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Fix traceback circular reference detection to pass test_raise.py #5995

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Fix traceback circular reference detection to pass test_raise.py #5995

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7f33100
Initial plan
Copilot Jul 17, 2025
9f4065c
Add circular reference detection to traceback.tb_next setter
Copilot Jul 17, 2025
ce5fe45
Implement general cycle detection algorithm for traceback.tb_next setter
Copilot Jul 17, 2025
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2 changes: 0 additions & 2 deletions Lib/test/test_raise.py
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Original file line number Diff line number Diff line change
Expand Up @@ -233,8 +233,6 @@ class TestTracebackType(unittest.TestCase):
def raiser(self):
raise ValueError

# TODO: RUSTPYTHON
@unittest.expectedFailure
def test_attrs(self):
try:
self.raiser()
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52 changes: 51 additions & 1 deletion vm/src/builtins/traceback.rs
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Original file line number Diff line number Diff line change
@@ -1,4 +1,5 @@
use rustpython_common::lock::PyMutex;
use std::ops::Deref;

use super::{PyType, PyTypeRef};
use crate::{
Expand Down Expand Up @@ -63,8 +64,57 @@ impl PyTraceback {
}

#[pygetset(setter)]
fn set_tb_next(&self, value: Option<PyRef<Self>>) {
fn set_tb_next(&self, value: Option<PyRef<Self>>, vm: &VirtualMachine) -> PyResult<()> {
// Check for circular references using Floyd's cycle detection algorithm
if let Some(ref _new_tb) = value {
// Temporarily make the assignment to simulate the new chain
let old_next = self.next.lock().clone();
*self.next.lock() = value.clone();

// Use Floyd's cycle detection on the chain starting from self
let has_cycle = Self::has_cycle_from(self);

// Restore the original state
*self.next.lock() = old_next;

if has_cycle {
return Err(vm.new_value_error("circular reference in traceback chain".to_owned()));
}
}

*self.next.lock() = value;
Ok(())
}

/// Detect cycles in traceback chain using Floyd's cycle detection algorithm
fn has_cycle_from(start: &PyTraceback) -> bool {
let mut slow = start.tb_next();
let mut fast = start.tb_next();

while let (Some(slow_tb), Some(fast_tb)) = (&slow, &fast) {
// Move slow pointer one step
slow = slow_tb.tb_next();

// Move fast pointer two steps
fast = fast_tb.tb_next();
if let Some(ref fast_tb2) = fast {
fast = fast_tb2.tb_next();
} else {
break;
}

// Check if slow and fast pointers meet (cycle detected)
if let (Some(slow_ptr), Some(fast_ptr)) = (&slow, &fast) {
if std::ptr::eq(
slow_ptr.deref().deref() as *const PyTraceback,
fast_ptr.deref().deref() as *const PyTraceback
) {
return true;
}
}
}

false
}
}

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