txn: implement staging for PipelinedMemDB (tikv#1230)

* feat: implement staging for PipelinedMemDB

Signed-off-by: ekexium <[email protected]>

* comment: explain staging methods

Signed-off-by: ekexium <[email protected]>

---------

Signed-off-by: ekexium <[email protected]>

internal/unionstore/pipelined_memdb.go

@@ -278,8 +278,14 @@ func (p *PipelinedMemDB) Flush(force bool) (bool, error) {
 	if p.flushFunc == nil {
 		return false, errors.New("flushFunc is not provided")
 	}
+
 	// invalidate the batch get cache whether the flush is really triggered.
 	p.batchGetCache = nil
+
+	if len(p.memDB.stages) > 0 {
+		return false, errors.New("there are stages unreleased when Flush is called")
+	}
+
 	if !force && !p.needFlush() {
 		return false, nil
 	}
@@ -446,19 +452,27 @@ func (p *PipelinedMemDB) SnapshotGetter() Getter {
 	panic("SnapshotGetter is not supported for PipelinedMemDB")
 }
 
-// Staging is not supported for PipelinedMemDB, it returns 0 handle.
+// NOTE about the Staging()/Cleanup()/Release() methods:
+// Its correctness is guaranteed by that no stage can exist when a Flush() is called.
+// We guarantee in TiDB side that every stage lives inside a flush batch,
+// which means the modifications all goes to the mutable memdb.
+// Then the staging of the whole PipelinedMemDB can be directly implemented by its mutable memdb.
+//
+// Checkpoint()/RevertToCheckpoint() is not supported for PipelinedMemDB.
+
+// Staging implements MemBuffer interface.
 func (p *PipelinedMemDB) Staging() int {
-	panic("Staging is not supported for PipelinedMemDB")
+	return p.memDB.Staging()
 }
 
 // Cleanup implements MemBuffer interface.
-func (p *PipelinedMemDB) Cleanup(int) {
-	panic("Cleanup is not supported for PipelinedMemDB")
+func (p *PipelinedMemDB) Cleanup(h int) {
+	p.memDB.Cleanup(h)
 }
 
 // Release implements MemBuffer interface.
-func (p *PipelinedMemDB) Release(int) {
-	panic("Release is not supported for PipelinedMemDB")
+func (p *PipelinedMemDB) Release(h int) {
+	p.memDB.Release(h)
 }
 
 // Checkpoint implements MemBuffer interface.