GUC assign hooks that look at external state in deciding whether a

setting is valid must ignore that state and permit the assignment anyway
when source is PGC_S_OVERRIDE.  Otherwise they may disallow a rollback
at transaction abort, which is The Wrong Thing.  Per example from
Michael Fuhr 12-Sep-04.

src/backend/commands/variable.c

@@ -9,7 +9,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/variable.c,v 1.103 2004/08/31 19:28:51 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/variable.c,v 1.104 2004/09/24 19:42:58 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -481,7 +481,8 @@ assign_XactIsoLevel(const char *value, bool doit, GucSource source)
 			ereport(ERROR,
 					(errcode(ERRCODE_ACTIVE_SQL_TRANSACTION),
 					 errmsg("SET TRANSACTION ISOLATION LEVEL must be called before any query")));
-		else
+		/* source == PGC_S_OVERRIDE means do it anyway, eg at xact abort */
+		else if (source != PGC_S_OVERRIDE)
 			return NULL;
 	}
 	if (IsSubTransaction())
@@ -490,7 +491,8 @@ assign_XactIsoLevel(const char *value, bool doit, GucSource source)
 			ereport(ERROR,
 					(errcode(ERRCODE_ACTIVE_SQL_TRANSACTION),
 					 errmsg("SET TRANSACTION ISOLATION LEVEL must not be called in a subtransaction")));
-		else
+		/* source == PGC_S_OVERRIDE means do it anyway, eg at xact abort */
+		else if (source != PGC_S_OVERRIDE)
 			return NULL;
 	}
 

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 1.238 2004/08/31 22:43:58 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/misc/guc.c,v 1.239 2004/09/24 19:43:03 tgl Exp $
  *
  *--------------------------------------------------------------------
  */
@@ -5532,7 +5532,8 @@ assign_stage_log_stats(bool newval, bool doit, GucSource source)
 			ereport(ERROR,
 					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 					 errmsg("cannot enable parameter when \"log_statement_stats\" is true")));
-		else
+		/* source == PGC_S_OVERRIDE means do it anyway, eg at xact abort */
+		else if (source != PGC_S_OVERRIDE)
 			return false;
 	}
 	return true;
@@ -5550,7 +5551,8 @@ assign_log_stats(bool newval, bool doit, GucSource source)
 					 errmsg("cannot enable \"log_statement_stats\" when "
 							"\"log_parser_stats\", \"log_planner_stats\", "
 							"or \"log_executor_stats\" is true")));
-		else
+		/* source == PGC_S_OVERRIDE means do it anyway, eg at xact abort */
+		else if (source != PGC_S_OVERRIDE)
 			return false;
 	}
 	return true;
@@ -5566,7 +5568,9 @@ assign_transaction_read_only(bool newval, bool doit, GucSource source)
 			ereport(ERROR,
 					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 					 errmsg("cannot set transaction read-write mode inside a read-only transaction")));
-		return false;
+		/* source == PGC_S_OVERRIDE means do it anyway, eg at xact abort */
+		else if (source != PGC_S_OVERRIDE)
+			return false;
 	}
 	return true;
 }