[lttng-dev] [RFC] Changes to the stop command

[David Goulet]

Mathieu Desnoyers:
> * David Goulet (dgoulet at efficios.com) wrote:
>> Hi everyone,
>>
>> We discovered a week ago a "broken guarantee" which is that when a
>> session is stopped by either using the lttng command or the API call
>> lttng_stop_session the traced data MUST be ready to be read.
>>
>> However, we don't offer that at all for now for both local storage and
>> network streaming. The stop command/call simply does _not_ wait for that
>> state.
>>
>> Here is the proposal to fix this issue before the 2.1 stable release.
>> Let's add a new API call (extending it) that probes the session daemon
>> for the trace files state (still writing, no more data, closed, ...).
>>
>> Ex: lttng_data_state(handle)
>>
>> This will bring a change to the default behavior of the stop command.
>> From now on, it will wait by default until the data is available to read
>> (for both network and local). This will however be done on the client
>> side in order to avoid blocking the session daemon client command sub
>> system for an unknown amount of time.
>>
>> The way I propose we proceed is to use the new API call (mention above)
>> on the liblttng-ctl side when a stop is done that requires it to wait.
>> Unfortunately, there is no clean way to do that other than an active
>> loop polling the session daemon...
>>
>> The "no wait" use case of the stop command will also be added with a
>> lttng_stop_session_no_wait or something like that.
>>
>> In a nutshell:
>>
>> (new) lttng_data_state(handle) --> name is NOT final, please chip in for
>> ideas! :)
> 
> ideas:
> 
> - lttng_data_pending()
> - lttng_data_available()

lttng_data_ready?

David

> 
> 
>> (new) lttng_stop_session_no_wait(session_name) --> naming NOT final.
> 
> lttng_stop_session_no_wait sounds ok to me.
> 
> The rest looks good. Let's see if others find better names ;)
> 
> Thanks,
> 
> Mathieu
> 
>>
>> (changes) lttng stop (and lttng_stop_session) will now wait for the data
>> to be available so babeltrace could be use right after for instance. A
>> --no-wait will be added as well to the UI command.
>>
>> I would like everyone opinion on that because this is an important issue
>> that _MUST_ be fixed in 2.1 stable or at least in the 2.1.x series.
>>
>> Thanks a lot!
>> David
>>
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>