From d1be38cc7d523a3ff1867cbb48ea75a270a44e72 Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Thu, 8 Feb 2007 18:37:43 +0000
Subject: [PATCH] Fix an ancient logic error in plpgsql's exec_stmt_block: it
 thought it could get away with not (re)initializing a local variable if the
 variable is marked "isconst" and not "isnull".  Unfortunately it makes this
 decision after having already freed the old value, meaning that something
 like

   for i in 1..10 loop
     declare c constant text := 'hi there';

leads to subsequent accesses to freed memory, and hence probably crashes.
(In particular, this is why Asif Ali Rehman's bug leads to crash and not
just an unexpectedly-NULL value for SQLERRM: SQLERRM is marked CONSTANT
and so triggers this error.)

The whole thing seems wrong on its face anyway: CONSTANT means that you can't
change the variable inside the block, not that the initializer expression is
guaranteed not to change value across successive block entries.  Hence,
remove the "optimization" instead of trying to fix it.
---
 src/pl/plpgsql/src/pl_exec.c | 74 ++++++++++++++++++++----------------
 1 file changed, 41 insertions(+), 33 deletions(-)

diff --git a/src/pl/plpgsql/src/pl_exec.c b/src/pl/plpgsql/src/pl_exec.c
index efce8004b93..3a381e16b6f 100644
--- a/src/pl/plpgsql/src/pl_exec.c
+++ b/src/pl/plpgsql/src/pl_exec.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/pl/plpgsql/src/pl_exec.c,v 1.180.2.3 2007/02/01 19:23:00 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/pl/plpgsql/src/pl_exec.c,v 1.180.2.4 2007/02/08 18:37:43 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -856,43 +856,43 @@ exec_stmt_block(PLpgSQL_execstate *estate, PLpgSQL_stmt_block *block)
 				{
 					PLpgSQL_var *var = (PLpgSQL_var *) (estate->datums[n]);
 
+					/* free any old value, in case re-entering block */
 					free_var(var);
-					if (!var->isconst || var->isnull)
-					{
-						if (var->default_val == NULL)
-						{
-							/* Initially it contains a NULL */
-							var->value = (Datum) 0;
-							var->isnull = true;
-							/*
-							 * If needed, give the datatype a chance to reject
-							 * NULLs, by assigning a NULL to the variable.
-							 * We claim the value is of type UNKNOWN, not the
-							 * var's datatype, else coercion will be skipped.
-							 * (Do this before the notnull check to be
-							 * consistent with exec_assign_value.)
-							 */
-							if (!var->datatype->typinput.fn_strict)
-							{
-								bool	valIsNull = true;
 
-								exec_assign_value(estate,
-												  (PLpgSQL_datum *) var,
-												  (Datum) 0,
-												  UNKNOWNOID,
-												  &valIsNull);
-							}
-							if (var->notnull)
-								ereport(ERROR,
+					/* Initially it contains a NULL */
+					var->value = (Datum) 0;
+					var->isnull = true;
+
+					if (var->default_val == NULL)
+					{
+						/*
+						 * If needed, give the datatype a chance to reject
+						 * NULLs, by assigning a NULL to the variable.
+						 * We claim the value is of type UNKNOWN, not the
+						 * var's datatype, else coercion will be skipped.
+						 * (Do this before the notnull check to be
+						 * consistent with exec_assign_value.)
+						 */
+						if (!var->datatype->typinput.fn_strict)
+						{
+							bool	valIsNull = true;
+
+							exec_assign_value(estate,
+											  (PLpgSQL_datum *) var,
+											  (Datum) 0,
+											  UNKNOWNOID,
+											  &valIsNull);
+						}
+						if (var->notnull)
+							ereport(ERROR,
 									(errcode(ERRCODE_NULL_VALUE_NOT_ALLOWED),
 									 errmsg("variable \"%s\" declared NOT NULL cannot default to NULL",
 											var->refname)));
-						}
-						else
-						{
-							exec_assign_expr(estate, (PLpgSQL_datum *) var,
-											 var->default_val);
-						}
+					}
+					else
+					{
+						exec_assign_expr(estate, (PLpgSQL_datum *) var,
+										 var->default_val);
 					}
 				}
 				break;
@@ -1026,7 +1026,9 @@ exec_stmt_block(PLpgSQL_execstate *estate, PLpgSQL_stmt_block *block)
 					rc = exec_stmts(estate, exception->action);
 
 					free_var(state_var);
+					state_var->value = (Datum) 0;
 					free_var(errm_var);
+					errm_var->value = (Datum) 0;
 					break;
 				}
 			}
@@ -4812,6 +4814,12 @@ plpgsql_subxact_cb(SubXactEvent event, SubTransactionId mySubid,
 	}
 }
 
+/*
+ * free_var --- pfree any pass-by-reference value of the variable.
+ *
+ * This should always be followed by some assignment to var->value,
+ * as it leaves a dangling pointer.
+ */
 static void
 free_var(PLpgSQL_var *var)
 {