pgsql: Fix possible crash reading pg_stat_activity.

Fix possible crash reading pg_stat_activity.

With the old code, a backend that read pg_stat_activity without ever
having executed a parallel query might see a backend in the midst of
executing one waiting on a DSA LWLock, resulting in a crash. The
solution is for backends to register the tranche at startup time, not
the first time a parallel query is executed.

Report by Andreas Seltenreich. Patch by me, reviewed by Thomas Munro.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/175ff6598e014b2fe84c06fa443161294fc2eed0

Modified Files
--------------
src/backend/executor/execParallel.c | 1 -
src/backend/storage/lmgr/lwlock.c | 2 ++
src/backend/utils/mmgr/dsa.c | 28 +++++++---------------------
src/include/utils/dsa.h | 5 ++---
4 files changed, 11 insertions(+), 25 deletions(-)