Provide a method to clone the tableReaderExecutorContext

[YangKeao]

Enhancement

We already have a small enough tableReaderExecutorContext to be cloned. Let's figure out how to clone each fields of it:

• *distsqlctx.DistSQLContext. Most of the fields in DistSQLContext is just a value and it's safe to clone. However, there are still some fields that need us to take care of:
  • (Clone) *tikvstore.Variables. I'm not sure why it's a pointer, maybe some fields of it will be changed during execution 🤔 .
  • (Optional) *stmtstats.KvExecCounter. It'll not be re-used for each statement, so it's safe to just use the reference directly.
  • (No) SessionMemTracker *memory.Tracker. As it always points to the memory tracker on session, so it's safe to use the reference directly.
  • (Optional) *execdetails.RuntimeStatsColl. It'll not be re-used, so use the reference directly is safe.
  • (TODO) *sqlkiller.SQLKiller.
  • (Optional) *execdetails.SyncExecDetails. It'll not be re-used, so it's safe to just use the reference directly.
• *rangerctx.RangerContext. Most of the fields are fine. We only need to re-build it with a cloned WarnHandler. We'll discuss WarnHandler field later.
• *planctx.BuildPBContext.
  • AppendWarning and AppendExtraWarning. We'll need to refactor them into the WarnHandler before considering clone it.
• exprctx.BuildContext. We already have StaticEvalContext https://github.com/pingcap/tidb/pull/52631/files
• (Clone) isctx.MetaOnlyInfoSchema. The base of MetaOnlyInfoSchema is returned from vars.SnapshotInfoschema. It's not safe to use by different statements concurrently. Therefore, it'll need to be cloned.
• (No) getDDLOwner. The current implementation of getDDLOwner only depends on ownerManager (of ddl), which is safe to be used in multiple statements.

Fields which are used across many contexts

• Warnhandler. It's widely used in errctx, typectx and many other contexts, which means that we'll need to re-build all these contexts according to the dependency.
• kv.Client. TODO. I'm not sure whether it can be used in parallel statement safely.