pgsql: Fix Windows implementation of PGSemaphoreLock.

Fix Windows implementation of PGSemaphoreLock.

The original coding failed to reset ImmediateInterruptOK before returning,
which would potentially allow a subsequent query-cancel interrupt to be
accepted at an unsafe point. This is a really nasty bug since it's so hard
to predict the consequences, but they could be unpleasant.

Also, ensure that signal handlers are serviced before this function
returns, even if the semaphore is already set. This should make the
behavior more like Unix.

Back-patch to all supported versions.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/ada8fa08fc6cf5f199b6df935b4d0a730aaa4fec

Modified Files
--------------
src/backend/port/win32_sema.c | 20 ++++++++++++--------
1 files changed, 12 insertions(+), 8 deletions(-)