*: add variables and support external timestamp read

[YangKeao]

What problem does this PR solve?

Issue Number: close #38274

Problem Summary:

TiDB doesn't have a way to allow staleread with a timestamp from external (globally).

What is changed and how it works?

1. Add two variables to control the timestamp (tidb_external_ts) and whether to enable this feature (tidb_enable_external_ts_read)
2. If there is no explicitly stale-read (e.g. select as of, start transaction as of, set tidb_read_staleness...), and the control variable is on, use the external timestamp to perform stale read.

TODO:

• Add variables
• Implement basic function through modifying staleread
• Add basic test about this function
• Fine tune the context.Context. They are all context.TODO() now 😢 . But it can be a little difficult (as the variables hook deep-callchain doesn't pass in a context)
  • Add context (context.Context) for planner preprocess #38359
  • Add context (context.Context) for global variable get/set #38378
• Add more unit tests / integration tests, especially with txn / stale-read.
• Integrate with the real oracle api (or mocked oracle api)

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Add two system variables `tidb_external_ts` and `tidb_enable_external_ts_read`. If the `tidb_enable_external_ts_read` is on, the tidb will be in read-only mode and read through the timestamp set in `tidb_external_ts`.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• bb7133
• lcwangchao

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[ngaut]

Does it necessary to change the definition of NewSessionVars, which introduced so many changes.

[YangKeao]

Sadly, these changes have been merged into master in #38227. @morgo Provided a better way to work around this 👍 . I'll revert these changes after #38182 merged 😃

[bb7133]

LGTM

[lcwangchao]

LGTM

[lcwangchao]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 24ba369

[sre-bot]

TiDB MergeCI notify

🔴 Bad News! New failing [1] after this pr merged.
These new failed integration tests seem to be caused by the current PR, please try to fix these new failed integration tests, thanks!

CI Name	Result	Duration	Compare with Parent commit
idc-jenkins-ci-tidb/integration-ddl-test	🟥 failed 1, success 5, total 6	28 min	New failing
idc-jenkins-ci/integration-cdc-test	🟢 all 38 tests passed	23 min	Existing passed
idc-jenkins-ci-tidb/integration-common-test	🟢 all 17 tests passed	18 min	Existing passed
idc-jenkins-ci-tidb/common-test	🟢 all 11 tests passed	11 min	Existing passed
idc-jenkins-ci-tidb/tics-test	🟢 all 1 tests passed	9 min 6 sec	Existing passed
idc-jenkins-ci-tidb/sqllogic-test-2	🟢 all 28 tests passed	6 min 51 sec	Existing passed
idc-jenkins-ci-tidb/integration-compatibility-test	🟢 all 1 tests passed	5 min 22 sec	Existing passed
idc-jenkins-ci-tidb/sqllogic-test-1	🟢 all 26 tests passed	5 min 12 sec	Existing passed
idc-jenkins-ci-tidb/mybatis-test	🟢 all 1 tests passed	3 min 33 sec	Existing passed
idc-jenkins-ci-tidb/plugin-test	🟢 build success, plugin test success	4min	Existing passed