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Support custom wait events for wait event type "Extension"

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Two backend routines are added to allow extension to allocate and define
custom wait events, all of these being allocated in the type
"Extension":
* WaitEventExtensionNew(), that allocates a wait event ID computed from
a counter in shared memory.
* WaitEventExtensionRegisterName(), to associate a custom string to the
wait event ID allocated.

Note that this includes an example of how to use this new facility in
worker_spi with tests in TAP for various scenarios, and some
documentation about how to use them.

Any code in the tree that currently uses WAIT_EVENT_EXTENSION could
switch to this new facility to define custom wait events.  This is left
as work for future patches.

Author: Masahiro Ikeda
Reviewed-by: Andres Freund, Michael Paquier, Tristan Partin, Bharath
Rupireddy
Discussion: https://postgr.es/m/b9f5411acda0cf15c8fbb767702ff43e@oss.nttdata.com
This commit is contained in:
Michael Paquier
2023-07-31 17:09:24 +09:00
parent 39055cb4cc
commit c9af054653
11 changed files with 393 additions and 23 deletions
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11
doc/src/sgml/monitoring.sgml
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@ -1117,11 +1117,14 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
<note>
<para>
Extensions can add <literal>LWLock</literal> types to the list shown in
<xref linkend="wait-event-lwlock-table"/>. In some cases, the name
Extensions can add <literal>Extension</literal> and
<literal>LWLock</literal> types
to the list shown in <xref linkend="wait-event-extension-table"/> and
<xref linkend="wait-event-lwlock-table"/>. In some cases, the name
assigned by an extension will not be available in all server processes;
so an <literal>LWLock</literal> wait event might be reported as
just <quote><literal>extension</literal></quote> rather than the
so an <literal>Extension</literal> or <literal>LWLock</literal> wait
event might be reported as just
<quote><literal>extension</literal></quote> rather than the
extension-assigned name.
</para>
</note>

45
doc/src/sgml/xfunc.sgml
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@ -3453,6 +3453,51 @@ if (!ptr)
</para>
</sect2>
<sect2 id="xfunc-addin-wait-events">
<title>Shared Memory and Custom Wait Events</title>
<para>
Add-ins can define custom wait events under the wait event type
<literal>Extension</literal>. The add-in's shared library must be
preloaded by specifying it in <literal>shared_preload_libraries</literal>,
and register a <literal>shmem_request_hook</literal> and a
<literal>shmem_startup_hook</literal> in its
<function>_PG_init</function> function.
<literal>shmem_request_hook</literal> can request a shared memory size
to be later used at startup by calling:
<programlisting>
void RequestAddinShmemSpace(int size)
</programlisting>
</para>
<para>
<literal>shmem_startup_hook</literal> can allocate in shared memory
custom wait events by calling while holding the LWLock
<function>AddinShmemInitLock</function> to avoid any race conditions:
<programlisting>
uint32 WaitEventExtensionNew(void)
</programlisting>
Next, each process needs to associate the wait event allocated previously
to a user-facing custom string, which is something done by calling:
<programlisting>
void WaitEventExtensionRegisterName(uint32 wait_event_info, const char *wait_event_name)
</programlisting>
An example can be found in <filename>src/test/modules/worker_spi</filename>
in the PostgreSQL source tree.
</para>
<para>
Custom wait events can be viewed in
<link linkend="monitoring-pg-stat-activity-view"><structname>pg_stat_activity</structname></link>:
<screen>
=# SELECT wait_event_type, wait_event FROM pg_stat_activity
WHERE backend_type ~ 'worker_spi';
wait_event_type | wait_event
-----------------+-----------------
Extension | worker_spi_main
(1 row)
</screen>
</para>
</sect2>
<sect2 id="extend-cpp">
<title>Using C++ for Extensibility</title>