Support loading of injection points

This can be used to load an injection point and prewarm the
backend-level cache before running it, to avoid issues if the point
cannot be loaded due to restrictions in the code path where it would be
run, like a critical section where no memory allocation can happen
(load_external_function() can do allocations when expanding a library
name).

Tests can use a macro called INJECTION_POINT_LOAD() to load an injection
point.  The test module injection_points gains some tests, and a SQL
function able to load an injection point.

Based on a request from Andrey Borodin, who has implemented a test for
multixacts requiring this facility.

Reviewed-by: Andrey Borodin
Discussion: https://postgr.es/m/ZkrBE1e2q2wGvsoN@paquier.xyz

1 files changed, 14 insertions, 0 deletions

diff --git a/doc/src/sgml/xfunc.sgml b/doc/src/sgml/xfunc.sgml
index f3a3e4e2f8f..756a9d07fb0 100644
--- a/doc/src/sgml/xfunc.sgml
+++ b/doc/src/sgml/xfunc.sgml
@@ -3619,6 +3619,20 @@ INJECTION_POINT(name);
     </para>
 
     <para>
+     An injection point with a given <literal>name</literal> can be loaded
+     using macro:
+<programlisting>
+INJECTION_POINT_LOAD(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.
+    </para>
+
+    <para>
      Add-ins can attach callbacks to an already-declared injection point by
      calling:
 <programlisting>