
    assert(!_PyFrame_IsIncomplete(frame->f_frame));
    int lasti = _PyInterpreterFrame_LASTI(frame->f_frame);
    if (lasti < 0) {
        return -1;
    }
    return lasti * sizeof(_Py_CODEUNIT);
}

PyObject *
PyFrame_GetGenerator(PyFrameObject *frame)
{
    assert(!_PyFrame_IsIncomplete(frame->f_frame));
    if (frame->f_frame->owner != FRAME_OWNED_BY_GENERATOR) {
        return NULL;
    }
    PyGenObject *gen = _PyFrame_GetGenerator(frame->f_frame);
    return Py_NewRef(gen);
}

PyObject*
_PyEval_BuiltinsFromGlobals(PyThreadState *tstate, PyObject *globals)
{
    PyObject *builtins = PyDict_GetItemWithError(globals, &_Py_ID(__builtins__));
    if (builtins) {
        if (PyModule_Check(builtins)) {
            builtins = _PyModule_GetDict(builtins);
            assert(builtins != NULL);
        }
        return builtins;
    }
    if (PyErr_Occurred()) {
        return NULL;
    }

    return _PyEval_GetBuiltins(tstate);
}