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Commit 09cf1d522 taught schedule_alarm() to not do anything if
the next requested event is after when we expect the next interrupt
to fire. However, if somehow an interrupt gets lost, we'll continue
to not do anything indefinitely, even after the "next interrupt" time
is obviously in the past. Thus, one missed interrupt can break
timeout scheduling for the life of the session. Michael Harris
reported a scenario where a bug in a user-defined function caused this
to happen, so you don't even need to assume kernel bugs exist to think
this is worth fixing. We can make things more robust at little cost
by detecting the case where signal_due_at is before "now" and forcing
a new setitimer call to occur. This isn't a completely bulletproof
fix of course; but in our typical usage pattern where we frequently set
timeouts and clear them before they are reached, the interrupt will
get re-enabled after at most one timeout interval, which with a little
luck will be before we really need it.
While here, let's mark signal_due_at as volatile, since the signal
handler can both examine and set it. I'm not sure there's any
actual risk given that signal_pending is already volatile, but
it's surely questionable.
Backpatch to v14 where this logic came in.
Michael Harris and Tom Lane
Discussion: https://postgr.es/m/CADofcAWbMrvgwSMqO4iG_iD3E2v8ZUrC-_crB41my=VMM02-CA@mail.gmail.com
PostgreSQL Database Management System ===================================== This directory contains the source code distribution of the PostgreSQL database management system. PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings. PostgreSQL has many language interfaces, many of which are listed here: https://www.postgresql.org/download/ See the file INSTALL for instructions on how to build and install PostgreSQL. That file also lists supported operating systems and hardware platforms and contains information regarding any other software packages that are required to build or run the PostgreSQL system. Copyright and license information can be found in the file COPYRIGHT. A comprehensive documentation set is included in this distribution; it can be read as described in the installation instructions. The latest version of this software may be obtained at https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.