Allow bgworkers to be terminated for database-related commands

Background workers gain a new flag, called BGWORKER_INTERRUPTIBLE, that
offers the possibility to terminate the workers when these are connected
to a database that is involved in one of the following commands:
ALTER DATABASE RENAME TO
ALTER DATABASE SET TABLESPACE
CREATE DATABASE
DROP DATABASE

This is useful to give background workers the same behavior as backends
and autovacuum workers, which are stopped when these commands are
executed.  The default behavior, that exists since 9.3, is still to
never terminate bgworkers connected to the database involved in any of
these commands.  The new flag has to be set to terminate the workers.

A couple of tests are added to worker_spi to track the commands that
impact the termination of the workers.  There is a test case for a
non-interruptible worker, additionally, that relies on an injection
point to make the wait time in CountOtherDBBackends() reduced from 5s to
0.3s for faster test runs.  The tests rely on the contents of the server
logs to check if a worker has been started or terminated:
- LOG generated by worker_spi_main() at startup, once connection to
database is done.
- FATAL in bgworker_die() when terminated.
A couple of tests run in the CI have showed that this method is stable
enough.  The safe_psql() calls that scan pg_stat_activity could be
replaced with some poll_query_until() for more stability, if the current
method proves to be an issue in the buildfarm.

Author: Aya Iwata <iwata.aya@fujitsu.com>
Reviewed-by: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Ryo Matsumura <matsumura.ryo@fujitsu.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Peter Smith <smithpb2250@gmail.com>
Discussion: https://postgr.es/m/OS7PR01MB11964335F36BE41021B62EAE8EAE4A@OS7PR01MB11964.jpnprd01.prod.outlook.com

src/include/postmaster/bgworker.h

@@ -59,6 +59,13 @@
  */
 #define BGWORKER_BACKEND_DATABASE_CONNECTION		0x0002
 
+/*
+ * Exit the bgworker if its database is involved in a CREATE, ALTER or DROP
+ * database command.  It requires BGWORKER_SHMEM_ACCESS and
+ * BGWORKER_BACKEND_DATABASE_CONNECTION.
+ */
+#define BGWORKER_INTERRUPTIBLE			0x0004
+
 /*
  * This class is used internally for parallel queries, to keep track of the
  * number of active parallel workers and make sure we never launch more than
@@ -129,6 +136,9 @@ extern const char *GetBackgroundWorkerTypeByPid(pid_t pid);
 /* Terminate a bgworker */
 extern void TerminateBackgroundWorker(BackgroundWorkerHandle *handle);
 
+/* Terminate background workers connected to database */
+extern void TerminateBackgroundWorkersForDatabase(Oid databaseId);
+
 /* This is valid in a running worker */
 extern PGDLLIMPORT BackgroundWorker *MyBgworkerEntry;