Create a multiplexing structure for signals to Postgres child processes.

This patch gets us out from under the Unix limitation of two user-defined
signal types.  We already had done something similar for signals directed to
the postmaster process; this adds multiplexing for signals directed to
backends and auxiliary processes (so long as they're connected to shared
memory).

As proof of concept, replace the former usage of SIGUSR1 and SIGUSR2
for backends with use of the multiplexing mechanism.  There are still some
hard-wired definitions of SIGUSR1 and SIGUSR2 for other process types,
but getting rid of those doesn't seem interesting at the moment.

Fujii Masao

src/include/storage/procsignal.h

@@ -0,0 +1,50 @@
+/*-------------------------------------------------------------------------
+ *
+ * procsignal.h
+ *	  Routines for interprocess signalling
+ *
+ *
+ * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * $PostgreSQL: pgsql/src/include/storage/procsignal.h,v 1.1 2009/07/31 20:26:23 tgl Exp $
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef PROCSIGNAL_H
+#define PROCSIGNAL_H
+
+#include "storage/backendid.h"
+
+
+/*
+ * Reasons for signalling a Postgres child process (a backend or an auxiliary
+ * process, like bgwriter).  We can cope with concurrent signals for different
+ * reasons.  However, if the same reason is signaled multiple times in quick
+ * succession, the process is likely to observe only one notification of it.
+ * This is okay for the present uses.
+ *
+ * Also, because of race conditions, it's important that all the signals be
+ * defined so that no harm is done if a process mistakenly receives one.
+ */
+typedef enum
+{
+	PROCSIG_CATCHUP_INTERRUPT,	/* sinval catchup interrupt */
+	PROCSIG_NOTIFY_INTERRUPT,	/* listen/notify interrupt */
+
+	NUM_PROCSIGNALS				/* Must be last! */
+} ProcSignalReason;
+
+/*
+ * prototypes for functions in procsignal.c
+ */
+extern Size ProcSignalShmemSize(void);
+extern void ProcSignalShmemInit(void);
+
+extern void ProcSignalInit(int pss_idx);
+extern int  SendProcSignal(pid_t pid, ProcSignalReason reason,
+						   BackendId backendId);
+
+extern void procsignal_sigusr1_handler(SIGNAL_ARGS);
+
+#endif   /* PROCSIGNAL_H */