Include the backend ID in the relpath of temporary relations.

This allows us to reliably remove all leftover temporary relation
files on cluster startup without reference to system catalogs or WAL;
therefore, we no longer include temporary relations in XLOG_XACT_COMMIT
and XLOG_XACT_ABORT WAL records.

Since these changes require including a backend ID in each
SharedInvalSmgrMsg, the size of the SharedInvalidationMessage.id
field has been reduced from two bytes to one, and the maximum number
of connections has been reduced from INT_MAX / 4 to 2^23-1.  It would
be possible to remove these restrictions by increasing the size of
SharedInvalidationMessage by 4 bytes, but right now that doesn't seem
like a good trade-off.

Review by Jaime Casanova and Tom Lane.

1 files changed, 22 insertions, 18 deletions

diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 97ed5b72474..dac704ee4cd 100644
--- a/src/backend/utils/misc/guc.c
+++ b/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 1.566 2010/08/06 14:51:33 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/misc/guc.c,v 1.567 2010/08/13 20:10:53 rhaas Exp $
  *
  *--------------------------------------------------------------------
  */
@@ -96,6 +96,16 @@
 #define MAX_KILOBYTES	(INT_MAX / 1024)
 #endif
 
+/*
+ * Note: MAX_BACKENDS is limited to 2^23-1 because inval.c stores the
+ * backend ID as a 3-byte signed integer.  Even if that limitation were
+ * removed, we still could not exceed INT_MAX/4 because some places compute
+ * 4*MaxBackends without any overflow check.  This is rechecked in
+ * assign_maxconnections, since MaxBackends is computed as MaxConnections
+ * plus autovacuum_max_workers plus one (for the autovacuum launcher).
+ */
+#define MAX_BACKENDS	0x7fffff
+
 #define KB_PER_MB (1024)
 #define KB_PER_GB (1024*1024)
 
@@ -1414,23 +1424,13 @@ static struct config_int ConfigureNamesInt[] =
 		30 * 1000, -1, INT_MAX / 1000, NULL, NULL
 	},
 
-	/*
-	 * Note: MaxBackends is limited to INT_MAX/4 because some places compute
-	 * 4*MaxBackends without any overflow check.  This check is made in
-	 * assign_maxconnections, since MaxBackends is computed as MaxConnections
-	 * plus autovacuum_max_workers plus one (for the autovacuum launcher).
-	 *
-	 * Likewise we have to limit NBuffers to INT_MAX/2.
-	 *
-	 * See also CheckRequiredParameterValues() if this parameter changes
-	 */
 	{
 		{"max_connections", PGC_POSTMASTER, CONN_AUTH_SETTINGS,
 			gettext_noop("Sets the maximum number of concurrent connections."),
 			NULL
 		},
 		&MaxConnections,
-		100, 1, INT_MAX / 4, assign_maxconnections, NULL
+		100, 1, MAX_BACKENDS, assign_maxconnections, NULL
 	},
 
 	{
@@ -1439,9 +1439,13 @@ static struct config_int ConfigureNamesInt[] =
 			NULL
 		},
 		&ReservedBackends,
-		3, 0, INT_MAX / 4, NULL, NULL
+		3, 0, MAX_BACKENDS, NULL, NULL
 	},
 
+	/*
+	 * We sometimes multiply the number of shared buffers by two without
+	 * checking for overflow, so we mustn't allow more than INT_MAX / 2.
+	 */
 	{
 		{"shared_buffers", PGC_POSTMASTER, RESOURCES_MEM,
 			gettext_noop("Sets the number of shared memory buffers used by the server."),
@@ -1618,7 +1622,7 @@ static struct config_int ConfigureNamesInt[] =
 			NULL
 		},
 		&max_prepared_xacts,
-		0, 0, INT_MAX / 4, NULL, NULL
+		0, 0, MAX_BACKENDS, NULL, NULL
 	},
 
 #ifdef LOCK_DEBUG
@@ -1782,7 +1786,7 @@ static struct config_int ConfigureNamesInt[] =
 			NULL
 		},
 		&max_wal_senders,
-		0, 0, INT_MAX / 4, NULL, NULL
+		0, 0, MAX_BACKENDS, NULL, NULL
 	},
 
 	{
@@ -2022,7 +2026,7 @@ static struct config_int ConfigureNamesInt[] =
 			NULL
 		},
 		&autovacuum_max_workers,
-		3, 1, INT_MAX / 4, assign_autovacuum_max_workers, NULL
+		3, 1, MAX_BACKENDS, assign_autovacuum_max_workers, NULL
 	},
 
 	{
@@ -7995,7 +7999,7 @@ show_tcp_keepalives_count(void)
 static bool
 assign_maxconnections(int newval, bool doit, GucSource source)
 {
-	if (newval + autovacuum_max_workers + 1 > INT_MAX / 4)
+	if (newval + autovacuum_max_workers + 1 > MAX_BACKENDS)
 		return false;
 
 	if (doit)
@@ -8007,7 +8011,7 @@ assign_maxconnections(int newval, bool doit, GucSource source)
 static bool
 assign_autovacuum_max_workers(int newval, bool doit, GucSource source)
 {
-	if (MaxConnections + newval + 1 > INT_MAX / 4)
+	if (MaxConnections + newval + 1 > MAX_BACKENDS)
 		return false;
 
 	if (doit)