LTTng » Babeltrace

Files to reproduce¶

1. Create `/tmp/babeltrace1.5-python`
2. Unzip `trace.tar.xz` into `/tmp/babeltrace1.5-python/trace/`
3. Create the following Python script `/tmp/babeltrace1.5-python/trace/repro_minimal.py`:

""" 
Minimal reproducer.

Opens an existing CTF/LTTng trace with babeltrace1's Python bindings and
iterates field names per event. On Python 3.14 / Ubuntu 26.04, this segfaults
inside `_bt_python_field_one_from_list` because the C wrapper returns a NULL
list pointer with an uninitialized count.

Reader is assumed to have the trace at the path below.
""" 
import sys
import faulthandler
faulthandler.enable()

import babeltrace

TRACE_DIR = '/tmp/babeltrace1.5-python/trace'

print(f'Python: {sys.version.splitlines()[0]}')
print(f'babeltrace: {babeltrace.__version__}')
print(f'Trace dir: {TRACE_DIR}')

tc = babeltrace.TraceCollection()
tc.add_traces_recursive(TRACE_DIR, 'ctf')

for event in tc.events:
    print(f'event {event.name!r} ts={event.timestamp}')
    print('  about to call event.keys() ...', flush=True)
    keys = event.keys()  # <-- crashes here on Python 3.14
    print(f'  keys = {keys}')
print('DONE')

Steps reproduce on Python 3.14 and Ubuntu 26.04¶

$ docker run --rm -it -v /tmp/babeltrace1.5-python:/tmp/babeltrace1.5-python ubuntu:26.04 bash -c '
  apt-get update -qq && DEBIAN_FRONTEND=noninteractive apt-get install -y -qq python3-babeltrace && python3 --version && python3 /tmp/babeltrace1.5-python/repro_minimal.py
'

Actual output output with Python 3.14 (Ubuntu 26.04): segfault¶

Python 3.14.4
Python: 3.14.4 (main, Apr  8 2026, 04:02:31) [GCC 15.2.0]
babeltrace: 1.5.11
Trace dir: /tmp/babeltrace1.5-python/trace
event 'lttng_ust_tracef:event' ts=1777503018164567458
  about to call event.keys() ...
Fatal Python error: Segmentation fault

Current thread 0x000072cf0dcc51c0 [python3] (most recent call first):
  File "/usr/lib/python3/dist-packages/babeltrace/babeltrace.py", line 83 in _bt_python_field_one_from_list
  File "/usr/lib/python3/dist-packages/babeltrace/babeltrace.py", line 839 in _field_list_with_scope
  File "/usr/lib/python3/dist-packages/babeltrace/babeltrace.py", line 719 in field_list_with_scope
  File "/usr/lib/python3/dist-packages/babeltrace/babeltrace.py", line 792 in keys
  File "/tmp/babeltrace1.5-python/repro_minimal.py", line 29 in <module>

Current thread's C stack trace (most recent call first):
  Binary file "python3", at _Py_DumpStack+0x4a [0x476e25]
  Binary file "python3" [0x504188]
  Binary file "/usr/lib/x86_64-linux-gnu/libc.so.6", at +0x45cb0 [0x72cf0dd0bcb0]
  Binary file "/usr/lib/python3/dist-packages/babeltrace/_babeltrace.cpython-314-x86_64-linux-gnu.so", at _bt_python_field_one_from_list+0x7 [0x72cf0d9bc627]
  Binary file "/usr/lib/python3/dist-packages/babeltrace/_babeltrace.cpython-314-x86_64-linux-gnu.so", at +0x156b5 [0x72cf0d9bc6b5]
  Binary file "python3" [0x5772b9]
  Binary file "python3", at _PyObject_MakeTpCall+0x36c [0x52f3fc]
  Binary file "python3", at _PyEval_EvalFrameDefault+0xe66 [0x54d936]
  Binary file "python3", at PyEval_EvalCode+0x106 [0x547bd6]
  Binary file "python3" [0x6cb701]
  Binary file "python3" [0x6c82b1]
  Binary file "python3" [0x6c795b]
  Binary file "python3" [0x6c7429]
  Binary file "python3", at Py_RunMain+0x349 [0x6c50e9]
  Binary file "python3", at Py_BytesMain+0x34 [0x6744b4]
  Binary file "/usr/lib/x86_64-linux-gnu/libc.so.6", at +0x2a601 [0x72cf0dcf0601]
  Binary file "/usr/lib/x86_64-linux-gnu/libc.so.6", at __libc_start_main+0x88 [0x72cf0dcf0718]
  Binary file "python3", at _start+0x25 [0x6738b5]

Extension modules: babeltrace._babeltrace (total: 1)

Expected output == actual output on Python 3.12 (Ubuntu 24.04): works¶

Just change the Docker image to `ubundu:24.04`:

Python 3.12.3
Python: 3.12.3 (main, Mar 23 2026, 19:04:32) [GCC 13.3.0]
babeltrace: 1.5.11
Trace dir: /tmp/babeltrace1.5-python/trace
event 'lttng_ust_tracef:event' ts=1777503018164567458
  about to call event.keys() ...
  keys = ['cpu_id', 'content_size', 'id', 'v', 'procname', 'vtid', 'packet_seq_num', 'uuid', 'timestamp_end', 'events_discarded', 'stream_instance_id', 'timestamp_begin', 'magic', 'stream_id', '_msg_length', 'vpid', 'msg', 'packet_size']
event 'lttng_ust_tracef:event' ts=1777503018178240108
  about to call event.keys() ...
  keys = ['cpu_id', 'content_size', 'id', 'v', 'procname', 'vtid', 'packet_seq_num', 'uuid', 'timestamp_end', 'events_discarded', 'stream_instance_id', 'timestamp_begin', 'magic', 'stream_id', '_msg_length', 'vpid', 'msg', 'packet_size']
event 'ros2:rmw_publisher_init' ts=1777503018183106175
...
DONE