Re: START_REPLICATION SLOT causing a crash in an assert build

Thanks!

At Mon, 26 Sep 2022 19:53:02 -0700, Andres Freund <andres@anarazel.de> wrote in 
> I wonder if the correct fix here wouldn't be to move the slotname out of
> PgStat_StatReplSlotEntry?

Ugh. Right. I thought its outer struct as purely the part for the
common header. But we can freely place anything after the header
part. I moved it to the outer struct. I didn't clear that part in
pgstat_create_relation() because it is filled in immediately.

The attached is that.

> On 2022-09-16 14:37:17 +0900, Kyotaro Horiguchi wrote:
...
> > @@ -307,6 +313,10 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
> >          .shared_size = sizeof(PgStatShared_ReplSlot),
> >          .shared_data_off = offsetof(PgStatShared_ReplSlot, stats),
> >          .shared_data_len = sizeof(((PgStatShared_ReplSlot *) 0)->stats),
> > +        /* reset doesn't wipe off slot name */
> > +        .reset_off = offsetof(PgStat_StatReplSlotEntry, spill_txns),
> > +        .reset_len = sizeof(((PgStatShared_ReplSlot *) 0)->stats),
> > +        offsetof(PgStat_StatReplSlotEntry, spill_txns),
> 
> I'm confused what this offsetof does here? It's not even assigned to a
> specific field? Am I missing something?
> 
> Also, wouldn't we need to subtract something of the size?

Yeah, I felt it confusing. The last line above is offset from just
after the header part (it is PgStat_, not PgStatShared_).  I first
wrote that as you suggested but rewrote to shorter representation.

> 
> > diff --git a/src/backend/utils/activity/pgstat_shmem.c b/src/backend/utils/activity/pgstat_shmem.c
> > index ac98918688..09a8c3873c 100644
> > --- a/src/backend/utils/activity/pgstat_shmem.c
> > +++ b/src/backend/utils/activity/pgstat_shmem.c
> > @@ -915,8 +915,9 @@ shared_stat_reset_contents(PgStat_Kind kind, PgStatShared_Common *header,
> >  {
> >      const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
> >  
> > -    memset(pgstat_get_entry_data(kind, header), 0,
> > -           pgstat_get_entry_len(kind));
> > +    memset((char *)pgstat_get_entry_data(kind, header) +
> > +           kind_info->reset_off, 0,
> > +           kind_info->reset_len);
> >  
> >      if (kind_info->reset_timestamp_cb)
> >          kind_info->reset_timestamp_cb(header, ts);
> 
> This likely doesn't quite conform to what pgindent wants...

In the first place, it's ugly...

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center