client,kv: new savepoint API

This patch introduces the KV savepoint API as discussed in the
savepoints RFC:

```go
	// CreateSavepoint establishes a savepoint.
	// This method is only valid when called on RootTxns.
	CreateSavepoint(context.Context) (SavepointToken, error)

	// RollbackToSavepoint rolls back to the given savepoint. The
	// savepoint must not have been rolled back or released already.
	// All savepoints "under" the savepoint being rolled back
	// are also rolled back and their token must not be used any more.
	// This method is only valid when called on RootTxns.
	RollbackToSavepoint(context.Context, SavepointToken) error

	// ReleaseSavepoint releases the given savepoint. The savepoint
	// must not have been rolled back or released already.
	// All savepoints "under" the savepoint being released
	// are also released and their token must not be used any more.
	// This method is only valid when called on RootTxns.
	ReleaseSavepoint(context.Context, SavepointToken) error
```

Release note: None

pkg/internal/client/mock_transactional_sender.go

@@ -135,6 +135,21 @@ func (m *MockTransactionalSender) IsSerializablePushAndRefreshNotPossible() bool
 	return false
 }
 
+// CreateSavepoint is part of the client.TxnSender interface.
+func (m *MockTransactionalSender) CreateSavepoint(context.Context) (SavepointToken, error) {
+	panic("unimplemented")
+}
+
+// RollbackToSavepoint is part of the client.TxnSender interface.
+func (m *MockTransactionalSender) RollbackToSavepoint(context.Context, SavepointToken) error {
+	panic("unimplemented")
+}
+
+// ReleaseSavepoint is part of the client.TxnSender interface.
+func (m *MockTransactionalSender) ReleaseSavepoint(context.Context, SavepointToken) error {
+	panic("unimplemented")
+}
+
 // Epoch is part of the TxnSender interface.
 func (m *MockTransactionalSender) Epoch() enginepb.TxnEpoch { panic("unimplemented") }
 

pkg/internal/client/sender.go

@@ -133,6 +133,24 @@ type TxnSender interface {
 	// TxnStatus exports the txn's status.
 	TxnStatus() roachpb.TransactionStatus
 
+	// CreateSavepoint establishes a savepoint.
+	// This method is only valid when called on RootTxns.
+	CreateSavepoint(context.Context) (SavepointToken, error)
+
+	// RollbackToSavepoint rolls back to the given savepoint. The
+	// savepoint must not have been rolled back or released already.
+	// All savepoints "under" the savepoint being rolled back
+	// are also rolled back and their token must not be used any more.
+	// This method is only valid when called on RootTxns.
+	RollbackToSavepoint(context.Context, SavepointToken) error
+
+	// ReleaseSavepoint releases the given savepoint. The savepoint
+	// must not have been rolled back or released already.
+	// All savepoints "under" the savepoint being released
+	// are also released and their token must not be used any more.
+	// This method is only valid when called on RootTxns.
+	ReleaseSavepoint(context.Context, SavepointToken) error
+
 	// SetFixedTimestamp makes the transaction run in an unusual way, at
 	// a "fixed timestamp": Timestamp and ReadTimestamp are set to ts,
 	// there's no clock uncertainty, and the txn's deadline is set to ts
@@ -274,6 +292,12 @@ const (
 	SteppingEnabled SteppingMode = true
 )
 
+// SavepointToken represents a savepoint.
+type SavepointToken interface {
+	// SavepointToken is a marker interface.
+	SavepointToken()
+}
+
 // TxnStatusOpt represents options for TxnSender.GetMeta().
 type TxnStatusOpt int
 

pkg/internal/client/txn.go

@@ -1217,3 +1217,33 @@ func (txn *Txn) ConfigureStepping(ctx context.Context, mode SteppingMode) (prevM
 	defer txn.mu.Unlock()
 	return txn.mu.sender.ConfigureStepping(ctx, mode)
 }
+
+// CreateSavepoint establishes a savepoint.
+// This method is only valid when called on RootTxns.
+func (txn *Txn) CreateSavepoint(ctx context.Context) (SavepointToken, error) {
+	txn.mu.Lock()
+	defer txn.mu.Unlock()
+	return txn.mu.sender.CreateSavepoint(ctx)
+}
+
+// RollbackToSavepoint rolls back to the given savepoint. The
+// savepoint must not have been rolled back or released already.
+// All savepoints "under" the savepoint being rolled back
+// are also rolled back and their token must not be used any more.
+// This method is only valid when called on RootTxns.
+func (txn *Txn) RollbackToSavepoint(ctx context.Context, s SavepointToken) error {
+	txn.mu.Lock()
+	defer txn.mu.Unlock()
+	return txn.mu.sender.RollbackToSavepoint(ctx, s)
+}
+
+// ReleaseSavepoint releases the given savepoint. The savepoint
+// must not have been rolled back or released already.
+// All savepoints "under" the savepoint being released
+// are also released and their token must not be used any more.
+// This method is only valid when called on RootTxns.
+func (txn *Txn) ReleaseSavepoint(ctx context.Context, s SavepointToken) error {
+	txn.mu.Lock()
+	defer txn.mu.Unlock()
+	return txn.mu.sender.ReleaseSavepoint(ctx, s)
+}