Serialize Undefined values to JSON for rpc requests

[drewbanin]

resolves #3464

Description

The rpc server uses a custom JSON encoder that doesn't know how to encode Undefined variables. As a result, the rpc server returns a 500 error w/ a RecursionError if an rpc response contains an Undefined object. See #3464 (comment) for a full diagnosis of the issue.

This PR addresses the issue by teaching the dbt JSON Encoder how to serialize an Undefined object to JSON - specifically by serializing it out as an empty string.

Note: need to update the changelog on this one still...

Checklist

• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have updated the CHANGELOG.md and added information about my change to the "dbt next" section.

[drewbanin]

is 0.21.0 the right place for this in the changelog? And more generally... how would I know which release a given commit merged to develop is going out in?

[jtcohen6]

is 0.21.0 the right place for this in the changelog? And more generally... how would I know which release a given commit merged to develop is going out in?

Yes, 0.21.0 is good. Since we've cut the first v0.21 prerelease (0.21.0-b1), thereby creating the 0.21.latest branch, you'll want to squash-merge into develop and then cherry-pick the squashed commit onto 0.21.latest.

Generally, you put your changelog entry under the release where that PR's changes are going out. If your change were for v0.20.1 instead of v0.21, you'd add a changelog entry under v0.20.1, and then also backport to 0.20.latest.

@leahwicz Does that sound right to you?

[leahwicz]

Yes that is correct

[drewbanin]

Yes, 0.21.0 is good. Since we've cut the first v0.21 prerelease (0.21.0-b1), thereby creating the 0.21.latest branch, you'll want to squash-merge into develop and then cherry-pick the squashed commit onto 0.21.latest.

Huh - is that what we ask external contributors to do too? That feels... onerous... to me!

[drewbanin]

Sorry - I hope I'm not being dense here - I just want to make sure I'm doing the right thing. I should squash+merge this into develop, then open a new PR to cherry pick the commit into 0.21.latest separately?

[jtcohen6]

Huh - is that what we ask external contributors to do too? That feels... onerous... to me!

We ask all external contributors to open PRs targeting develop. We determine which upcoming release the change will be going into, and we ask the external contributor to add a changelog entry under that release. When the PR is ready, we take care of squash-merging into develop and opening a back-port PR to the appropriate x.y.latest branch.

I should squash+merge this into develop, then open a new PR to cherry pick the commit into 0.21.latest separately?

That's right! I'm also happy to merge this PR and take it from there, let me know :)

[drewbanin]

@jtcohen6 if you don't mind helping me out with that, it would be supremely helpful!