Re: pgsql: Validate page level checksums in base backups

On 4/10/18 5:24 PM, Tomas Vondra wrote:

I think there's a bug in sendFile(). We do check checksums on all pages
that pass this LSN check:

     /*
      * Only check pages which have not been modified since the
      * start of the base backup. Otherwise, they might have been
      * written only halfway and the checksum would not be valid.
      * However, replaying WAL would reinstate the correct page in
      * this case.
      */
     if (PageGetLSN(page) < startptr)
     {
         ...
     }

Now, imagine the page is new, i.e. all-zeroes. That means the LSN is 0/0
too, and we'll try to verify the checksum - but we actually do not set
checksums on empty pages.

So I think it should be something like this:

     if ((!PageIsNew(page)) && (PageGetLSN(page) < startptr))
     {
         ...
     }

It might be worth verifying that the page is actually all-zeroes (and
not just with corrupted pd_upper value. Not sure it's worth it.

I've found this by fairly trivial stress testing - running pgbench and
pg_basebackup in a loop. It was failing pretty reliably (~75% of runs).
With the proposed change I see no further failures.

Good catch, Tomas!

I should have seen this since I had the same issue when I implemented this feature in pgBackRest.

Anyway, I agree that your fix looks correct.