Look up backend type in pg_signal_backend() more cheaply.

Commit ccd38024bc, which introduced the pg_signal_autovacuum_worker
role, added a call to pgstat_get_beentry_by_proc_number() for the
purpose of determining whether the process is an autovacuum worker.
This function calls pgstat_read_current_status(), which can be
fairly expensive and may return cached, out-of-date information.
Since we just need to look up the target backend's BackendType, and
we already know its ProcNumber, we can instead inspect the
BackendStatusArray directly, which is much less expensive and
possibly more up-to-date.  There are some caveats with this
approach (which are documented in the code), but it's still
substantially better than before.

Reported-by: Andres Freund
Reviewed-by: Andres Freund
Discussion: https://postgr.es/m/ujenaa2uabzfkwxwmfifawzdozh3ljr7geozlhftsuosgm7n7q%40g3utqqyyosb6

src/backend/storage/ipc/signalfuncs.c

@@ -88,9 +88,9 @@ pg_signal_backend(int pid, int sig)
 	if (!OidIsValid(proc->roleId) || superuser_arg(proc->roleId))
 	{
 		ProcNumber	procNumber = GetNumberFromPGProc(proc);
-		PgBackendStatus *procStatus = pgstat_get_beentry_by_proc_number(procNumber);
+		BackendType backendType = pgstat_get_backend_type_by_proc_number(procNumber);
 
-		if (procStatus && procStatus->st_backendType == B_AUTOVAC_WORKER)
+		if (backendType == B_AUTOVAC_WORKER)
 		{
 			if (!has_privs_of_role(GetUserId(), ROLE_PG_SIGNAL_AUTOVACUUM_WORKER))
 				return SIGNAL_BACKEND_NOAUTOVAC;

src/backend/utils/activity/backend_status.c

@@ -1036,6 +1036,31 @@ pgstat_get_my_query_id(void)
 	return MyBEEntry->st_query_id;
 }
 
+/* ----------
+ * pgstat_get_backend_type_by_proc_number() -
+ *
+ *	Return the type of the backend with the specified ProcNumber.  This looks
+ *	directly at the BackendStatusArray, so the return value may be out of date.
+ *	The only current use of this function is in pg_signal_backend(), which is
+ *	inherently racy, so we don't worry too much about this.
+ *
+ *	It is the caller's responsibility to use this wisely; at minimum, callers
+ *	should ensure that procNumber is valid and perform the required permissions
+ *	checks.
+ * ----------
+ */
+BackendType
+pgstat_get_backend_type_by_proc_number(ProcNumber procNumber)
+{
+	volatile PgBackendStatus *status = &BackendStatusArray[procNumber];
+
+	/*
+	 * We bypass the changecount mechanism since fetching and storing an int
+	 * is almost certainly atomic.
+	 */
+	return status->st_backendType;
+}
+
 /* ----------
  * cmp_lbestatus
  *

src/include/utils/backend_status.h

@@ -323,6 +323,7 @@ extern const char *pgstat_get_backend_current_activity(int pid, bool checkUser);
 extern const char *pgstat_get_crashed_backend_activity(int pid, char *buffer,
 													   int buflen);
 extern uint64 pgstat_get_my_query_id(void);
+extern BackendType pgstat_get_backend_type_by_proc_number(ProcNumber procNumber);
 
 
 /* ----------