LTTng » LTTng-tools

Ericsson, myself, and Christian stumbled on this issue, and each of us thought it was a bug, while it was indeed a bad usage. We have two things here: the default behavior of enable-event (without -c parameter) is to create a "default" channel, even if a channel has already been created. The second issue is a misleading error message.

First, when a --buffers-uid channel has been created in a session, we should probably then use --buffers-uid as default buffer type, else the enable-command fails without -c parameter.

We might also want to consider that enable-event command performed after an enable-channel command, should probably not spawn a default channel, because this is unexpected.

Also, this failure scenario is misleading:

  512  lttng create
  513  lttng enable-channel -u --buffers-uid test
  514  lttng enable-event -u -a --filter '$ctx.procname=="abc"'

Ret filter: -24
Error: Setting filter: '$ctx.procname=="abc"'

It leads the user to think that there is something wrong with the filter expression, while in fact it's the default channel that has the wrong buffer type (pid buffers).

So both the error message and the behavior should be fixed here I think.