pgsql: Fix locking while setting flags in MySerializableXact.

Fix locking while setting flags in MySerializableXact.

Even if a flag is modified only by the backend owning the transaction, it's
not safe to modify it without a lock. Another backend might be setting or
clearing a different flag in the flags field concurrently, and that
operation might be lost because setting or clearing a bit in a word is not
atomic.

Make did-write flag a simple backend-private boolean variable, because it
was only set or tested in the owning backend (except when committing a
prepared transaction, but it's not worthwhile to optimize for the case of a
read-only prepared transaction). This also eliminates the need to add
locking where that flag is set.

Also, set the did-write flag when doing DDL operations like DROP TABLE or
TRUNCATE -- that was missed earlier.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/cb2d158c5808c72cbd12cbffe0b7f5dbe830760b

Modified Files
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src/backend/storage/lmgr/predicate.c | 85 ++++++++++++++++++-----------
src/include/storage/predicate_internals.h | 15 +++---
2 files changed, 61 insertions(+), 39 deletions(-)