Allow GRANT on pg_log_backend_memory_contexts().

Remove superuser check, allowing any user granted permissions on
pg_log_backend_memory_contexts() to log the memory contexts of any
backend.

Note that this could allow a privileged non-superuser to log the
memory contexts of a superuser backend, but as discussed, that does
not seem to be a problem.

Reviewed-by: Nathan Bossart, Bharath Rupireddy, Michael Paquier, Kyotaro Horiguchi, Andres Freund
Discussion: https://postgr.es/m/e5cf6684d17c8d1ef4904ae248605ccd6da03e72.camel@j-davis.com

doc/src/sgml/func.sgml

@@ -25332,7 +25332,6 @@ SELECT collation for ('foo' COLLATE "de_DE");
         (See <xref linkend="runtime-config-logging"/> for more information),
         but will not be sent to the client regardless of
         <xref linkend="guc-client-min-messages"/>.
-        Only superusers can request to log the memory contexts.
        </para></entry>
       </row>
 

src/backend/catalog/system_functions.sql

@@ -699,6 +699,8 @@ REVOKE EXECUTE ON FUNCTION pg_ls_dir(text) FROM public;
 
 REVOKE EXECUTE ON FUNCTION pg_ls_dir(text,boolean,boolean) FROM public;
 
+REVOKE EXECUTE ON FUNCTION pg_log_backend_memory_contexts(integer) FROM PUBLIC;
+
 --
 -- We also set up some things as accessible to standard roles.
 --

src/backend/utils/adt/mcxtfuncs.c

@@ -162,10 +162,10 @@ pg_get_backend_memory_contexts(PG_FUNCTION_ARGS)
  * pg_log_backend_memory_contexts
  *		Signal a backend process to log its memory contexts.
  *
- * Only superusers are allowed to signal to log the memory contexts
+ * By default, only superusers are allowed to signal to log the memory
- * because allowing any users to issue this request at an unbounded
+ * contexts because allowing any users to issue this request at an unbounded
- * rate would cause lots of log messages and which can lead to
+ * rate would cause lots of log messages and which can lead to denial of
- * denial of service.
+ * service. Additional roles can be permitted with GRANT.
  *
  * On receipt of this signal, a backend sets the flag in the signal
  * handler, which causes the next CHECK_FOR_INTERRUPTS() to log the
@@ -177,12 +177,6 @@ pg_log_backend_memory_contexts(PG_FUNCTION_ARGS)
 	int			pid = PG_GETARG_INT32(0);
 	PGPROC	   *proc;
 
-	/* Only allow superusers to log memory contexts. */
-	if (!superuser())
-		ereport(ERROR,
-				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
-				 errmsg("must be a superuser to log memory contexts")));
-
 	proc = BackendPidGetProc(pid);
 
 	/*

src/include/catalog/catversion.h

@@ -53,6 +53,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	202109101
+#define CATALOG_VERSION_NO	202110260
 
 #endif

src/test/regress/expected/misc_functions.out

@@ -138,14 +138,43 @@ HINT:  No function matches the given name and argument types. You might need to
 --
 -- Memory contexts are logged and they are not returned to the function.
 -- Furthermore, their contents can vary depending on the timing. However,
--- we can at least verify that the code doesn't fail.
+-- we can at least verify that the code doesn't fail, and that the
+-- permissions are set properly.
 --
-SELECT * FROM pg_log_backend_memory_contexts(pg_backend_pid());
+SELECT pg_log_backend_memory_contexts(pg_backend_pid());
  pg_log_backend_memory_contexts 
 --------------------------------
  t
 (1 row)
 
+CREATE ROLE regress_log_memory;
+SELECT has_function_privilege('regress_log_memory',
+  'pg_log_backend_memory_contexts(integer)', 'EXECUTE'); -- no
+ has_function_privilege 
+------------------------
+ f
+(1 row)
+
+GRANT EXECUTE ON FUNCTION pg_log_backend_memory_contexts(integer)
+  TO regress_log_memory;
+SELECT has_function_privilege('regress_log_memory',
+  'pg_log_backend_memory_contexts(integer)', 'EXECUTE'); -- yes
+ has_function_privilege 
+------------------------
+ t
+(1 row)
+
+SET ROLE regress_log_memory;
+SELECT pg_log_backend_memory_contexts(pg_backend_pid());
+ pg_log_backend_memory_contexts 
+--------------------------------
+ t
+(1 row)
+
+RESET ROLE;
+REVOKE EXECUTE ON FUNCTION pg_log_backend_memory_contexts(integer)
+  FROM regress_log_memory;
+DROP ROLE regress_log_memory;
 --
 -- Test some built-in SRFs
 --

src/test/regress/sql/misc_functions.sql

@@ -35,9 +35,31 @@ SELECT num_nulls();
 --
 -- Memory contexts are logged and they are not returned to the function.
 -- Furthermore, their contents can vary depending on the timing. However,
--- we can at least verify that the code doesn't fail.
+-- we can at least verify that the code doesn't fail, and that the
+-- permissions are set properly.
 --
-SELECT * FROM pg_log_backend_memory_contexts(pg_backend_pid());
+
+SELECT pg_log_backend_memory_contexts(pg_backend_pid());
+
+CREATE ROLE regress_log_memory;
+
+SELECT has_function_privilege('regress_log_memory',
+  'pg_log_backend_memory_contexts(integer)', 'EXECUTE'); -- no
+
+GRANT EXECUTE ON FUNCTION pg_log_backend_memory_contexts(integer)
+  TO regress_log_memory;
+
+SELECT has_function_privilege('regress_log_memory',
+  'pg_log_backend_memory_contexts(integer)', 'EXECUTE'); -- yes
+
+SET ROLE regress_log_memory;
+SELECT pg_log_backend_memory_contexts(pg_backend_pid());
+RESET ROLE;
+
+REVOKE EXECUTE ON FUNCTION pg_log_backend_memory_contexts(integer)
+  FROM regress_log_memory;
+
+DROP ROLE regress_log_memory;
 
 --
 -- Test some built-in SRFs