[yuri12344]

Problem

On Python 3.14, round-tripping code with two consecutive async for loops whose bodies contain a try/except fails during stack-size computation:

async def f(a, b):
    async for x in a:
        try: pass
        except Exception: pass
    async for y in b:
        try: pass
        except Exception: pass

Bytecode.from_code(f.__code__).to_code()
# 0.19.0: AssertionError at `assert self._current_try_begin is None` (cfg.py)
# 0.17.0: RuntimeError: Failed to compute stacksize, got negative size / KeyError

A single such loop works, and Python 3.11–3.13 are unaffected. We hit this in production through ddtrace's Code Origin for Spans, which rewrites functions with this library.

Root cause

Since 3.14, END_ASYNC_FOR has an oparg and is in opcode.hasjump, and since #179 bytecode treats it as a conditional backward jump. The interpreter never takes that jump: _END_ASYNC_FOR in Python/bytecodes.c does JUMPBY(0); // Pretend jump as we need source offset for monitoring. The oparg only lets INSTRUMENTED_END_ASYNC_FOR report the branch source to sys.monitoring. CPython's own stack-depth pass skips this edge (Python/flowgraph.c, calculate_stackdepth: HAS_TARGET(opcode) && opcode != END_ASYNC_FOR).

_StackSizeComputer followed the edge into the loop's SEND/END_SEND block at the wrong depth and with the enclosing TryBegin still open. For the second loop that block starts with another TryBegin, which trips the non-nesting assertion.

Fix

• Add MONITORING_ONLY_JUMP_OPS ({END_ASYNC_FOR} on 3.14+, empty otherwise) in instr.py.
• Skip the jump-taken exploration for these opcodes in _StackSizeComputer.run.
• has_jump() and the other jump classifications are unchanged, because the target is still needed for labels and oparg encoding.

Tests

• New test_stack_size_computation_consecutive_async_for_try_except. It fails without the fix on 3.14 and passes on 3.12, 3.13 and 3.14.
• The full suite passes on 3.12, 3.13 and 3.14.
• Computed stack sizes match co_stacksize for nested async for, async comprehensions, and async with + async for with try/finally.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.57%. Comparing base (9eea626) to head (e5564e0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #214   +/-   ##
=======================================
  Coverage   95.57%   95.57%           
=======================================
  Files           7        7           
  Lines        2147     2148    +1     
  Branches      461      461           
=======================================
+ Hits         2052     2053    +1     
  Misses         53       53           
  Partials       42       42           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[MatthieuDartiailh]

Thanks for the very well explained issue and patch. I will try to release next week.