pgsql: Tighten up application of parallel mode checks.

Tighten up application of parallel mode checks.

Commit 924bcf4f16d54c55310b28f77686608684734f42 failed to enforce
parallel mode checks during the commit of a parallel worker, because
we exited parallel mode prior to ending the transaction so that we
could pop the active snapshot.  Re-establish parallel mode during
parallel worker commit.  Without this, it's far too easy for unsafe
actions during the pre-commit sequence to crash the server instead of
hitting the error checks as intended.

Just to be extra paranoid, adjust a couple of the sanity checks in
xact.c to check not only IsInParallelMode() but also
IsParallelWorker().

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/d43e3adc7572d34988967475900dcd4f9e0a7b89

Modified Files
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src/backend/access/transam/xact.c |   12 +++++++-----
1 file changed, 7 insertions(+), 5 deletions(-)