pgsql: Don't let timeout interrupts happen unless ImmediateInterruptOK

Don't let timeout interrupts happen unless ImmediateInterruptOK is set.

Serious oversight in commit 16e1b7a1b7f7ffd8a18713e83c8cd72c9ce48e07:
we should not allow an interrupt to take control away from mainline code
except when ImmediateInterruptOK is set. Just to be safe, let's adopt
the same save-clear-restore dance that's been used for many years in
HandleCatchupInterrupt and HandleNotifyInterrupt, so that nothing bad
happens if a timeout handler invokes code that tests or even manipulates
ImmediateInterruptOK.

Per report of "stuck spinlock" failures from Christophe Pettus, though
many other symptoms are possible. Diagnosis by Andres Freund.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/e8312b4f03315a98a987997e05a6e9d6d8d84ddf

Modified Files
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src/backend/utils/misc/timeout.c | 36 +++++++++++++++++++++++-------------
1 file changed, 23 insertions(+), 13 deletions(-)