Fix: sessiond: ust session is inactive during ust_app_global_update

Fix: sessiond: ust session is inactive during ust_app_global_update

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

The following scenario leads to an abort of lttng-sessiond.

 lttng-sessiond (with kernel tracing available)
 lttng create system-trace --snapshot -U /tmp/snapshot
 lttng enable-channel -k system-trace --subbuf-size=4k --num-subbuf=256
 lttng enable-event -c system-trace -k 'sched_wak*' -s system-trace
 lttng start system-trace

 lttng enable-event -u -a

Fails as expected with:
 Error: Events: The command tried to enable an event in a new domain for
 a session that has already been started once. (channel channel0,
 session system-trace)

Launch any ust app such as easy_ust from the lttng-ust repository.

The following backtrace is generated:

 (gdb) bt
 #0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
 #1  0x00007ffff7af0859 in __GI_abort () at abort.c:79
 #2  0x00007ffff7af0729 in __assert_fail_base (fmt=0x7ffff7c86588 "%s%s%s:%u: %s%sAssertion `%s' failed.\n%n", assertion=0x55555564b765 "usess->active", file=0x555555649a60 "ust-app.c", line
 #3  0x00007ffff7b01f36 in __GI___assert_fail (assertion=0x55555564b765 "usess->active", file=0x555555649a60 "ust-app.c", line=5123, function=0x55555564ecf0 <__PRETTY_FUNCTION__.14199> "ust_
 #4  0x00005555555d1f5e in ust_app_global_update (usess=0x7fffe001fb90, app=0x7fffac000b80) at ust-app.c:5123
 #5  0x00005555555b60d4 in update_ust_app (app_sock=82) at dispatch.c:71
 #6  0x00005555555b7025 in thread_dispatch_ust_registration (data=0x5555556a07f0) at dispatch.c:409
 #7  0x00005555555ad5ab in launch_thread (data=0x5555556a0810) at thread.c:65
 #8  0x00007ffff7ce6609 in start_thread (arg=<optimized out>) at pthread_create.c:477
 #9  0x00007ffff7bed293 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

This also happens for the track command. You can replace the `lttng
enable-event -u -a` with `lttng track --userspace --vuid=0` then launch
an app and the same backtrace gets generated.

Cause
=====

During `process_client_msg` the `create_ust_session` function is called
and a ust session is assigned to the "system_trace" session with a
state of `active` set to 0 (false). This is not a problem.

The problem seems to lie with a single call site for
`ust_app_global_update` in `update_ust_app`. The status of the ust
session is not checked before calling the `ust_app_global_update`. It is
important to note that all `ust_app_global_update_all` callsites guard
the call with a check against the status of the session.

Solution
========

Guard the call to `ust_app_global_update` with a check of the ust
session active state.

Known drawbacks
=========

None.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I14d25d99d0609689247cdfa86130bd0219613581