Re: Inconsistency in determining the timestamp of the db statfile. - Mailing list pgsql-hackers

On Tue, Sep 8, 2020 at 7:03 PM Magnus Hagander <magnus@hagander.net> wrote:
>
> On Tue, Sep 8, 2020 at 3:11 PM Fujii Masao <masao.fujii@oss.nttdata.com> wrote:
>>
>>
>>
>> On 2020/09/08 19:28, Magnus Hagander wrote:
>> >
>> >
>> > On Tue, Sep 8, 2020 at 8:10 AM Amit Kapila <amit.kapila16@gmail.com <mailto:amit.kapila16@gmail.com>> wrote:
>> >
>> >     We use the timestamp of the global statfile if we are not able to
>> >     determine it for a particular database either because the entry for
>> >     that database doesn't exist or there is an error while reading the
>> >     specific database entry. This was not taken care of while reading
>> >     other entries like ArchiverStats or SLRUStats. See
>> >     pgstat_read_db_statsfile_timestamp. I have observed this while
>> >     reviewing Sawada-San's patch related to logical replication stats [1].
>> >
>> >     I think this can only happen if due to some reason the statfile got
>> >     corrupt or we
>> >     have some bug in stats writing code, the chances of both are rare and even
>> >     if that happens we will use stale statistics.
>> >
>> >     The attached patch by Sawada-San will fix this issue. As the chances of this
>> >     the problem is rare and nobody has reported any issue related to this,
>> >     so it might be okay not to backpatch this.
>> >
>> >     Thoughts?
>> >
>> >
>> > Why are we reading the archiver statis and and slru stats in "pgstat_read_db_statsfile_timestamp" in the first
place?
>>
>> Maybe because they are written before database stats entries? That is,
>> to read the target database stats entry and get the timestamp from it,
>> we need to read (or lseek) all the global stats entries written before them.
>>
>
> Oh meh. Yeah, I'm reading this thing completely wrong :/ Ignore my comments :)
>
>
>
>> > That seems well out of scope for that function.
>> >
>> > If nothing else the comment at the top of that function is out of touch with reality. We do seem to read it into
localbuffers and then ignore the contents. I guess we're doing it just to verify that it's not corrupt -- so perhaps
thefunction should actually have a different name than it does now, since it certainly seems to do more than actually
readthe statsfile timestamp. 
>> >
>> > Specifically in this patch it looks wrong -- in the case of say the SLRU stats being corrupt, we will now return
thetimestamp of the global stats file even if there is one existing for the database requested, which definitely breaks
thecontract of the function. 
>>
>> Yes.
>> We should return false when fread() for database entry fails, instead? That is,
>>
>> 1. If corrupted stats file is found, the function always returns false.
>> 2. If the file is not currupted and the target database entry is found, its timestamp is returned.
>> 3. If the file is not corrupted and the target is NOT found, the timestamp of global entry is returned
>
>
> Yeah, with more coffee and re-reading it, I'm not sure how we could have the database stats being OK if the archiver
orslru stats are not. 
>
> That said, I think it still makes sense to return false if the stats file is corrupt. How much can we trust the first
block,if the block right after it is corrupt? So yeah, I think your described order seems correct. But that's also what
thecode already did before this patch, isn't it? 
>

No, before patch as well, if we can't read the DB entry say because
the file is corrupt, we return true and use timestamp of global stats
file and this is what is established by the original commit 187492b6.
So, if we consider that as correct then the functionality is something
like once we have read the timestamp of the global statfile, we use
that if we can't read the actual db entry for whatever reason. It
seems if we return false, the caller will call this function again in
the loop. Now, I see the point that if we can't read some part of the
file we should return false instead but not sure if that is helpful
from the perspective of the caller of
pgstat_read_db_statsfile_timestamp.

I have included Alvaro as he is a committer for 187492b6, so he might
remember something and let us know if this is a mistake or there is
some reason for doing so (return true even when the db entry we are
trying to read is corrupt).

--
With Regards,
Amit Kapila.