Reverse scan fail to find key when range spans multi region

[yzhang43]

[ti-chi-bot]

Welcome @yzhang43! It looks like this is your first issue to tikv/client-rust 🎉

[pingyu]

Please provide details for the issue.

E.g.

• What's the version (git hash) are using.
• What "fail" it is, backtrace, error logs, or anything else.
• In what condition the "fail" produced. How to reproduce the issue.

[yzhang43]

Running at 54487d0.
The failure is key_not_in_region when doing reverse_scan in raw kv. As long as the region for reverse scan is cross-region, this will show up.

What i think is happening when we do scan_reverse for [lower_bound, upper_bound] range

1. region_for_key selects the region based on lower_bound https://github.com/tikv/client-rust/blob/master/src/raw/client.rs#L811
2. raw scan request is constructed with the start as upper_bound and end as lower_bound https://github.com/tikv/client-rust/blob/master/src/raw/client.rs#L813
3. do_store_scan throws region not found error because the region returned in 1 is for lower_bound and it is try to scan from upper_bound https://github.com/tikv/client-rust/blob/master/src/raw/client.rs#L820

[pingyu]

You are right. We should scan regions from end_key and then go backward.