Fix: relayd: live: Catch short lived applications for attached viewers

Fix: relayd: live: Catch short lived applications for attached viewers

Observed issue
==============

When a live viewer is attached to a session and a new application
starts, emits events, and exits the viewer may not see the produced
events.

With per-UID buffer allocation, the application needs to run as a new
user that hasn't had streams allocated before. With per-PID buffers,
spawning a new traced application is sufficient.

Cause
=====

When the new relay streams are created, associated viewer streams are
not immediately created. As a result, there is a gap between in which
the session may start being destroyed and/or the relay streams
unpublished and the time at which the live viewer sends a GET_NEW_STREAMS
command. When the relay streams are unpublished for any reason, the
reference to the relay stream in the ctf_trace is removed. The new
and unsent streams iterate over the relay streams in each ctf_trace.
Therefore, relay streams that were created and unpublished while
the live viewer was already attached to the session can be completely
missed.

Solution
========

The solution has three main aspects:

1. When new relayd streams are published and a viewer is attached for the
corresponding relay session or when a live viewer session attaches to
an existing relay session the viewer streams are created immediately.

2. The unsent viewer streams are tracked in a per-viewer session
list so that there continues to be a reference (via the
viewer_stream->stream backreference) held for the relay stream, and that
unpublished relay streams can be found without iterating over the
entire relay streams hashtable.

3. To cover cases where a relay stream has been closed but there are
still known trace chunks available, an additional check has been added
to the `get_next_index` viewer stream transition checks. When the
seen rotation count and relay stream rotation count are the same and
that the relay stream no longer has an active trace chunk, the
viewer stream is not forcibly rotated. This stops the final drop to
the trace chunk reference (via
viewerstream->stream_file->trace_chunk). Later, when the relay stream
is fully closed, there is a final rotation that is performed.

Known drawbacks
===============

The current implementation adds a global hash table which holds
references to created viewer sessions. When searching to determine if
new viewer streams should be created, the search is O(N*M) where N is the
number of viewer sessons and M is the number of relay sessions.

A different approach to recording references from relay sessions to
viewer sessions (if any exist) could reduce the search space.

Change-Id: Ie8f00697a4dafd5c9b0bfe60a872d1c1882f6944
Signed-off-by: Kienan Stewart <kstewart@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>