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

Author: michaelpq

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>

src/backend/utils/misc/injection_point.c

@@ -553,3 +553,20 @@ InjectionPointRun(const char *name)
 	elog(ERROR, "Injection points are not supported by this build");
 #endif
 }
+
+/*
+ * Execute an injection point directly from the cache, if defined.
+ */
+void
+InjectionPointCached(const char *name)
+{
+#ifdef USE_INJECTION_POINTS
+	InjectionPointCacheEntry *cache_entry;
+
+	cache_entry = injection_point_cache_get(name);
+	if (cache_entry)
+		cache_entry->callback(name, cache_entry->private_data);
+#else
+	elog(ERROR, "Injection points are not supported by this build");
+#endif
+}

src/include/utils/injection_point.h

@@ -17,9 +17,11 @@
 #ifdef USE_INJECTION_POINTS
 #define INJECTION_POINT_LOAD(name) InjectionPointLoad(name)
 #define INJECTION_POINT(name) InjectionPointRun(name)
+#define INJECTION_POINT_CACHED(name) InjectionPointCached(name)
 #else
 #define INJECTION_POINT_LOAD(name) ((void) name)
 #define INJECTION_POINT(name) ((void) name)
+#define INJECTION_POINT_CACHED(name) ((void) name)
 #endif
 
 /*
@@ -38,6 +40,7 @@ extern void InjectionPointAttach(const char *name,
 								 int private_data_size);
 extern void InjectionPointLoad(const char *name);
 extern void InjectionPointRun(const char *name);
+extern void InjectionPointCached(const char *name);
 extern bool InjectionPointDetach(const char *name);
 
 #endif							/* INJECTION_POINT_H */

src/test/modules/injection_points/expected/injection_points.out

@@ -129,6 +129,12 @@ SELECT injection_points_detach('TestInjectionLog2');
 (1 row)
 
 -- Loading
+SELECT injection_points_cached('TestInjectionLogLoad'); -- nothing in cache
+ injection_points_cached 
+-------------------------
+ 
+(1 row)
+
 SELECT injection_points_load('TestInjectionLogLoad'); -- nothing
  injection_points_load 
 -----------------------
@@ -147,6 +153,13 @@ SELECT injection_points_load('TestInjectionLogLoad'); -- nothing happens
 
 (1 row)
 
+SELECT injection_points_cached('TestInjectionLogLoad'); -- runs from cache
+NOTICE:  notice triggered for injection point TestInjectionLogLoad
+ injection_points_cached 
+-------------------------
+ 
+(1 row)
+
 SELECT injection_points_run('TestInjectionLogLoad'); -- runs from cache
 NOTICE:  notice triggered for injection point TestInjectionLogLoad
  injection_points_run 

src/test/modules/injection_points/injection_points--1.0.sql

@@ -34,6 +34,16 @@ RETURNS void
 AS 'MODULE_PATHNAME', 'injection_points_run'
 LANGUAGE C STRICT PARALLEL UNSAFE;
 
+--
+-- injection_points_cached()
+--
+-- Executes the action attached to the injection point, from local cache.
+--
+CREATE FUNCTION injection_points_cached(IN point_name TEXT)
+RETURNS void
+AS 'MODULE_PATHNAME', 'injection_points_cached'
+LANGUAGE C STRICT PARALLEL UNSAFE;
+
 --
 -- injection_points_wakeup()
 --

src/test/modules/injection_points/injection_points.c

@@ -333,6 +333,20 @@ injection_points_run(PG_FUNCTION_ARGS)
 	PG_RETURN_VOID();
 }
 
+/*
+ * SQL function for triggering an injection point from cache.
+ */
+PG_FUNCTION_INFO_V1(injection_points_cached);
+Datum
+injection_points_cached(PG_FUNCTION_ARGS)
+{
+	char	   *name = text_to_cstring(PG_GETARG_TEXT_PP(0));
+
+	INJECTION_POINT_CACHED(name);
+
+	PG_RETURN_VOID();
+}
+
 /*
  * SQL function for waking up an injection point waiting in injection_wait().
  */

src/test/modules/injection_points/sql/injection_points.sql

@@ -42,9 +42,11 @@ SELECT injection_points_run('TestInjectionLog2'); -- notice
 SELECT injection_points_detach('TestInjectionLog2');
 
 -- Loading
+SELECT injection_points_cached('TestInjectionLogLoad'); -- nothing in cache
 SELECT injection_points_load('TestInjectionLogLoad'); -- nothing
 SELECT injection_points_attach('TestInjectionLogLoad', 'notice');
 SELECT injection_points_load('TestInjectionLogLoad'); -- nothing happens
+SELECT injection_points_cached('TestInjectionLogLoad'); -- runs from cache
 SELECT injection_points_run('TestInjectionLogLoad'); -- runs from cache
 SELECT injection_points_detach('TestInjectionLogLoad');