Tracing
=======

This subsystem will provide a mechanism to get structured tracing info from gstreamer
applications. This can be used for post-run analysis as well as for life
introspection.

We are going to introduce a GstTracer object. There will be only a single instance
per process or none if tracing is off.
Certain GStreamer core function (such as gst_pad_push or gst_element_add_pad) will
call into the tracer. The tracer will dispatch into loaded tracing plugins.
Developers will be able to select a list of plugins by setting an environment
variable, such as GST_TRACE="meminfo,dbus". When then plugins are loaded, they will
add them to certain hooks. Another env var GST_TRACE_CHANNEL can be used to send
the tracing to a file or a socket (Do the same for GST_DEBUG_CHANNEL). The syntax
could be GST_XXX_CHANNEL=file:///path/to/file or GST_XXX_CHANNEL=tcp://<ip>:<port>.
If no channel is set, the tracing goes to stderr like the debug logging.

TODO(ensonic): we might want to have GST_{DEBUG|TRACE)_FORMAT envars as well. These
could be raw, ansi-color, binary, ...

Hooks
-----
e.g. gst_pad_push() will do add this line:
GST_TRACER_PUSH_BUFFER (pad, buffer)

If tracing is disable at compile time the macro will evaluate to nothing. Otherwise
it will become somthing along the lines of:
if (__tracer) {
  gst_tracer_push_buffer (pad, buffer);
}

Plugins can attach to a specific (named) hook or to groups of them. Groups could be:
all, topology, communication. When a hook such as gst_tracer_push_buffer () is called
it will take a timstamp and call all attached plugins from all, each group and this
hook. Hooks will be called from misc threads. The trace plugins should only consume
the provided data. Most trace plugins will log data to a trace channel.

TODO(ensonic): use GSignal for the hooks?

Plugins
=======

meminfo
-------
- register to the "all" group.
- call mallinfo() and log memory usage

rusage
------
- register to the "all" group.
- call getrusage() and log ressource usage

dbus
----
- provide a dbus iface to announe applications that are traced
- tracing UIs can use the dbus iface to find the channels where logging and tracing
  is getting logged to


UI
==

gst-debug-viewer
----------------
gst-debug-viewer could be given the tracelog in addition to the debug log.
Alternatively it would show a dialog that shows all local apps (if the dbus plugin
is loaded) and read the log streams from the sockets/files that are configured for
the app.

gst-trace-stats
---------------
Such a tool could read a trace and sumarize the content like gst-tracelib did for
stats in 0.10.


Problems / Open items
=====================
- when connecting to a running app, we cant easily get the 'current' state if logging
is using a socket, as past events are not stored