Make stats_temp_directory PGC_SIGHUP, and document how it may cause a temporary

"outage" of the statistics views.

This requires making the stats collector respond to SIGHUP, like the other
utility processes already did.

doc/src/sgml/config.sgml

-<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.187 2008/08/22 18:47:07 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.188 2008/08/25 15:11:00 mha Exp $ -->
 
 <chapter Id="runtime-config">
   <title>Server Configuration</title>
@@ -3418,7 +3418,9 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
         path relative to the data directory or an absolute path. The default is
         <filename>pg_stat_tmp</filename>. Pointing this at a RAM based filesystem
         will decrease physical I/O requirements and can lead to increased 
-        performance. This parameter can only be set at server start.
+        performance. If this parameter is changed when the system is running,
+        the statistics functions might return no information until a new 
+        file has been written, which typically happens twice per second.
        </para>
       </listitem>
      </varlistentry>

src/backend/postmaster/pgstat.c

@@ -13,7 +13,7 @@
  *
  *	Copyright (c) 2001-2008, PostgreSQL Global Development Group
  *
- *	$PostgreSQL: pgsql/src/backend/postmaster/pgstat.c,v 1.179 2008/08/15 08:37:39 mha Exp $
+ *	$PostgreSQL: pgsql/src/backend/postmaster/pgstat.c,v 1.180 2008/08/25 15:11:00 mha Exp $
  * ----------
  */
 #include "postgres.h"
@@ -203,6 +203,7 @@ static PgStat_GlobalStats globalStats;
 
 static volatile bool need_exit = false;
 static volatile bool need_statwrite = false;
+static volatile bool got_SIGHUP = false;
 
 /*
  * Total time charged to functions so far in the current backend.
@@ -224,6 +225,7 @@ NON_EXEC_STATIC void PgstatCollectorMain(int argc, char *argv[]);
 static void pgstat_exit(SIGNAL_ARGS);
 static void force_statwrite(SIGNAL_ARGS);
 static void pgstat_beshutdown_hook(int code, Datum arg);
+static void pgstat_sighup_handler(SIGNAL_ARGS);
 
 static PgStat_StatDBEntry *pgstat_get_db_entry(Oid databaseid, bool create);
 static void pgstat_write_statsfile(bool permanent);
@@ -2571,7 +2573,7 @@ PgstatCollectorMain(int argc, char *argv[])
 	 * Ignore all signals usually bound to some action in the postmaster,
 	 * except SIGQUIT and SIGALRM.
 	 */
-	pqsignal(SIGHUP, SIG_IGN);
+	pqsignal(SIGHUP, pgstat_sighup_handler);
 	pqsignal(SIGINT, SIG_IGN);
 	pqsignal(SIGTERM, SIG_IGN);
 	pqsignal(SIGQUIT, pgstat_exit);
@@ -2634,6 +2636,15 @@ PgstatCollectorMain(int argc, char *argv[])
 		if (need_exit)
 			break;
 
+		/*
+		 * Reload configuration if we got SIGHUP from the postmaster.
+		 */
+		if (got_SIGHUP)
+		{
+			ProcessConfigFile(PGC_SIGHUP);
+			got_SIGHUP = false;
+		}
+
 		/*
 		 * If time to write the stats file, do so.	Note that the alarm
 		 * interrupt isn't re-enabled immediately, but only after we next
@@ -2834,6 +2845,13 @@ force_statwrite(SIGNAL_ARGS)
 	need_statwrite = true;
 }
 
+/* SIGHUP handler for collector process */
+static void
+pgstat_sighup_handler(SIGNAL_ARGS)
+{
+	got_SIGHUP = true;
+}
+
 
 /*
  * Lookup the hash table entry for the specified database. If no hash

src/backend/postmaster/postmaster.c

@@ -37,7 +37,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/postmaster/postmaster.c,v 1.561 2008/06/26 02:47:19 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/postmaster/postmaster.c,v 1.562 2008/08/25 15:11:01 mha Exp $
  *
  * NOTES
  *
@@ -1923,7 +1923,8 @@ SIGHUP_handler(SIGNAL_ARGS)
 			signal_child(PgArchPID, SIGHUP);
 		if (SysLoggerPID != 0)
 			signal_child(SysLoggerPID, SIGHUP);
-		/* PgStatPID does not currently need SIGHUP */
+		if (PgStatPID != 0)
+			signal_child(PgStatPID, SIGHUP);
 
 		/* Reload authentication config files too */
 		load_hba();

src/backend/utils/misc/guc.c

@@ -10,7 +10,7 @@
  * Written by Peter Eisentraut <peter_e@gmx.net>.
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/misc/guc.c,v 1.469 2008/08/22 18:47:07 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/misc/guc.c,v 1.470 2008/08/25 15:11:00 mha Exp $
  *
  *--------------------------------------------------------------------
  */
@@ -2470,7 +2470,7 @@ static struct config_string ConfigureNamesString[] =
 	},
 
 	{
-		{"stats_temp_directory", PGC_POSTMASTER, STATS_COLLECTOR,
+		{"stats_temp_directory", PGC_SIGHUP, STATS_COLLECTOR,
 			gettext_noop("Writes temporary statistics files to the specified directory."),
 			NULL,
 			GUC_SUPERUSER_ONLY