Add INJECTION_POINT_CACHED() to run injection points directly from cache

This new macro is able to perform a direct lookup from the local cache
of injection points (refreshed each time a point is loaded or run),
without touching the shared memory state of injection points at all.

This works in combination with INJECTION_POINT_LOAD(), and it is better
than INJECTION_POINT() in a critical section due to the fact that it
would avoid all memory allocations should a concurrent detach happen
since a LOAD(), as it retrieves a callback from the backend-private
memory.

The documentation is updated to describe in more details how to use this
new macro with a load.  Some tests are added to the module
injection_points based on a new SQL function that acts as a wrapper of
INJECTION_POINT_CACHED().

Based on a suggestion from Heikki Linnakangas.

Author: Heikki Linnakangas, Michael Paquier
Discussion: https://postgr.es/m/58d588d0-e63f-432f-9181-bed29313dece@iki.fi

1 files changed, 10 insertions, 7 deletions

diff --git a/doc/src/sgml/xfunc.sgml b/doc/src/sgml/xfunc.sgml
index 756a9d07fb0..7e92e898460 100644
--- a/doc/src/sgml/xfunc.sgml
+++ b/doc/src/sgml/xfunc.sgml
@@ -3619,17 +3619,20 @@ INJECTION_POINT(name);
     </para>
 
     <para>
-     An injection point with a given <literal>name</literal> can be loaded
-     using macro:
+     Executing an injection point can require allocating a small amount of
+     memory, which can fail. If you need to have an injection point in a
+     critical section where dynamic allocations are not allowed, you can use
+     a two-step approach with the following macros:
 <programlisting>
 INJECTION_POINT_LOAD(name);
+INJECTION_POINT_CACHED(name);
 </programlisting>
 
-     This will load the injection point callback into the process cache,
-     doing all memory allocations at this stage without running the callback.
-     This is useful when an injection point is attached in a critical section
-     where no memory can be allocated: load the injection point outside the
-     critical section, then run it in the critical section.
+     Before entering the critical section,
+     call <function>INJECTION_POINT_LOAD</function>. It checks the shared
+     memory state, and loads the callback into backend-private memory if it is
+     active. Inside the critical section, use
+     <function>INJECTION_POINT_CACHED</function> to execute the callback.
     </para>
 
     <para>