Change the built-in default socket directory to be /var/run/postgresql.
For backwards compatibility with (probably non-libpq-based) clients that
might still expect to find the socket in /tmp, also create a socket in
/tmp.  This is to resolve communication problems with clients operating
under systemd's PrivateTmp environment, which won't be using the same
global /tmp directory as the server; see bug #825448.

Note that we apply the socket directory change at the level of the
hard-wired defaults in the C code, not by just twiddling the setting in
postgresql.conf.sample; this is so that the change will take effect on
server package update, without requiring any existing postgresql.conf
to be updated.  (Of course, a user who dislikes this behavior can still
override it via postgresql.conf.)

diff -ur postgresql-16.1/src/backend/utils/misc/guc_tables.c pg_patched/src/backend/utils/misc/guc_tables.c
--- postgresql-16.1/src/backend/utils/misc/guc_tables.c	2023-11-06 23:04:27.000000000 +0100
+++ pg_patched/src/backend/utils/misc/guc_tables.c	2023-12-05 12:02:46.000000000 +0100
@@ -4271,7 +4271,7 @@
 			GUC_LIST_INPUT | GUC_LIST_QUOTE | GUC_SUPERUSER_ONLY
 		},
 		&Unix_socket_directories,
-		DEFAULT_PGSOCKET_DIR,
+		DEFAULT_PGSOCKET_DIR ", /tmp",
 		NULL, NULL, NULL
 	},

diff -ur postgresql-16.1/src/bin/initdb/initdb.c pg_patched/src/bin/initdb/initdb.c
--- postgresql-16.1/src/bin/initdb/initdb.c	2023-11-06 23:04:27.000000000 +0100
+++ pg_patched/src/bin/initdb/initdb.c	2023-12-05 12:04:19.000000000 +0100
@@ -1287,7 +1287,7 @@
 	 * these are indeed defaults, keep the postgresql.conf lines commented.
 	 */
 	conflines = replace_guc_value(conflines, "unix_socket_directories",
-								  DEFAULT_PGSOCKET_DIR, true);
+								  DEFAULT_PGSOCKET_DIR ", /tmp", true);

 	conflines = replace_guc_value(conflines, "port",
 								  DEF_PGPORT_STR, true);
diff -ur postgresql-16.1/src/include/pg_config_manual.h pg_patched/src/include/pg_config_manual.h
--- postgresql-16.1/src/include/pg_config_manual.h	2023-11-06 23:04:27.000000000 +0100
+++ pg_patched/src/include/pg_config_manual.h	2023-12-05 11:45:20.000000000 +0100
@@ -206,7 +206,7 @@
  * support them yet.
  */
 #ifndef WIN32
-#define DEFAULT_PGSOCKET_DIR  "/tmp"
+#define DEFAULT_PGSOCKET_DIR  "/var/run/postgresql"
 #else
 #define DEFAULT_PGSOCKET_DIR ""
 #endif