Fix: src.ctf.lttng-live: incomplete metadata packet is an error

Fix: src.ctf.lttng-live: incomplete metadata packet is an error

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

While investigating the issue described in [1], I noticed that
babeltrace2 falls into a retry loop when the src.ctf.lttng-live
component encounters unparseable metadata.

The src.ctf.lttng-live reports parsing errors on every subsequent
reception of a metadata packet. The relay daemon eventually sends
binary data which fails to be decoded, ending the graph's execution
with a binary decoding error, which is not the "real" issue.

Cause
=====

Due to a (now fixed) bug in LTTng [1], unparseable (incomplete)
metadata can be made visible to live clients. This bug fix doesn't
involve the clients; it resulted in an illegal state in the lttng-live
protocol.

When the relay daemon notifies the live client that new metadata is
available, lttng_live_metadata_update() receives all the metadata made
available by the relay daemon in a memory stream and then uses the
ctf_metadata_decoder to append the new content to the existing trace
class.

However, if the decoder returns
`CTF_METADATA_DECODER_STATUS_INCOMPLETE`,
`LTTNG_LIVE_ITERATOR_STATUS_AGAIN` is returned to the caller,
resulting in the graph retrying to invoke the live iterator's `next`
method until another error eventually prevents the successful
completion of the graph.

I am assuming that the use of the `AGAIN` status code may have been a
failed attempt at fixing [1] in the live component rather than
adressing the underlying problem.

Solution
========

To my knowledge there are no provisions made for incomplete metadata
in the lttng-live protocol as of the current version. This may have
been done in anticipation of a future change (?), but it currently
obscures the error reported when a corrupted/incomplete/unparseable
metadata packet is received by the src.ctf.lttng-live component.

The current approach doesn't work anyhow as
lttng_live_metadata_update() creates a "fresh" memory stream on each
invocation. If the intention was to accumulate partial metadata until
it can be successfully parsed, the accumulated metadata would have to
be preserved from one invocation to the next.

The conversion of the status code from `INCOMPLETE` to `AGAIN` is
removed to fail immediately during the current invocation of the
iterator's `next` method.

Drawbacks
=========

None.

References
==========

[1] https://github.com/lttng/lttng-tools/commit/f5ba75b4f0c0b44092c76bc931b25b24a2e62718

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I8a379ea5d838786a6731199dd5f03bbf70ec13f5
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3586
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>