pgsql: Fix unsafe order of operations in foreign-table DDL commands.

Fix unsafe order of operations in foreign-table DDL commands.

When updating or deleting a system catalog tuple, it's necessary to acquire
RowExclusiveLock on the catalog before looking up the tuple; otherwise a
concurrent VACUUM FULL on the catalog might move the tuple to a different
TID before we can apply the update. Coding patterns that find the tuple
via a table scan aren't at risk here, but when obtaining the tuple from a
catalog cache, correct ordering is important; and several routines in
foreigncmds.c got it wrong. Noted while running the regression tests in
parallel with VACUUM FULL of assorted system catalogs.

For consistency I moved all the heap_open calls to the starts of their
functions, including a couple for which there was no actual bug.

Back-patch to 8.4 where foreigncmds.c was added.

Branch
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REL8_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/3c96f5c647852d1c77f9854cf1f78dfb01af8631

Modified Files
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src/backend/commands/foreigncmds.c | 31 ++++++++++++++-----------------
1 files changed, 14 insertions(+), 17 deletions(-)