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Commitfe705ef

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doc: Expand section related to LWLocks and shared memory
The documentation includes a section describing how to define customLWLocks in extensions using the shmem hooks. However, it has nevermentioned the second, more flexible method based on the followingroutines:- LWLockNewTrancheId() to allocate a tranche ID.- LWLockRegisterTranche() to associate a name to a tranche ID.- LWLockInitialize() to initialize a LWLock with a tranche ID.autoprewarm.c is the only example of extension in the tree thatallocates a LWLock this way.This commit adds some documentation about all that. While on it, acomment is added about the need of AddinShmemInitLock. This is requiredespecially for EXEC_BACKEND builds (aka Windows, normally), as per aremark from Alexander, because backends can execute shmem initializationpaths concurrently.Author: Aleksander Alekseev, Michael PaquierDiscussion:https://postgr.es/m/CAJ7c6TPKhFgL+54cdTD9yGpG4+sNcyJ+N1GvQqAxgWENAOa3VA@mail.gmail.com
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‎doc/src/sgml/xfunc.sgml

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@@ -3428,6 +3428,29 @@ void RequestNamedLWLockTranche(const char *tranche_name, int num_lwlocks)
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<filename>contrib/pg_stat_statements/pg_stat_statements.c</filename> in the
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<productname>PostgreSQL</productname> source tree.
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</para>
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<para>
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There is another, more flexible method of obtaining LWLocks. First,
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allocate a <literal>tranche_id</literal> from a shared counter by
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calling:
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<programlisting>
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int LWLockNewTrancheId(void)
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</programlisting>
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Next, each individual process using the <literal>tranche_id</literal>
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should associate it with a <literal>tranche_name</literal> by calling:
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<programlisting>
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void LWLockRegisterTranche(int tranche_id, const char *tranche_name)
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</programlisting>
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It is also required to call <function>LWLockInitialize</function> once
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per LWLock, passing the <literal>tranche_id</literal> as argument:
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<programlisting>
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void LWLockInitialize(LWLock *lock, int tranche_id)
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</programlisting>
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A complete usage example of <function>LWLockNewTrancheId</function>,
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<function>LWLockInitialize</function> and
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<function>LWLockRegisterTranche</function> can be found in
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<filename>contrib/pg_prewarm/autoprewarm.c</filename> in the
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<productname>PostgreSQL</productname> source tree.
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</para>
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<para>
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To avoid possible race-conditions, each backend should use the LWLock
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<function>AddinShmemInitLock</function> when connecting to and initializing
@@ -3451,6 +3474,13 @@ if (!ptr)
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}
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</programlisting>
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</para>
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<para>
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It is convenient to use <literal>shmem_startup_hook</literal> which allows
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placing all the code responsible for initializing shared memory in one
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place. When using <literal>shmem_startup_hook</literal> the extension
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still needs to acquire <function>AddinShmemInitLock</function> in order to
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work properly on all the supported platforms.
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</para>
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</sect2>
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<sect2 id="xfunc-addin-wait-events">

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