Fix worker_spi when launching workers without shared_preload_libraries

Currently, the database name to connect is initialized only when the
module is loaded with shared_preload_libraries, causing any call of
worker_spi_launch() to fail if the library is not loaded for a dynamic
bgworker launch.  Rather than making the GUC defining the database to
connect to a PGC_POSTMASTER, this commit switches worker_spi.database to
PGC_SIGHUP, loaded even if the module's library is loaded dynamically
for a worker.

We have been discussing about the integration of more advanced tests in
this module, with and without shared_preload_libraries set, so this
eases a bit the work planned in this area.

No backpatch is done as, while this is a bug, it changes the definition
of worker_spi.database.

Author: Masahiro Ikeda
Reviewed-by: Bharath Rupireddy
Discussion: https://postgr.es/m/d30d3ea7d21cb7c9e1e3cc47e301f1b6@oss.nttdata.com

src/test/modules/worker_spi/worker_spi.c

@@ -283,6 +283,11 @@ _PG_init(void)
 	BackgroundWorker worker;
 
 	/* get the configuration */
+
+	/*
+	 * These GUCs are defined even if this library is not loaded with
+	 * shared_preload_libraries, for worker_spi_launch().
+	 */
 	DefineCustomIntVariable("worker_spi.naptime",
 							"Duration between each check (in seconds).",
 							NULL,
@@ -296,6 +301,15 @@ _PG_init(void)
 							NULL,
 							NULL);
 
+	DefineCustomStringVariable("worker_spi.database",
+							   "Database to connect to.",
+							   NULL,
+							   &worker_spi_database,
+							   "postgres",
+							   PGC_SIGHUP,
+							   0,
+							   NULL, NULL, NULL);
+
 	if (!process_shared_preload_libraries_in_progress)
 		return;
 
@@ -312,15 +326,6 @@ _PG_init(void)
 							NULL,
 							NULL);
 
-	DefineCustomStringVariable("worker_spi.database",
-							   "Database to connect to.",
-							   NULL,
-							   &worker_spi_database,
-							   "postgres",
-							   PGC_POSTMASTER,
-							   0,
-							   NULL, NULL, NULL);
-
 	MarkGUCPrefixReserved("worker_spi");
 
 	/* set up common data for all our workers */