[lttng-dev] RFC LTTng session and daemon configuration save and restore

[Thibault, Daniel]

> Date: Wed, 4 Dec 2013 16:33:09 -0500
>
> Version:
> - v0.2: 04/12/2013
> [...]
> Daemon configuration files are, by default, either stored system-wide under /etc/lttng/*.conf or in the user's home directory under $HOME/.lttng/*.conf.

   Transient files  (lttng-sessiond.pid, etc., but not .lttngrc) are currently created under $HOME/.lttng/ (match) and /run/lttng/ (mismatch).  But I guess the difference between /run and /etc makes sense in this case.

> Load sessions. If url the is a directory, all .lttng files it contains will be loaded. If session_name is NULL, all sessions found in url are restored.

« If url the is a directory »?

> int lttng_save_sessions(const char *url, const char *session_name int flags/struct lttng_session_save_attr *attr);

   Missing commas.

> If a session was saved as active, the tracing for that session will be activated automatically on load. [...]
> Tracing sessions saved in an active state will be resumed on load.

   But what if the trace files have gone missing in the mean time?  I think newly-loaded sessions should create new trace folders altogether (let the user decide how to merge the trace folders later).  This would be a good occasion to resolve the recurring problem of trace folder collisions (e.g. when a user-space app being traced per-processID dlcloses and dlopens its trace provider within a one-second window).  A simple solution is to use the snapshot approach, and have a "-%d" sequential sub-index added to the trace folder name to avoid the collision.
   Or is the intent to have the conf files store only the "name" part of the sessions, without the timestamp part?  This prevents collisions only when the session is not configured with an output or set-url option.

Daniel U. Thibault
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