Some SwitchMode improvements

[kennytm]

What problem does this PR solve?

Fix #134 and AskTUG-33070.

What is changed and how it works?

1. During switch mode, if we encounter a store that returns Unimplemented (mainly TiFlash nowadays), simply ignore the error.
2. Added ./tidb-lightning-ctl --fetch-mode subcommand to print the mode.

Check List

Tests

• Unit test
• Manual test (add detailed scripts or steps below)

Side effects

Related changes

• Need to update the documentation
• Need to be included in the release note
  • Document the new --fetch-mode flag.

[IANTHEREAL]

take a time to fix ci 😄

[IANTHEREAL]

This pr just ignores these error logs temporarily. If ideal, the tikv client should be able to handle the incomplete implementation of these interfaces, right?

[IANTHEREAL]

LGTM

[kennytm]

This pr just ignores these error logs temporarily. If ideal, the tikv client should be able to handle the incomplete implementation of these interfaces, right?

No that's not intended to be temporary. The reason is that failing to switch mode won't cause import to fail (just slower), so ignoring the "Unimplemented" error permanently is fine.

Yes TiFlash should implement the SwitchMode API (by doing nothing at least), which will make the error go truly away.

[kennytm]

take a time to fix ci 😄

[2020-03-23T21:43:10.004Z] tests/run.sh: line 30: 17815 Aborted                 (core dumped) bin/tikv-server -C "$TEST_DIR/tikv-config.toml"

We're always enabling TLS and thus hit tikv/tikv#7209 😭. Either we block on tikv/tikv#7251 or disable the TLS temporarily.

---

/run-all-tests tikv=release-3.1 pd=release-3.1 tidb=release-3.1

[kennytm]

/run-all-tests

[kennytm]

PTAL @3pointer