pgsql: When restoring GUCs in parallel workers, show an error context.

When restoring GUCs in parallel workers, show an error context.

Otherwise it can be hard to see where an error is coming from, when
the parallel worker sets all the GUCs that it received from the
leader.  Bug #15726.  Back-patch to 9.5, where RestoreGUCState()
appeared.

Reported-by: Tiago Anastacio
Reviewed-by: Daniel Gustafsson, Tom Lane
Discussion: https://postgr.es/m/15726-6d67e4fa14f027b3%40postgresql.org

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/3af7c64fe70c0b9d1072c660ddb1cb87dbe0a9d3

Modified Files
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src/backend/utils/misc/guc.c | 29 +++++++++++++++++++++++++++++
1 file changed, 29 insertions(+)