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Add a new shmem_request_hook hook.

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Currently, preloaded libraries are expected to request additional
shared memory and LWLocks in _PG_init().  However, it is not unusal
for such requests to depend on MaxBackends, which won't be
initialized at that time.  Such requests could also depend on GUCs
that other modules might change.  This introduces a new hook where
modules can safely use MaxBackends and GUCs to request additional
shared memory and LWLocks.

Furthermore, this change restricts requests for shared memory and
LWLocks to this hook.  Previously, libraries could make requests
until the size of the main shared memory segment was calculated.
Unlike before, we no longer silently ignore requests received at
invalid times.  Instead, we FATAL if someone tries to request
additional shared memory or LWLocks outside of the hook.

Nathan Bossart and Julien Rouhaud

Discussion: https://postgr.es/m/20220412210112.GA2065815%40nathanxps13
Discussion: https://postgr.es/m/Yn2jE/lmDhKtkUdr@paquier.xyz
This commit is contained in:
Robert Haas
2022-05-13 09:31:06 -04:00
parent 8c8d307f82
commit 4f2400cb3f
9 changed files with 81 additions and 38 deletions
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12
doc/src/sgml/xfunc.sgml
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@ -3402,22 +3402,30 @@ CREATE FUNCTION make_array(anyelement) RETURNS anyarray
startup. The add-in's shared library must be preloaded by specifying
it in
<xref linkend="guc-shared-preload-libraries"/><indexterm><primary>shared_preload_libraries</primary></indexterm>.
The shared library should register a <literal>shmem_request_hook</literal>
in its <function>_PG_init</function> function. This
<literal>shmem_request_hook</literal> can reserve LWLocks or shared memory.
Shared memory is reserved by calling:
<programlisting>
void RequestAddinShmemSpace(int size)
</programlisting>
from your <function>_PG_init</function> function.
from your <literal>shmem_request_hook</literal>.
</para>
<para>
LWLocks are reserved by calling:
<programlisting>
void RequestNamedLWLockTranche(const char *tranche_name, int num_lwlocks)
</programlisting>
from <function>_PG_init</function>. This will ensure that an array of
from your <literal>shmem_request_hook</literal>. This will ensure that an array of
<literal>num_lwlocks</literal> LWLocks is available under the name
<literal>tranche_name</literal>. Use <function>GetNamedLWLockTranche</function>
to get a pointer to this array.
</para>
<para>
An example of a <literal>shmem_request_hook</literal> can be found in
<filename>contrib/pg_stat_statements/pg_stat_statements.c</filename> in the
<productname>PostgreSQL</productname> source tree.
</para>
<para>
To avoid possible race-conditions, each backend should use the LWLock
<function>AddinShmemInitLock</function> when connecting to and initializing