Dead code or buggy code?

On Wed, Sep 18, 2013 at 6:20 PM, Greg Stark <stark@mit.edu> wrote:

The following code is in the ProcSleep at proc.c:1138.
GetBlockingAutoVacuumPgproc() should presumably always return a vacuum
pgproc entry since the deadlock state says it's blocked by autovacuum.
But I'm not really familiar enough with this codepath to know whether
there's not a race condition here where it can sometimes return null.
The following code checks autovac != NULL but the PGXACT initializer
would have seg faulted if it returned NULL if that's possible.

if (deadlock_state == DS_BLOCKED_BY_AUTOVACUUM &&
allow_autovacuum_cancel)
{
PGPROC *autovac = GetBlockingAutoVacuumPgproc();
PGXACT *autovac_pgxact =
&ProcGlobal->allPgXact[autovac->pgprocno];

LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);

/*
* Only do it if the worker is not working to protect against Xid
* wraparound.
*/
if ((autovac != NULL) &&
(autovac_pgxact->vacuumFlags & PROC_IS_AUTOVACUUM) &&
!(autovac_pgxact->vacuumFlags & PROC_VACUUM_FOR_WRAPAROUND))
{

Hmm, yeah. I remember noticing this some time ago but never got
around to fixing it. +1 for rearranging things there somehow.

--
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So I'm just going to make the code defensive and assume NULL is possible
when if maybe it isn't.

In case it's not clear, this is one of the thing's Xi Wang's took picked
up. There not to many but it turns out they are indeed not all in the adt
code so I might wait until after the commit fest to commit it to avoid
causing bit churn.

--
greg
On 19 Sep 2013 12:52, "Robert Haas" <robertmhaas@gmail.com> wrote:

On Fri, Sep 20, 2013 at 12:13:18AM +0100, Greg Stark wrote:

So I'm just going to make the code defensive and assume NULL is possible when
if maybe it isn't.

In case it's not clear, this is one of the thing's Xi Wang's took picked up.
There not to many but it turns out they are indeed not all in the adt code so I
might wait until after the commit fest to commit it to avoid causing bit churn.

Uh, where are we on this?

---------------------------------------------------------------------------

--
greg

On 19 Sep 2013 12:52, "Robert Haas" <robertmhaas@gmail.com> wrote:

On Wed, Sep 18, 2013 at 6:20 PM, Greg Stark <stark@mit.edu> wrote:

The following code is in the ProcSleep at proc.c:1138.
GetBlockingAutoVacuumPgproc() should presumably always return a vacuum
pgproc entry since the deadlock state says it's blocked by autovacuum.
But I'm not really familiar enough with this codepath to know whether
there's not a race condition here where it can sometimes return null.
The following code checks autovac != NULL but the PGXACT initializer
would have seg faulted if it returned NULL if that's possible.

if (deadlock_state == DS_BLOCKED_BY_AUTOVACUUM &&
allow_autovacuum_cancel)
{
PGPROC *autovac = GetBlockingAutoVacuumPgproc();
PGXACT *autovac_pgxact =
&ProcGlobal->allPgXact[autovac->pgprocno];

LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);

/*
* Only do it if the worker is not working to protect against

Xid

* wraparound.
*/
if ((autovac != NULL) &&
(autovac_pgxact->vacuumFlags & PROC_IS_AUTOVACUUM) &&
!(autovac_pgxact->vacuumFlags &

PROC_VACUUM_FOR_WRAPAROUND))

{

Hmm, yeah. I remember noticing this some time ago but never got
around to fixing it. +1 for rearranging things there somehow.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

--
Bruce Momjian <bruce@momjian.us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ Everyone has their own god. +

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