Currently, when advance_and_await() fails to allocate the new gate
object, it will throw bad_alloc and leave the phased_barrier object in
an invalid state. Calling advance_and_await() again on it will result
in undefined behavior (typically SIGSEGV) beacuse _gate will be
disengaged.
One place affected by this is table::seal_active_memtable(), which
calls _flush_barrier.advance_and_await(). If this throws, subsequent
flush attempts will SIGSEGV.
This patch rearranges the code so that advance_and_await() has strong
exception guarantees.
Message-Id: <1542645562-20932-1-git-send-email-tgrabiec@scylladb.com>
Fixes#3931.
(cherry picked from commit 57e25fa0f8)
c0ffc9a2b7