BUG #16038: Alter table - SegFault

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BUG #16038: Alter table - SegFault

PG Bug reporting form
The following bug has been logged on the website:

Bug reference:      16038
Logged by:          death lock
Email address:      [hidden email]
PostgreSQL version: 12.0
Operating system:   WinXP (VisualStudio 2013) , Debian11 gcc9
Description:        

create schema if not exists test ;
drop table test.testa;
CREATE TABLE test.testa (
    code character varying(2) NOT NULL,
    namez character varying(255) NOT NULL,
    descr character varying(1024),
    wersja numeric(1,0),
    modt timestamp without time zone,
    crtt timestamp without time zone,
    crt integer,
    mod integer,
    -- ida numeric(10,0) DEFAULT nextval(('test.sq_testa'::text)::regclass)
NOT NULL,
    -- fk_tmp character varying(20),
    CONSTRAINT test_pk PRIMARY KEY (code)
);

INSERT INTO test.testa VALUES ('0', 'AA', NULL, NULL, '2018-02-19
09:40:38.776875', '2008-05-20 10:06:28.171', 1001, 1001 /*,  1, 'DUMMYA'
*/);
-- INSERT INTO test.testa VALUES ('1', 'BB', NULL, NULL, '2018-02-19
09:40:38.776875', '2008-05-20 10:06:28.171', 1001, 1001 /*,  2, 'DUMMYB'
*/);

drop sequence if exists test.sq_testb;
create SEQUENCE test.sq_testb;

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

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Re: BUG #16038: Alter table - SegFault

Alvaro Herrera-9
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:

#0  0x0000561459590e9f in heap_compute_data_size (tupleDesc=tupleDesc@entry=0x7faa37b16618,
    values=values@entry=0x56145b26ba30, isnull=isnull@entry=0x56145b26ba80)
    at /pgsql/source/REL_12_STABLE/src/backend/access/common/heaptuple.c:138
#1  0x000056145959232b in heap_form_tuple (tupleDescriptor=0x7faa37b16618, values=0x56145b26ba30,
    isnull=0x56145b26ba80) at /pgsql/source/REL_12_STABLE/src/backend/access/common/heaptuple.c:1061
#2  0x000056145975f881 in tts_buffer_heap_materialize (slot=0x56145b26b9c0)
    at /pgsql/source/REL_12_STABLE/src/backend/executor/execTuples.c:713
#3  0x0000561459761083 in ExecFetchSlotHeapTuple (slot=0x56145b26b9c0, materialize=<optimized out>,
    shouldFree=0x7ffffc491bbf) at /pgsql/source/REL_12_STABLE/src/backend/executor/execTuples.c:1618
#4  0x00005614595e09c4 in heapam_tuple_insert (relation=0x7faa37b159e0, slot=0x56145b26b9c0, cid=8,
    options=2, bistate=0x56145b26bd68)
    at /pgsql/source/REL_12_STABLE/src/backend/access/heap/heapam_handler.c:250
#5  0x0000561459715614 in table_tuple_insert (bistate=0x56145b26bd68, options=2, cid=8,
    slot=0x56145b26b9c0, rel=0x7faa37b159e0)
    at /pgsql/source/REL_12_STABLE/src/include/access/tableam.h:1126
#6  ATRewriteTable (tab=tab@entry=0x56145b25b5d8, OIDNewHeap=OIDNewHeap@entry=16396,
    lockmode=lockmode@entry=8) at /pgsql/source/REL_12_STABLE/src/backend/commands/tablecmds.c:5039
#7  0x000056145972a9b8 in ATRewriteTables (lockmode=8, wqueue=0x7ffffc491d08,
    parsetree=0x56145b24d7c0) at /pgsql/source/REL_12_STABLE/src/backend/commands/tablecmds.c:4641
#8  ATController (parsetree=parsetree@entry=0x56145b24d7c0, rel=<optimized out>,
    cmds=<optimized out>, recurse=<optimized out>, lockmode=lockmode@entry=8)
    at /pgsql/source/REL_12_STABLE/src/backend/commands/tablecmds.c:3848

--
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services


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Re: BUG #16038: Alter table - SegFault

Andres Freund
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


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Re: BUG #16038: Alter table - SegFault

Andres Freund
Hi,

On 2019-10-04 08:26:24 -0700, Andres Freund wrote:
> 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

Thanks for the bug report!  I've pushed a fix, which will be included in
the next minor release. If you need a workaround until then, you can
avoid the crash by only adding one column add a time.



> > 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);
>          }

Pushed, after adding some tests.

Greetings,

Andres Freund