Re: backup manifests

On Mon, Mar 30, 2020 at 7:24 PM David Steele <david(at)pgmasters(dot)net> wrote:
> > I'm confused as to why you're not seeing that. What's the exact
> > sequence of steps?
>
> $ pg_basebackup -D test/backup5 --manifest-checksums=SHA256
>
> $ vi test/backup5/backup_manifest
> * Add 'X' to the checksum of backup_label
>
> $ pg_validatebackup test/backup5
> pg_validatebackup: fatal: invalid checksum for file "backup_label":
> "a98e9164fd59d498d14cfdf19c67d1c2208a30e7b939d1b4a09f524c7adfc11fX"
>
> No mention of the manifest checksum being invalid. But if I remove the
> backup label file from the manifest:
>
> pg_validatebackup: fatal: manifest checksum mismatch

Oh, I see what's happening now. If the checksum is not an even-length
string of hexademical characters, it's treated as a syntax error, so
it bails out at that point. Generally, a syntax error in the manifest
file is treated as a fatal error, and you just die right there. You'd
get the same behavior if you had malformed JSON, like a stray { or }
or [ or ] someplace that it doesn't belong according to the rules of
JSON. On the other hand, if you corrupt the checksum by adding AA or
EE or 54 or some other even-length string of hex characters, then you
have (in this code's view) a semantic error rather than a syntax
error, so it will finish loading all the manifest data and then bail
because the checksum doesn't match.

We really can't avoid bailing out early sometimes, because if the file
is totally malformed at the JSON level, there's just no way to
continue. We could cause this particular error to get treated as a
semantic error rather than a syntax error, but I don't really see much
advantage in so doing. This way was easier to code, and I don't think
it really matters which error we find first.

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