LTTng » LTTng-tools

Some Python tests have been failing randomly for some time on the CI. It seems that the reason for the failure is that Babeltrace tries to open a trace which turns out to be empty.

These tests basically all operate the same way:

1. create a session
2. enable all events
3. start the tracing
4. launch a test application
5. stop the session
6. destroy the session
7. launch Babeltrace to read the trace

Stopping the trace is currently done by launching a sub-process that invokes "lttng stop". However, it does not wait for the process to finish before trying to read the trace.

The patch attached to this bug report forgoes this mechanism completely and calls lttng_stop_tracing() via the Python bindings. However, the error still occurs which leads me to believe the lttng_stop_tracing() call does not really ensure the trace data is flushed to disk.

Note that this patch also modifies the lttng-tools/tests/regressions/ust/fork/test_fork test to print Babeltrace's stderr output as diagnostic.

$ ./test_fork 
1..6
ok 1 - Fork test application exited normally
Waiting for data availability
not ok 2 - Resulting trace is readable
#[error] Encountered an empty file, but expecting a trace packet header.
#[error] Stream index creation error.
#[error] Open file stream error.
#[warning] [Context] Cannot open_trace of format ctf at path /tmp/tmp68ecst/b99f4e80-efe5-11e2-b4ca-00248c0e622f/ust/pid/fork2-19302-20130718-160751.
#[warning] [Context] cannot open trace "/tmp/tmp68ecst/b99f4e80-efe5-11e2-b4ca-00248c0e622f/ust/pid/fork2-19302-20130718-160751" from /tmp/tmp68ecst/b99f4e80-efe5-11e2-b4ca-00248c0e622f for reading.
#[warning] errors occurred when opening trace "/tmp/tmp68ecst/b99f4e80-efe5-11e2-b4ca-00248c0e622f" for reading, continuing anyway.
#
Bail out!
# Unreadable trace; can't proceed with analysis.