A simpler implement for snapshot custom retriever

[lcwangchao]

Considering comments in #27854 , we want some simpler implement for custom data retrieving in snapshot. The new implement have several considerations:

• Make a simple code in snapshot to avoid potential bugs.
• If possible, code in snapshot should be abstract enough for future extension.
• We can suppose snapshot data is empty for custom retriever's ranges to reduce design complexity （avioding oneByOneIter）

A preliminary proposal:

Firstly introduce a new interface txn.SnapshotInterceptor to snapshot. The definition is:

type SnapshotInterceptor interface {
    OnGet(ctx context.Context, snapshot kv.Snapshot, key kv.Key) ([]byte, error)
    OnBatchGet(ctx context.Context, snapshot kv.Snapshot, keys []kv.Key) (map[string][]byte, error)
    OnIter(ctx context.Context, snapshot kv.Snapshot, k []kv.Key, upper []kv.Key) (kv.Iterator, error)
    OnIterReverse(ctx context.Context, snapshot kv.Snapshot, k []kv.Key) (kv.Iterator, error)
}

We can set a interceptor to snapshot like this:

snapshot.SetOption(kv.SnapshotInterceptor, interceptor)

Take Get as example，the inner tikvSnapshot code is:

func (s *tikvSnapshot) Get(ctx context.Context, k kv.Key) ([]byte, error) {
        if s.interceptor != nil {
            return s.interceptor.OnGet(ctx, NewSnapshot(s.KVSnapshot), k)
        }

	data, err := s.KVSnapshot.Get(ctx, k)
	return data, extractKeyErr(err)
}

It means that if an interceptor is set, snapshot will delegate its read to the interceptor passing a kv.Snapshot parameter to it. Then the interceptor can do any thing to read the data.

For temporary table case, we can create a interceptor called TemporaryTableSnapshotInteceptor and its OnGet looks like this:

func (i *TemporaryTableSnapshotInteceptor) OnGet(ctx context.Context, snapshot kv.Snapshot, key kv.Key) ([]byte, error) {
    if tblInfo, ok := i.getAccessTableInfoFromKey(key); ok {
        if tblInfo.TempTableType == model.TempTableGlobal {
            return nil, kv.ErrNotExist
        }

        if tblInfo.TempTableType == model.TempTableLocal {
            val, err := i.memBuffer.Get(key)
            if err == nil && len(val) == 0 {
                return nil, kv.ErrNotExist
            }
            return val, err
        }
    }

    // fallback to snapshot read if the key is not a temporary table key
    return snapshot.Get(ctx, key)
}

It checks if it is accessing a temporary table's key. If it is, get data from memory otherwise fallback to default snapshot