src.ctf.fs: add and use medops to iterate on a ds_file_group using the index

src.ctf.fs: add and use medops to iterate on a ds_file_group using the index

This patch solves the problem of reading multiple snapshots of the same
tracing session taken quickly.  Taking the snapshots quickly enough can
cause them to overlap, in which case some complete / identical packets
will be found in different snapshots.

As an example, imagine we have three snapshots, and packets labeled from
A to G belonging the same stream instance in all snapshots.  We could
have the following sequence of packets in each snapshot:

  - snapshot 1: A B C D
  - snapshot 2:     C D E F
  - snapshot 3:       D E F G

Babeltrace 1 reads these three snapshots successfully.  In fact, it just
considers them as three different traces, so it will order events
individually.  As a result, events from packet D will be present three
times in the output.  So while it works (as in Babeltrace exits with
status 0), it's probably not what a user would want.

Babeltrace 2 (before this patch) hits the following assert, which
validates that messages produced by iterators never go back in time:

    11-11 15:13:23.874  8329  8329 F LIB/MSG-ITER call_iterator_next_method@iterator.c:872 Babeltrace 2 library postcondition not satisfied; error is:
    11-11 15:13:23.874  8329  8329 F LIB/MSG-ITER call_iterator_next_method@iterator.c:872 Clock snapshots are not monotonic
    11-11 15:13:23.874  8329  8329 F LIB/MSG-ITER call_iterator_next_method@iterator.c:872 Aborting...

This is because Babeltrace 2 groups all CTF traces sharing the same UUID
(which is the case for our three snapshots) and gives them to the same
src.ctf.fs component to read.  The component groups data stream files
from the various snapshots by stream instance id, and sorts them
according to the timestamp of their first event.  It then reads them
sequentially, from end to end, assuming that the start of the second
data stream file is after the end of the first data stream file.  Using
our example above, the src.ctf.fs component would therefore try to read
packets in this order:

    A B C D C D E F D E F G
           ^
	   `- ouch!

In this case, we want to read all packets exactly once, in the right
order.

The solution brought by this patch is to iterate on the packets by
following the index, instead of reading all data files from end to end.
Index entries were already de-duplicated by commit

    ctf: de-duplicate index entries

So the index already refers to a single instance of each packet.  We can
therefore use it as a playlist of the packets to read.

The change mainly revolves around adding a new kind of CTF message
iterator medium operations, called ctf_fs_ds_group_medops.  Instead of
the medium being a single data stream file, like in
ctf_fs_ds_file_medops, this new medium is conceptually the sequence of
all packets described by the index of a ctf_fs_ds_group, possibly spread
out in different data stream files.

A new optional medium operation called `switch_packet` is added.  When
the CTF message iterator is done reading a packet, it calls this method,
indicating to the medium that it is at the frontier of two packets.  If
the medium is aware of the packets (like ctf_fs_ds_group_medops is) and
knows that the following packet is not contiguous with the previous
packet, it can reposition its "read head" at the right place (open the
new file, go to the right offset).  Immediatly after calling
`switch_packet`, the message iterator calls the `request_bytes` method,
which allows the medium to return a buffer containing the bytes of the
next packet.

When the packet-aware medium has its `switch_packet` method called but
there are no more packets to read, it returns
CTF_MSG_ITER_MEDIUM_STATUS_EOF.  That brings the message iterator in the
STATE_CHECK_EMIT_MSG_STREAM_END state, which will close the stream.

If the `switch_packet` method is not provided by the medium, the message
iterator just continues, assuming that the bytes of the next packet are
contiguous with those of the previous packet.

The ctf_fs_ds_file_medops medium operations are still necessary for
reading individual ctf_fs_ds_files, when initializing src.ctf.fs
components.  This is needed when building the index from a stream or for
applying tracer fixups.

This simplifies a little bit the interaction between the src.ctf.fs
iterator and the ctf_msg_iter.  Previously, we would read each data
stream file until the message iterator returned EOF.  If it wasn't the
last data stream file, we would reset the iterator to read the next data
stream file.  Functions
ctf_msg_iter_set_emit_stream_{beginning,end}_message were necessary to
indicate to the message iterator whether to send the stream beginning or
end messages, because it had otherwise no idea of whether the data
stream file it is reading is the first one or last one.  The function
ctf_msg_iter_set_medops_data was necessary to swap the data of the
ctf_fs_ds_file_medops to point to the new data stream file.

With the ctf_fs_ds_group_medops, the CTF message iterator only returns
EOF when the stream is done.  The data passed to the
ctf_fs_ds_group_medops has everything it needs to go through all the
packets, so it doesn't need to change.

Change-Id: I72f6d1e09b87414fb83f68cb57abb1f2dc61b439
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>