gh-151424: Fix impossible stack traces in RemoteUnwinder(..., cache_frames=True) by copying chunks on cache miss

[maurycy]

The issue is caused by memory walks with cache_frames=True when there's a cache miss.

If I understand the whole machniery correctly.

Basically, we don't copy chunks in cache mode:

cpython/Modules/_remote_debugging/threads.c

Lines 506 to 512 in f4f1020

	// In cache mode, copying stack chunks is more expensive than direct memory reads
	if (!unwinder->cache_frames) {
	if (copy_stack_chunks(unwinder, *current_tstate, &chunks) < 0) {
	set_exception_cause(unwinder, PyExc_RuntimeError, "Failed to copy stack chunks");
	goto error;
	}
	}

So there are never any chunks in the cache mode, not a problem with full cache hits:

cpython/Modules/_remote_debugging/frames.c

Lines 328 to 351 in f4f1020

	if (ctx->chunks && ctx->chunks->count > 0) {
	if (parse_frame_from_chunks(unwinder, &frame, frame_addr, &next_frame_addr, &stackpointer, ctx->chunks) == 0) {
	goto parsed_frame;
	}
	PyErr_Clear();
	}
	{
	uintptr_t address_of_code_object = 0;
	int parse_result;
	if (ctx->prefetch.frame && ctx->prefetch.frame_addr == frame_addr) {
	parse_result = parse_frame_buffer(
	unwinder, &frame, ctx->prefetch.frame,
	&address_of_code_object, &next_frame_addr);
	}
	else {
	parse_result = parse_frame_object(
	unwinder, &frame, frame_addr,
	&address_of_code_object, &next_frame_addr);
	}
	if (parse_result < 0) {
	set_exception_cause(unwinder, PyExc_RuntimeError, "Failed to parse frame object in chain");
	return -1;
	}
	}

But when there's no cache hit:

cpython/Modules/_remote_debugging/frames.c

Lines 578 to 585 in f4f1020

	int full_hit = try_full_cache_hit(unwinder, ctx, thread_id);
	if (full_hit != 0) {
	return full_hit < 0 ? -1 : 0;
	}
	Py_ssize_t frames_before = PyList_GET_SIZE(ctx->frame_info);
	if (process_frame_chain(unwinder, ctx) < 0) {

it falls back to many memory reads:

cpython/Modules/_remote_debugging/frames.c

Line 315 in f4f1020

while ((void*)frame_addr != NULL) {

cpython/Modules/_remote_debugging/frames.c

Lines 236 to 242 in f4f1020

	char frame[SIZEOF_INTERP_FRAME];
	Py_ssize_t bytes_read = _Py_RemoteDebug_ReadRemoteMemory(
	&unwinder->handle,
	address,
	SIZEOF_INTERP_FRAME,
	frame
	);

which isn't very atomic... the target mutates between reads and we're lost.

That's definitely the cause. Is there a cleaner fix ?

One side-note - let's not tighten the validation yet?

Generally, it's a statistical profiler so we should have some statistical tests for correctness, too.

Proof

Reusing

def leaf_a(): return sum(range(50))
def leaf_b(): return sum(range(50))
def hot_a(): return leaf_a()
def hot_b(): return leaf_b()
while True:
    hot_a(); hot_b();

from #151424 we get

2026-06-13T03:14:37.706845000+0200 maurycy@gimel /Users/maurycy/src/github.com/maurycy/cpython (gh-151424 141f623*?) % sudo ./python.exe -m profiling.sampling run --collapsed -o /tmp/stacks.txt -d 10 repro.py
Captured 10,000 samples in 10.00 seconds
Sample rate: 999.99 samples/sec
Error rate: 14.91
Collapsed stack output written to /tmp/stacks.txt
2026-06-13T03:14:51.568490000+0200 maurycy@gimel /Users/maurycy/src/github.com/maurycy/cpython (gh-151424 141f623*?) % cat /tmp/stacks.txt 
tid:13199240;<frozen runpy>:_run_module_as_main:201;<frozen runpy>:_run_code:87;repro.py:<module>:6;repro.py:hot_a:3;repro.py:leaf_a:1 3324
tid:13199240;<frozen runpy>:_run_module_as_main:201;<frozen runpy>:_run_code:87;repro.py:<module>:6;repro.py:hot_b:4;repro.py:leaf_b:2 3269
tid:13199240;<frozen runpy>:_run_module_as_main:201;<frozen runpy>:_run_code:87;repro.py:<module>:6 1234
tid:13199240;<frozen runpy>:_run_module_as_main:201;<frozen runpy>:_run_code:87;repro.py:<module>:6;repro.py:hot_a:3 341
tid:13199240;<frozen runpy>:_run_module_as_main:201;<frozen runpy>:_run_code:87;repro.py:<module>:6;repro.py:hot_b:4 324
tid:13199240;<frozen runpy>:_run_module_as_main:201;<frozen runpy>:_run_code:87;repro.py:<module>:6;repro.py:hot_b:4;repro.py:leaf_b:-1 8
tid:13199240;<frozen runpy>:_run_module_as_main:201;<frozen runpy>:_run_code:87;repro.py:<module>:6;repro.py:hot_a:3;repro.py:leaf_a:-1 6
tid:13199240;<frozen runpy>:_run_module_as_main:201;<frozen runpy>:_run_code:87;repro.py:<module>:6;repro.py:hot_a:3;repro.py:leaf_b:2 1
tid:13199240;<frozen runpy>:_run_module_as_main:201;<frozen runpy>:_run_code:87;repro.py:<module>:6;repro.py:leaf_a:1 1
tid:13199240;<frozen runpy>:_run_module_as_main:201;<frozen runpy>:_run_code:87;repro.py:<module>:6;repro.py:leaf_b:2 1
2026-06-13T03:15:14.324823000+0200 maurycy@gimel /Users/maurycy/src/github.com/maurycy/cpython (gh-151424 141f623*?) % 

The impossible rate went to just 0,20%.