Add new GUC, max_worker_processes, limiting number of bgworkers.

In 9.3, there's no particular limit on the number of bgworkers;
instead, we just count up the number that are actually registered,
and use that to set MaxBackends.  However, that approach causes
problems for Hot Standby, which needs both MaxBackends and the
size of the lock table to be the same on the standby as on the
master, yet it may not be desirable to run the same bgworkers in
both places.  9.3 handles that by failing to notice the problem,
which will probably work fine in nearly all cases anyway, but is
not theoretically sound.

A further problem with simply counting the number of registered
workers is that new workers can't be registered without a
postmaster restart.  This is inconvenient for administrators,
since bouncing the postmaster causes an interruption of service.
Moreover, there are a number of applications for background
processes where, by necessity, the background process must be
started on the fly (e.g. parallel query).  While this patch
doesn't actually make it possible to register new background
workers after startup time, it's a necessary prerequisite.

Patch by me.  Review by Michael Paquier.

1 files changed, 2 insertions, 1 deletions

diff --git a/src/include/access/xlog_internal.h b/src/include/access/xlog_internal.h
index ee12d1a110a..c3e173106fc 100644
--- a/src/include/access/xlog_internal.h
+++ b/src/include/access/xlog_internal.h
@@ -55,7 +55,7 @@ typedef struct BkpBlock
 /*
  * Each page of XLOG file has a header like this:
  */
-#define XLOG_PAGE_MAGIC 0xD075	/* can be used as WAL version indicator */
+#define XLOG_PAGE_MAGIC 0xD076	/* can be used as WAL version indicator */
 
 typedef struct XLogPageHeaderData
 {
@@ -205,6 +205,7 @@ typedef XLogLongPageHeaderData *XLogLongPageHeader;
 typedef struct xl_parameter_change
 {
 	int			MaxConnections;
+	int			max_worker_processes;
 	int			max_prepared_xacts;
 	int			max_locks_per_xact;
 	int			wal_level;