Placement Policy support for dumpling

[morgo]

Feature Request

This is part of #18030

Is your feature request related to a problem? Please describe:

Currently dumpling supports dumping tables + data, but it does not yet support correctly dumping tables with a PLACEMENT POLICY defined.

Placement Policies can be thought of as similar to "Roles". They are globally scoped and save a set of placement options under a common name. See pingcap/docs#6618 for docs.

Describe the feature you'd like:

Dumpling should use the following procedure to dump placement policies:

1. Build a list of distinct placement policy names (select distinct policy_name from information_schema.placement_rules;)
2. Run SHOW CREATE PLACEMENT POLICY foo to get the recreatable version of the policy. This should be written to the top of the dump file.
3. Append SET PLACEMENT_CHECKS=0 to the start of the dump file. This feature was developed with dumpling in mind, since it should be possible to disable failures if a dump is reimported into a newer server which has stricter interpretations of how placement policies should be validated/specified. It also helps if the labels differ on the physical server where data is being restored (since this should not prevent restore - that could be disasterous).

Describe alternatives you've considered:

None

Teachability, Documentation, Adoption, Migration Strategy:

It should automatically work assuming the server is a TiDB server.

[sylzd]

@morgo @kennytm Can I try this? any idea or suggestion for me? 🤣

[sylzd]

/assign

[morgo]

Please try! @lichunzhu is a good person to ask for suggestions.

[sylzd]

@morgo It seems done already, any other work left?

[morgo]

@morgo It seems done already, any other work left?

I have added several new issues to #18030 just today (see the "Other" section).