gstreamer/subprojects/gst-devtools/tracer

# Add a python api for tracer analyzers

The python framework will parse the tracer log and aggregate information.
the tool writer will subclass from the Analyzer class and override methods:

  'handle_tracer_class(self, entry)'
  'handle_tracer_entry(self, entry)'

Each of those is optional. The entry field is the parsed log line. In most cases
the tools will parse the structure contained in event[Parser.F_MESSAGE].

TODO: maybe do apply_tracer_entry() and revert_tracer_entry() - 'apply' will
patch the shared state forward and 'revert' will 'apply' the inverse. This would
let us go back from a state. An application should still take snapshots to allow
for efficient jumping around. If that is the case we could also always go forward
from a snapshot.

A tool will use an AnalysisRunner to chain one or more analyzers and iterate the
log. A tool can also replay the log multiple times. If it does, it won't work in
'streaming' mode though (streaming mode can offer live stats).

## TODO
### gst shadow types
Do we want to provide classes like GstBin, GstElement, GstPad, ... to aggregate
info. One way to get them would be to have a GstLogAnalyzer that knows
about data from the log tracer and populates the classes. Tools then can
do e.g.

  pad.name()             # pad name
  pad.parent().name()    # element name
  pad.peer().parent()    # peer element
  pad.parent().state()   # element state

This would allow us to e.g. get a pipeline graph at any point in the log.

### improve class handling
We already parse the tracer classes. Add helpers that for numeric values that
extract them, and aggregate min/max/avg. Consider other statistical information
(std. deviation) and provide a rolling average for live view.

## Examples
### Sequence chart generator (mscgen)

1.) Write file header

2.) collect element order
Replay the log and use pad_link_pre to collect pad->peer_pad relationship.
Build a sequence of element names and write to msc file.

3.) collect event processing
Replay the log and use pad_push_event_pre to output message lines to mscfile.

4.) write footer and run the tool.

## Latency stats

1.) collect per sink-latencies and for each sink per source latencies
Calculate min, max, avg. Consider streaming interface, where we update the stats
e.g. once a sec

2.) in non-streaming mode write final statistic

## cpu load stats

Like latency stats, for cpu load. Process cpu load + per thread cpu load.

## top

Combine various stats tools into one.


# todo
## all tools
* need some (optional) progress reporting

## structure parser
* add an optional compiled regexp matcher an constructor param
* then we'll parse the whole structure with a single regexp
  * this will only parse the top-level structure, we'd then check if there are
    nested substructure and handle them

# Improve tracers
## log
* the log tracer logs args and results into misc categories
* issues
  * not easy/reliable to detect its output among other trace output
  * not easy to match pre/post lines
  * uses own do_log method, instead of gst_tracer_record_log
    * if we also log structures, we need to log the 'function' as the
      structure-name, also fields would be key=(type)val, instead of key=value
    * if we switch to gst_tracer_record_log, we'd need to register 27 formats :/
## object ids
When logging GstObjects in PTR_FORMAT, we log the name. Unfortunately the name
is not neccesarilly unique over time. Same goes for the object address.
When logging a tracer record we need a way for the scope fileds to uniquely
relate to objects.
a) parse object creation and destruction and build <name:id>-maps in the tracer
   tools:
   new-element message: gst_util_seqnum_next() and assoc with name
   <new stats>: get id by name and get data record via id

   if we go this way, the stats tracer would log name in regullar record (which
   makes them more readable).

FIXME:
- if we use stats or log and latency, do we log latency messages twice?
  grep -c ":: latency, " logs/trace.all.log
  8365

  grep ":: event, " logs/trace.all.log | grep -c "name=(string)latency"
  63

  seems to not happen, regardless of order in GST_TRACERS="latency;stats"

- why do we log element-ix for buffer, event, ... log-entries in the stats
  tracer? We log new-pad, when the pad get added to a parent, so we should know
  the element already