Re: making the backend's json parser work in frontend code

On Thu, Jan 23, 2020 at 1:22 PM Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> Yes, good point. I think my one concern is that someone might specify
> both keys in the JSON, which would be very odd.

I think that if a tool other than PostgreSQL chooses to generate a
PostreSQL backup manifest, it must take care to do it in a manner that
is compatible with what PostgreSQL would generate. If it doesn't,
well, that sucks for them, but we can't prevent other people from
writing bad code. On a very good day, we can prevent ourselves from
writing bad code.

There is in general the question of how rigorous PostgreSQL ought to
be when validating a backup manifest. The proposal on the table is to
store four (4) fields per file: name, size, last modification time,
and checksum. So a JSON object representing a file should have four
keys, say "path", "size", "mtime", and "checksum". The "checksum" key
could perhaps be optional, in case the user disables checksums, or we
could represent that case in some other way, like "checksum" => null,
"checksum" => "", or "checksum" => "NONE". There is an almost
unlimited scope for bike-shedding here, but let's leave that to one
side for the moment.

Suppose that someone asks PostgreSQL's backup manifest verification
tool to validate a backup manifest where there's an extra key. Say, in
addition to the four keys listed in the previous paragraph, there is
an additional key, "momjian". On the one hand, our backup manifest
verification tool could take this as a sign that the manifest is
invalid, and accordingly throw an error. Or, it could assume that some
third-party backup tool generated the backup manifest and that the
"momjian" field is there to track something which is of interest to
that tool but not to PostgreSQL core, in which case it should just be
ignored.

Incidentally, some research seems to suggest that the problem of
filenames which don't form a valid UTF-8 sequence cannot occur on
Windows. This blog post may be helpful in understanding the issues:

http://beets.io/blog/paths.html

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