pgsql: Don't send protocol messages to a shm_mq that no longer exists.

Don't send protocol messages to a shm_mq that no longer exists.

Commit 2bd9e412f92bc6a68f3e8bcb18e04955cc35001d introduced a mechanism
for relaying protocol messages from a background worker to another
backend via a shm_mq.  However, there was no provision for shutting
down the communication channel.  Therefore, a protocol message sent
late in the shutdown sequence, such as a DEBUG message resulting from
cranking up log_min_messages, could crash the server.  To fix, install
an on_dsm_detach callback that disables sending messages to the shm_mq
when the associated DSM is detached.

Branch
------
master

Details
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http://git.postgresql.org/pg/commitdiff/2ad5c27bb565c26a4b12ea3343331c80f121f269

Modified Files
--------------
src/backend/access/transam/parallel.c |    2 +-
src/backend/libpq/pqmq.c              |   28 ++++++++++++++++++++++++++--
src/include/libpq/pqmq.h              |    2 +-
3 files changed, 28 insertions(+), 4 deletions(-)