lib: Drain rbuff before close

ssm_rbuff_close used to unmap the SHM page immediately, leaving any
in-flight peer-process thread that was inside pthread_mutex_lock or
pthread_cond_wait on the SHM-resident sync primitives reading freed
memory.  Adds an n_users counter, bumped on entry and dropped on exit
of every function that touches the mutex / cond vars (write, write_b,
read, read_b, fini), and have ssm_rbuff_close poll-spin until the
counter drains before tearing down.

ssm_rbuff_read now re-checks IS_EMPTY after taking the mutex, plugging
a TOCTOU where two readers could both pass a lock-free fast path and
the loser would read a stale TAIL.

Signed-off-by: Dimitri Staessens <dimitri@ouroboros.rocks>
Signed-off-by: Sander Vrijders <sander@ouroboros.rocks>

0 files changed, 0 insertions, 0 deletions