Making git-reader return 206 for older expected than since values

[alexcottner]

Git-reader should return a 206 instead of a 400 when clients request data with an older _expected value than the _since value.
This will mirror the existing v1 behavior, which is what clients are expecting.

Client-side code (link) in remote-settings.sys.mjs is expecting a 400 to mean the etag should be cleared so it can resync. But this is a behavior change from v1.

[leplatrem]

This will mirror the existing v1 behavior,

I can't recall the reason why we did use 206 in the first place. Wasn't it because of GCP caching limitation on 400?

This will mirror the existing v1 behavior

206 Partial Content seems like a hack. If we can stick with 400 I think it matches more what we want for a future-proof v2

which is what clients are expecting.

In what sense the client expects 206?

Couldn't we switch v1 to 400 instead?

If we really have no choice, then yeah let's do this :)

[alexcottner]

Added a comment to the jira ticket (link) but the evidence seems pretty strong here.

[alexcottner]

I can't recall the reason why we did use 206 in the first place. Wasn't it because of GCP caching limitation on 400?

I think the GCP limitation was the big reason before. But I don't think we were returning 400 responses for long enough to encounter this secondary problem.

In what sense the client expects 206?

I think the main thing here is if we return a 206, the clients don't do anything and keep working with their current data. And will catch up on the next sync. But if we return a 400, the etag value is cleared and they could go backwards.