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.

diff --git a/src/pl/plpgsql/src/pl_exec.c b/src/pl/plpgsql/src/pl_exec.c
index abeea96ef4d23f253d5f34fc38fd1897ac399a66..44fe284c4fdbe420a1175e8c090899ad301f1f89 100644 (file)
--- a/src/pl/plpgsql/src/pl_exec.c
+++ b/src/pl/plpgsql/src/pl_exec.c
@@ -866,29 +866,29 @@ 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 */
                                        if (var->freeval)
                                        {
                                                pfree((void *) (var->value));
                                                var->freeval = false;
                                        }
 
-                                       if (!var->isconst || var->isnull)
+                                       /* Initially it contains a NULL */
+                                       var->value = (Datum) 0;
+                                       var->isnull = true;
+
+                                       if (var->default_val == NULL)
+                                       {
+                                               if (var->notnull)
+                                                       ereport(ERROR,
+                                                                       (errcode(ERRCODE_NULL_VALUE_NOT_ALLOWED),
+                                                                        errmsg("variable \"%s\" declared NOT NULL cannot default to NULL",
+                                                                                       var->refname)));
+                                       }
+                                       else
                                        {
-                                               if (var->default_val == NULL)
-                                               {
-                                                       var->value = (Datum) 0;
-                                                       var->isnull = true;
-                                                       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);
-                                               }
+                                               exec_assign_expr(estate, (PLpgSQL_datum *) var,
+                                                                                var->default_val);
                                        }
                                }
                                break;