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  • 4 commits
  • 87 files changed
  • 2 contributors

Commits on Dec 30, 2024

  1. Replace Latches with Interrupts 

    The Latch was a flag, with an inter-process signalling mechanism that
allowed a process to wait for the Latch to be set. My original vision
for Latches was that you could have different latches for different
things that you need to wait for, but in practice, almost all code
used one "process latch" that was shared for all wakeups. The only
exception was the "recoveryWakeupLatch". I think the reason that we
ended up just sharing the same latch for all wakeups is that it was
cumbersome in practice to deal with multiple latches. For starters,
there was no machinery for waiting for multiple latches at the same
time. Secondly, changing the "ownership" of a latch needed extra
locking or other coordination. Thirdly, each inter-process latch
needed to be initialized at postmaster startup time.

This patch embraces the reality of how Latches were used, and replaces
the Latches with a per-process interrupt mask. You can no longer
allocate Latches in arbitrary shared memory structs and an interrupt
is always directed at a particular process, addressed by its
ProcNumber.  Each process has a bitmask of pending interrupts in
PGPROC.

This commit introduces two interrupt bits. INTERRUPT_GENERAL replaces
the general-purpose per-process latch. All code that previously set a
process's process latch now sets its INTERRUPT_GENERAL interrupt bit
instead.

The second interrupt bit, INTERRUPT_RECOVERY_CONTINUE, replaces
recoveryWakeupLatch. With this new interface, the code can easily wait
for the regular interrupts (INTERRUPT_GENERAL) at the same time as
INTERRUPT_RECOVERY_CONTINUE. Previously, the code that waited on
recoveryWakeupLatch had to resort to timeouts to react to other
signals, because it was not possible to wait for two latches at the
same time. Previous attempt at unifying those wakeups was in commit
ac22929, but it was reverted in commit 00f690a because it caused
a lot of spurious wakeups when the startup process was waiting for
recovery conflicts. The new machinery avoids that problem by making it
easy to wait for two interrupts at the same time.

More interrupt bits are planned for followup patches, to replace the
various ProcSignal bits, as well as ConfigReloadPending and
ShutdownRequestPending.

This also moves the WaitEventSet functions to a different source file,
waiteventset.c. This separates the platform-dependent code waiting and
signalling code from the platform-independent parts.

Reviewed-by: Thomas Munro, Robert Haas, Álvaro Herrera
Discussion: https://www.postgresql.org/message-id/476672e7-62f1-4cab-a822-f3a8e949dd3f@iki.fi
    @hlinnaka
    hlinnaka authored and Commitfest Bot committed Dec 30, 2024
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  2. Fix lost wakeup issue in logical replication launcher 

    Fix it by using a separate interrupt bit for subscription changes.

Discussion: https://www.postgresql.org/message-id/476672e7-62f1-4cab-a822-f3a8e949dd3f@iki.fi
Discussion: https://www.postgresql.org/message-id/ff0663d9-8011-420f-a169-efbf57327cb5@iki.fi
    @hlinnaka
    hlinnaka authored and Commitfest Bot committed Dec 30, 2024
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  3. Use INTERRUPT_GENERAL for bgworker state change notification 

    As part of a project to remove the reliance on PIDs to identify
backends, backends that register dynamic background workers will now
receive INTERRUPT_GENERAL sent directly by the postmaster when the
worker state changes.  Previously, the postmaster would send a SIGUSR1
signal, which relied on the ProcSignal system's handler setting the
interrupt flag, with the same end effect.  Now that intermediate step
is skipped.

The field bgw_notify_pid still exists for backwards compatibility, but
has no effect and may be removed in a later release.
RegisterBackgroundWorker() now automatically assumes that the caller
would like state change notifications delivered to its proc number,
unless BGW_NO_NOTIFY is set in bgw_flags.

Removing other cases of PIDs from the bgworker API is left for later
work; this patch is concerned only with removing a dependency on
ProcSignal infrastructure that is due to be removed by a later commit.

This represents the first use of SendInterrupt() in the postmaster.
It might seem more natural to use condition variables in the
wait-for-state-change functions, so that anyone with a handle can
wait, but condition variables as currently implemented would be a lot
less robust than simple interrupts.

Author: Thomas Munro <thomas.munro@gmail.com>
Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://postgr.es/m/CA%2BhUKG%2B3MkS21yK4jL4cgZywdnnGKiBg0jatoV6kzaniBmcqbQ%40mail.gmail.com
    @hlinnaka
    hlinnaka authored and Commitfest Bot committed Dec 30, 2024
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  4. [CF 51/5118] v5 - SendProcSignal(), SetLatch() → SendInterrupt() 

    This commit was automatically generated by a robot at cfbot.cputube.org.
It is based on patches submitted to the PostgreSQL mailing lists and
registered in the PostgreSQL Commitfest application.

This branch will be overwritten each time a new patch version is posted to
the email thread, and also periodically to check for bitrot caused by changes
on the master branch.

Commitfest entry: https://commitfest.postgresql.org/51/5118
Patch(es): https://www.postgresql.org/message-id/4ddab51d-e932-40c5-b6fa-18c52db0e082@iki.fi
Author(s): Heikki Linnakangas, Thomas Munro
    Commitfest Bot committed Dec 30, 2024
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