LTTng » Babeltrace

If I use the trace_handle from the TraceCollection.add_trace or if I recover it from a piece of code like:

    for event in trace_collection.events:
        trace1 = event.handle
        break

    while True:
        n_decl = 0
        for decl in trace_handle.events:
            n_decl += 1
        print("   {0:d} declarations".format(n_decl))

Due to limitations of the native Babeltrace API, only one event may be "alive" at a given time, i.e. a user should never store a copy of the events returned by this function for ulterior use. Users shall make sure to copy the information they need from an event before accessing the next one.

Here's an even simpler case:

def some_events(trace_collection, number):
    n_events = 0
    path = None
    for event in trace_collection.events:
        th = event.handle
        n_events += 1
        print("Trace handles obtained: {0:d}".format(n_events))

This is a very annoying bug that is getting in the way of our trace analyses (and should be treated as high priority). For instance, reading a bunch of sequential (non-overlapping in time) traces and then trying to treat them one at a time with:

traces = babeltrace.TraceCollection()
trace_handles = traces.add_traces_recursive(traces_path, 'ctf')
handle = None
for event in traces.events:
    if (handle is None) or (handle.id != event.handle.id):
        # New trace (they don't overlap)
        handle = event.handle
        print("Trace {0}".format(handle.path))
    ...

I agree that the warning should be clarified.
Do you have a trace that reproduces the problem?

Here's a self-contained script and a small (kernel system call) trace that chokes on the third pass through the EventDeclarations. You'll have to adapt the trace_path

#!/usr/bin/python3
import sys, babeltrace

def some_events(trace_collection, number):
    n_names = 0
    for event in trace_collection.events:
        n_names += 1
        print("{0:s}:".format(event.name))
        n_decl = 0
        for decl in event.handle.events:
            n_decl += 1
        print("   {0:d} declarations".format(n_decl))
        if (number is not None) and (n_names >= number):
            break

def main(argv):
    """ 
    The main function.
    :param argv: the list of command-line arguments, excluding the executable name
    :return: Nothing
    """ 
    trace_path = "/home/daniel/lttng-traces/smallsession-20160803-143625/kernel" 

    trace_collection = babeltrace.TraceCollection()

    trace_handle = trace_collection.add_trace(trace_path, 'ctf')

    print("first three events:")
    some_events(trace_collection, 3)
    print("some_events done.")

    print("All done.")

if __name__ == "__main__":
    main(sys.argv[1:])

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