Re: BUG #16038: Alter table - SegFault

Hi,

On 2019-10-04 11:43:52 -0300, Alvaro Herrera wrote:
> On 2019-Oct-04, PG Bug reporting form wrote:
> 
> > alter table test.testa
> > add column idb numeric(10,0) NOT NULL DEFAULT nextval('test.sq_testb'),
> > add column fk_tmpb varchar(20);
> > 
> > server process (PID 21884) was terminated by signal 11: Segmentation fault
> > - empty table - alter goes ok , split alter into 2 add col - works too
> 
> Hmm, confirmed, here's the stack trace:

The trailing attributes in the new slot in AtRewriteTable() aren't
necessarily set to isnull=true. So to trigger the problem, one needs a
rewrite triggered by *another* column (otherwise we'll not hit this
path), combined with a new column that doesn't have a default value.  I
think that's probably my bug.

This seems to fix the problem:

diff --git i/src/backend/commands/tablecmds.c w/src/backend/commands/tablecmds.c
index 05593f33162..6f72b08a87d 100644
--- i/src/backend/commands/tablecmds.c
+++ w/src/backend/commands/tablecmds.c
@@ -4890,6 +4890,14 @@ ATRewriteTable(AlteredTableInfo *tab, Oid OIDNewHeap, LOCKMODE lockmode)
                                                table_slot_callbacks(oldrel));
             newslot = MakeSingleTupleTableSlot(newTupDesc,
                                                table_slot_callbacks(newrel));
+
+            /*
+             * Set all columns in the new slot to NULL initially, to ensure
+             * columns added as part of the rewrite are initialized to
+             * NULL. That's necessary as tab->newvals will not contain an
+             * expression for columns with a NULL default.
+             */
+            ExecStoreAllNullTuple(newslot);
         }
         else
         {


Greetings,

Andres Freund