transaction.go

/*
Copyright 2017 Google Inc. All Rights Reserved.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package spanner

import (
	"sync"
	"time"

	"golang.org/x/net/context"

	"google.golang.org/api/iterator"
	sppb "google.golang.org/genproto/googleapis/spanner/v1"
	"google.golang.org/grpc"
	"google.golang.org/grpc/codes"
	"google.golang.org/grpc/metadata"
)

// transactionID stores a transaction ID which uniquely identifies a transaction in Cloud Spanner.
type transactionID []byte

// txReadEnv manages a read-transaction environment consisting of a session handle and a transaction selector.
type txReadEnv interface {
	// acquire returns a read-transaction environment that can be used to perform a transactional read.
	acquire(ctx context.Context) (*sessionHandle, *sppb.TransactionSelector, error)
	// sets the transaction's read timestamp
	setTimestamp(time.Time)
	// release should be called at the end of every transactional read to deal with session recycling.
	release(error)
}

// txReadOnly contains methods for doing transactional reads.
type txReadOnly struct {
	// read-transaction environment for performing transactional read operations.
	txReadEnv
}

// errSessionClosed returns error for using a recycled/destroyed session
func errSessionClosed(sh *sessionHandle) error {
	return spannerErrorf(codes.FailedPrecondition,
		"session is already recycled / destroyed: session_id = %q, rpc_client = %v", sh.getID(), sh.getClient())
}

// Read returns a RowIterator for reading multiple rows from the database.
func (t *txReadOnly) Read(ctx context.Context, table string, keys KeySet, columns []string) *RowIterator {
	// ReadUsingIndex will use primary index if an empty index name is provided.
	return t.ReadUsingIndex(ctx, table, "", keys, columns)
}

// ReadUsingIndex returns a RowIterator for reading multiple rows from the database
// using an index.
//
// Currently, this function can only read columns that are part of the index
// key, part of the primary key, or stored in the index due to a STORING clause
// in the index definition.
func (t *txReadOnly) ReadUsingIndex(ctx context.Context, table, index string, keys KeySet, columns []string) *RowIterator {
	var (
		sh  *sessionHandle
		ts  *sppb.TransactionSelector
		err error
	)
	kset, err := keys.keySetProto()
	if err != nil {
		return &RowIterator{err: err}
	}
	if sh, ts, err = t.acquire(ctx); err != nil {
		return &RowIterator{err: err}
	}
	// Cloud Spanner will return "Session not found" on bad sessions.
	sid, client := sh.getID(), sh.getClient()
	if sid == "" || client == nil {
		// Might happen if transaction is closed in the middle of a API call.
		return &RowIterator{err: errSessionClosed(sh)}
	}
	return stream(
		contextWithOutgoingMetadata(ctx, sh.getMetadata()),
		func(ctx context.Context, resumeToken []byte) (streamingReceiver, error) {
			return client.StreamingRead(ctx,
				&sppb.ReadRequest{
					Session:     sid,
					Transaction: ts,
					Table:       table,
					Index:       index,
					Columns:     columns,
					KeySet:      kset,
					ResumeToken: resumeToken,
				})
		},
		t.setTimestamp,
		t.release,
	)
}

// errRowNotFound returns error for not being able to read the row identified by key.
func errRowNotFound(table string, key Key) error {
	return spannerErrorf(codes.NotFound, "row not found(Table: %v, PrimaryKey: %v)", table, key)
}

// ReadRow reads a single row from the database.
//
// If no row is present with the given key, then ReadRow returns an error where
// spanner.ErrCode(err) is codes.NotFound.
func (t *txReadOnly) ReadRow(ctx context.Context, table string, key Key, columns []string) (*Row, error) {
	iter := t.Read(ctx, table, key, columns)
	defer iter.Stop()
	row, err := iter.Next()
	switch err {
	case iterator.Done:
		return nil, errRowNotFound(table, key)
	case nil:
		return row, nil
	default:
		return nil, err
	}
}

// Query executes a query against the database. It returns a RowIterator
// for retrieving the resulting rows.
func (t *txReadOnly) Query(ctx context.Context, statement Statement) *RowIterator {
	var (
		sh  *sessionHandle
		ts  *sppb.TransactionSelector
		err error
	)
	if sh, ts, err = t.acquire(ctx); err != nil {
		return &RowIterator{err: err}
	}
	// Cloud Spanner will return "Session not found" on bad sessions.
	sid, client := sh.getID(), sh.getClient()
	if sid == "" || client == nil {
		// Might happen if transaction is closed in the middle of a API call.
		return &RowIterator{err: errSessionClosed(sh)}
	}
	req := &sppb.ExecuteSqlRequest{
		Session:     sid,
		Transaction: ts,
		Sql:         statement.SQL,
	}
	if err := statement.bindParams(req); err != nil {
		return &RowIterator{err: err}
	}
	return stream(
		contextWithOutgoingMetadata(ctx, sh.getMetadata()),
		func(ctx context.Context, resumeToken []byte) (streamingReceiver, error) {
			req.ResumeToken = resumeToken
			return client.ExecuteStreamingSql(ctx, req)
		},
		t.setTimestamp,
		t.release)
}

// txState is the status of a transaction.
type txState int

const (
	// transaction is new, waiting to be initialized.
	txNew txState = iota
	// transaction is being initialized.
	txInit
	// transaction is active and can perform read/write.
	txActive
	// transaction is closed, cannot be used anymore.
	txClosed
)

// errRtsUnavailable returns error for read transaction's read timestamp being unavailable.
func errRtsUnavailable() error {
	return spannerErrorf(codes.Internal, "read timestamp is unavailable")
}

// errTxNotInitialized returns error for using an uninitialized transaction.
func errTxNotInitialized() error {
	return spannerErrorf(codes.InvalidArgument, "cannot use a uninitialized transaction")
}

// errTxClosed returns error for using a closed transaction.
func errTxClosed() error {
	return spannerErrorf(codes.InvalidArgument, "cannot use a closed transaction")
}

// errUnexpectedTxState returns error for transaction enters an unexpected state.
func errUnexpectedTxState(ts txState) error {
	return spannerErrorf(codes.FailedPrecondition, "unexpected transaction state: %v", ts)
}

// ReadOnlyTransaction provides a snapshot transaction with guaranteed
// consistency across reads, but does not allow writes.  Read-only
// transactions can be configured to read at timestamps in the past.
//
// Read-only transactions do not take locks. Instead, they work by choosing a
// Cloud Spanner timestamp, then executing all reads at that timestamp. Since they do
// not acquire locks, they do not block concurrent read-write transactions.
//
// Unlike locking read-write transactions, read-only transactions never
// abort. They can fail if the chosen read timestamp is garbage collected;
// however, the default garbage collection policy is generous enough that most
// applications do not need to worry about this in practice. See the
// documentation of TimestampBound for more details.
//
// A ReadOnlyTransaction consumes resources on the server until Close() is
// called.
type ReadOnlyTransaction struct {
	// txReadOnly contains methods for performing transactional reads.
	txReadOnly

	// singleUse indicates that the transaction can be used for only one read.
	singleUse bool

	// sp is the session pool for allocating a session to execute the read-only transaction. It is set only once during initialization of the ReadOnlyTransaction.
	sp *sessionPool
	// mu protects concurrent access to the internal states of ReadOnlyTransaction.
	mu sync.Mutex
	// tx is the transaction ID in Cloud Spanner that uniquely identifies the ReadOnlyTransaction.
	tx transactionID
	// txReadyOrClosed is for broadcasting that transaction ID has been returned by Cloud Spanner or that transaction is closed.
	txReadyOrClosed chan struct{}
	// state is the current transaction status of the ReadOnly transaction.
	state txState
	// sh is the sessionHandle allocated from sp.
	sh *sessionHandle
	// rts is the read timestamp returned by transactional reads.
	rts time.Time
	// tb is the read staleness bound specification for transactional reads.
	tb TimestampBound
}

// errTxInitTimeout returns error for timeout in waiting for initialization of the transaction.
func errTxInitTimeout() error {
	return spannerErrorf(codes.Canceled, "timeout/context canceled in waiting for transaction's initialization")
}

// getTimestampBound returns the read staleness bound specified for the ReadOnlyTransaction.
func (t *ReadOnlyTransaction) getTimestampBound() TimestampBound {
	t.mu.Lock()
	defer t.mu.Unlock()
	return t.tb
}

// begin starts a snapshot read-only Transaction on Cloud Spanner.
func (t *ReadOnlyTransaction) begin(ctx context.Context) error {
	var (
		locked bool
		tx     transactionID
		rts    time.Time
		sh     *sessionHandle
		err    error
	)
	defer func() {
		if !locked {
			t.mu.Lock()
			// Not necessary, just to make it clear that t.mu is being held when locked == true.
			locked = true
		}
		if t.state != txClosed {
			// Signal other initialization routines.
			close(t.txReadyOrClosed)
			t.txReadyOrClosed = make(chan struct{})
		}
		t.mu.Unlock()
		if err != nil && sh != nil {
			// Got a valid session handle, but failed to initalize transaction on Cloud Spanner.
			if shouldDropSession(err) {
				sh.destroy()
			}
			// If sh.destroy was already executed, this becomes a noop.
			sh.recycle()
		}
	}()
	sh, err = t.sp.take(ctx)
	if err != nil {
		return err
	}
	err = runRetryable(contextWithOutgoingMetadata(ctx, sh.getMetadata()), func(ctx context.Context) error {
		res, e := sh.getClient().BeginTransaction(ctx, &sppb.BeginTransactionRequest{
			Session: sh.getID(),
			Options: &sppb.TransactionOptions{
				Mode: &sppb.TransactionOptions_ReadOnly_{
					ReadOnly: buildTransactionOptionsReadOnly(t.getTimestampBound(), true),
				},
			},
		})
		if e != nil {
			return e
		}
		tx = res.Id
		if res.ReadTimestamp != nil {
			rts = time.Unix(res.ReadTimestamp.Seconds, int64(res.ReadTimestamp.Nanos))
		}
		return nil
	})
	t.mu.Lock()
	locked = true            // defer function will be executed with t.mu being held.
	if t.state == txClosed { // During the execution of t.begin(), t.Close() was invoked.
		return errSessionClosed(sh)
	}
	// If begin() fails, this allows other queries to take over the initialization.
	t.tx = nil
	if err == nil {
		t.tx = tx
		t.rts = rts
		t.sh = sh
		// State transite to txActive.
		t.state = txActive
	}
	return err
}

// acquire implements txReadEnv.acquire.
func (t *ReadOnlyTransaction) acquire(ctx context.Context) (*sessionHandle, *sppb.TransactionSelector, error) {
	if err := checkNestedTxn(ctx); err != nil {
		return nil, nil, err
	}
	if t.singleUse {
		return t.acquireSingleUse(ctx)
	}
	return t.acquireMultiUse(ctx)
}

func (t *ReadOnlyTransaction) acquireSingleUse(ctx context.Context) (*sessionHandle, *sppb.TransactionSelector, error) {
	t.mu.Lock()
	defer t.mu.Unlock()
	switch t.state {
	case txClosed:
		// A closed single-use transaction can never be reused.
		return nil, nil, errTxClosed()
	case txNew:
		t.state = txClosed
		ts := &sppb.TransactionSelector{
			Selector: &sppb.TransactionSelector_SingleUse{
				SingleUse: &sppb.TransactionOptions{
					Mode: &sppb.TransactionOptions_ReadOnly_{
						ReadOnly: buildTransactionOptionsReadOnly(t.tb, true),
					},
				},
			},
		}
		sh, err := t.sp.take(ctx)
		if err != nil {
			return nil, nil, err
		}
		// Install session handle into t, which can be used for readonly operations later.
		t.sh = sh
		return sh, ts, nil
	}
	us := t.state
	// SingleUse transaction should only be in either txNew state or txClosed state.
	return nil, nil, errUnexpectedTxState(us)
}

func (t *ReadOnlyTransaction) acquireMultiUse(ctx context.Context) (*sessionHandle, *sppb.TransactionSelector, error) {
	for {
		t.mu.Lock()
		switch t.state {
		case txClosed:
			t.mu.Unlock()
			return nil, nil, errTxClosed()
		case txNew:
			// State transit to txInit so that no further TimestampBound change is accepted.
			t.state = txInit
			t.mu.Unlock()
			continue
		case txInit:
			if t.tx != nil {
				// Wait for a transaction ID to become ready.
				txReadyOrClosed := t.txReadyOrClosed
				t.mu.Unlock()
				select {
				case <-txReadyOrClosed:
					// Need to check transaction state again.
					continue
				case <-ctx.Done():
					// The waiting for initialization is timeout, return error directly.
					return nil, nil, errTxInitTimeout()
				}
			}
			// Take the ownership of initializing the transaction.
			t.tx = transactionID{}
			t.mu.Unlock()
			// Begin a read-only transaction.
			// TODO: consider adding a transaction option which allow queries to initiate transactions by themselves. Note that this option might not be
			// always good because the ID of the new transaction won't be ready till the query returns some data or completes.
			if err := t.begin(ctx); err != nil {
				return nil, nil, err
			}
			// If t.begin() succeeded, t.state should have been changed to txActive, so we can just continue here.
			continue
		case txActive:
			sh := t.sh
			ts := &sppb.TransactionSelector{
				Selector: &sppb.TransactionSelector_Id{
					Id: t.tx,
				},
			}
			t.mu.Unlock()
			return sh, ts, nil
		}
		state := t.state
		t.mu.Unlock()
		return nil, nil, errUnexpectedTxState(state)
	}
}

func (t *ReadOnlyTransaction) setTimestamp(ts time.Time) {
	t.mu.Lock()
	defer t.mu.Unlock()
	if t.rts.IsZero() {
		t.rts = ts
	}
}

// release implements txReadEnv.release.
func (t *ReadOnlyTransaction) release(err error) {
	t.mu.Lock()
	sh := t.sh
	t.mu.Unlock()
	if sh != nil { // sh could be nil if t.acquire() fails.
		if shouldDropSession(err) {
			sh.destroy()
		}
		if t.singleUse {
			// If session handle is already destroyed, this becomes a noop.
			sh.recycle()
		}
	}
}

// Close closes a ReadOnlyTransaction, the transaction cannot perform any reads after being closed.
func (t *ReadOnlyTransaction) Close() {
	if t.singleUse {
		return
	}
	t.mu.Lock()
	if t.state != txClosed {
		t.state = txClosed
		close(t.txReadyOrClosed)
	}
	sh := t.sh
	t.mu.Unlock()
	if sh == nil {
		return
	}
	// If session handle is already destroyed, this becomes a noop.
	// If there are still active queries and if the recycled session is reused before they complete, Cloud Spanner will cancel them
	// on behalf of the new transaction on the session.
	if sh != nil {
		sh.recycle()
	}
}

// Timestamp returns the timestamp chosen to perform reads and
// queries in this transaction. The value can only be read after some
// read or query has either returned some data or completed without
// returning any data.
func (t *ReadOnlyTransaction) Timestamp() (time.Time, error) {
	t.mu.Lock()
	defer t.mu.Unlock()
	if t.rts.IsZero() {
		return t.rts, errRtsUnavailable()
	}
	return t.rts, nil
}

// WithTimestampBound specifies the TimestampBound to use for read or query.
// This can only be used before the first read or query is invoked. Note:
// bounded staleness is not available with general ReadOnlyTransactions; use a
// single-use ReadOnlyTransaction instead.
//
// The returned value is the ReadOnlyTransaction so calls can be chained.
func (t *ReadOnlyTransaction) WithTimestampBound(tb TimestampBound) *ReadOnlyTransaction {
	t.mu.Lock()
	defer t.mu.Unlock()
	if t.state == txNew {
		// Only allow to set TimestampBound before the first query.
		t.tb = tb
	}
	return t
}

// ReadWriteTransaction provides a locking read-write transaction.
//
// This type of transaction is the only way to write data into Cloud Spanner;
// (*Client).Apply and (*Client).ApplyAtLeastOnce use transactions
// internally. These transactions rely on pessimistic locking and, if
// necessary, two-phase commit. Locking read-write transactions may abort,
// requiring the application to retry. However, the interface exposed by
// (*Client).ReadWriteTransaction eliminates the need for applications to write
// retry loops explicitly.
//
// Locking transactions may be used to atomically read-modify-write data
// anywhere in a database. This type of transaction is externally consistent.
//
// Clients should attempt to minimize the amount of time a transaction is
// active. Faster transactions commit with higher probability and cause less
// contention. Cloud Spanner attempts to keep read locks active as long as the
// transaction continues to do reads.  Long periods of inactivity at the client
// may cause Cloud Spanner to release a transaction's locks and abort it.
//
// Reads performed within a transaction acquire locks on the data being
// read. Writes can only be done at commit time, after all reads have been
// completed. Conceptually, a read-write transaction consists of zero or more
// reads or SQL queries followed by a commit.
//
// See (*Client).ReadWriteTransaction for an example.
//
// Semantics
//
// Cloud Spanner can commit the transaction if all read locks it acquired are still
// valid at commit time, and it is able to acquire write locks for all
// writes. Cloud Spanner can abort the transaction for any reason. If a commit
// attempt returns ABORTED, Cloud Spanner guarantees that the transaction has not
// modified any user data in Cloud Spanner.
//
// Unless the transaction commits, Cloud Spanner makes no guarantees about how long
// the transaction's locks were held for. It is an error to use Cloud Spanner locks
// for any sort of mutual exclusion other than between Cloud Spanner transactions
// themselves.
//
// Aborted transactions
//
// Application code does not need to retry explicitly; RunInTransaction will
// automatically retry a transaction if an attempt results in an abort. The
// lock priority of a transaction increases after each prior aborted
// transaction, meaning that the next attempt has a slightly better chance of
// success than before.
//
// Under some circumstances (e.g., many transactions attempting to modify the
// same row(s)), a transaction can abort many times in a short period before
// successfully committing. Thus, it is not a good idea to cap the number of
// retries a transaction can attempt; instead, it is better to limit the total
// amount of wall time spent retrying.
//
// Idle transactions
//
// A transaction is considered idle if it has no outstanding reads or SQL
// queries and has not started a read or SQL query within the last 10
// seconds. Idle transactions can be aborted by Cloud Spanner so that they don't hold
// on to locks indefinitely. In that case, the commit will fail with error
// ABORTED.
//
// If this behavior is undesirable, periodically executing a simple SQL query
// in the transaction (e.g., SELECT 1) prevents the transaction from becoming
// idle.
type ReadWriteTransaction struct {
	// txReadOnly contains methods for performing transactional reads.
	txReadOnly
	// sh is the sessionHandle allocated from sp. It is set only once during the initialization of ReadWriteTransaction.
	sh *sessionHandle
	// tx is the transaction ID in Cloud Spanner that uniquely identifies the ReadWriteTransaction.
	// It is set only once in ReadWriteTransaction.begin() during the initialization of ReadWriteTransaction.
	tx transactionID
	// mu protects concurrent access to the internal states of ReadWriteTransaction.
	mu sync.Mutex
	// state is the current transaction status of the read-write transaction.
	state txState
	// wb is the set of buffered mutations waiting to be commited.
	wb []*Mutation
}

// BufferWrite adds a list of mutations to the set of updates that will be
// applied when the transaction is committed. It does not actually apply the
// write until the transaction is committed, so the operation does not
// block. The effects of the write won't be visible to any reads (including
// reads done in the same transaction) until the transaction commits.
//
// See the example for Client.ReadWriteTransaction.
func (t *ReadWriteTransaction) BufferWrite(ms []*Mutation) error {
	t.mu.Lock()
	defer t.mu.Unlock()
	if t.state == txClosed {
		return errTxClosed()
	}
	if t.state != txActive {
		return errUnexpectedTxState(t.state)
	}
	t.wb = append(t.wb, ms...)
	return nil
}

// acquire implements txReadEnv.acquire.
func (t *ReadWriteTransaction) acquire(ctx context.Context) (*sessionHandle, *sppb.TransactionSelector, error) {
	ts := &sppb.TransactionSelector{
		Selector: &sppb.TransactionSelector_Id{
			Id: t.tx,
		},
	}
	t.mu.Lock()
	defer t.mu.Unlock()
	switch t.state {
	case txClosed:
		return nil, nil, errTxClosed()
	case txActive:
		return t.sh, ts, nil
	}
	return nil, nil, errUnexpectedTxState(t.state)
}

// release implements txReadEnv.release.
func (t *ReadWriteTransaction) release(err error) {
	t.mu.Lock()
	sh := t.sh
	t.mu.Unlock()
	if sh != nil && shouldDropSession(err) {
		sh.destroy()
	}
}

func beginTransaction(ctx context.Context, sid string, client sppb.SpannerClient) (transactionID, error) {
	var tx transactionID
	err := runRetryable(ctx, func(ctx context.Context) error {
		res, e := client.BeginTransaction(ctx, &sppb.BeginTransactionRequest{
			Session: sid,
			Options: &sppb.TransactionOptions{
				Mode: &sppb.TransactionOptions_ReadWrite_{
					ReadWrite: &sppb.TransactionOptions_ReadWrite{},
				},
			},
		})
		if e != nil {
			return e
		}
		tx = res.Id
		return nil
	})
	if err != nil {
		return nil, err
	}
	return tx, nil
}

// begin starts a read-write transacton on Cloud Spanner, it is always called before any of the public APIs.
func (t *ReadWriteTransaction) begin(ctx context.Context) error {
	if t.tx != nil {
		t.state = txActive
		return nil
	}
	tx, err := beginTransaction(contextWithOutgoingMetadata(ctx, t.sh.getMetadata()), t.sh.getID(), t.sh.getClient())
	if err == nil {
		t.tx = tx
		t.state = txActive
		return nil
	}
	if shouldDropSession(err) {
		t.sh.destroy()
	}
	return err
}

// commit tries to commit a readwrite transaction to Cloud Spanner. It also returns the commit timestamp for the transactions.
func (t *ReadWriteTransaction) commit(ctx context.Context) (time.Time, error) {
	var ts time.Time
	t.mu.Lock()
	t.state = txClosed // No futher operations after commit.
	mPb, err := mutationsProto(t.wb)
	t.mu.Unlock()
	if err != nil {
		return ts, err
	}
	// In case that sessionHandle was destroyed but transaction body fails to report it.
	sid, client := t.sh.getID(), t.sh.getClient()
	if sid == "" || client == nil {
		return ts, errSessionClosed(t.sh)
	}
	err = runRetryable(contextWithOutgoingMetadata(ctx, t.sh.getMetadata()), func(ctx context.Context) error {
		var trailer metadata.MD
		res, e := client.Commit(ctx, &sppb.CommitRequest{
			Session: sid,
			Transaction: &sppb.CommitRequest_TransactionId{
				TransactionId: t.tx,
			},
			Mutations: mPb,
		}, grpc.Trailer(&trailer))
		if e != nil {
			return toSpannerErrorWithMetadata(e, trailer)
		}
		if tstamp := res.GetCommitTimestamp(); tstamp != nil {
			ts = time.Unix(tstamp.Seconds, int64(tstamp.Nanos))
		}
		return nil
	})
	if shouldDropSession(err) {
		t.sh.destroy()
	}
	return ts, err
}

// rollback is called when a commit is aborted or the transaction body runs into error.
func (t *ReadWriteTransaction) rollback(ctx context.Context) {
	t.mu.Lock()
	// Forbid further operations on rollbacked transaction.
	t.state = txClosed
	t.mu.Unlock()
	// In case that sessionHandle was destroyed but transaction body fails to report it.
	sid, client := t.sh.getID(), t.sh.getClient()
	if sid == "" || client == nil {
		return
	}
	err := runRetryable(contextWithOutgoingMetadata(ctx, t.sh.getMetadata()), func(ctx context.Context) error {
		_, e := client.Rollback(ctx, &sppb.RollbackRequest{
			Session:       sid,
			TransactionId: t.tx,
		})
		return e
	})
	if shouldDropSession(err) {
		t.sh.destroy()
	}
	return
}

// runInTransaction executes f under a read-write transaction context.
func (t *ReadWriteTransaction) runInTransaction(ctx context.Context, f func(context.Context, *ReadWriteTransaction) error) (time.Time, error) {
	var (
		ts  time.Time
		err error
	)
	if err = f(context.WithValue(ctx, transactionInProgressKey{}, 1), t); err == nil {
		// Try to commit if transaction body returns no error.
		ts, err = t.commit(ctx)
	}
	if err != nil {
		if isAbortErr(err) {
			// Retry the transaction using the same session on ABORT error.
			// Cloud Spanner will create the new transaction with the previous one's wound-wait priority.
			err = errRetry(err)
			return ts, err
		}
		// Not going to commit, according to API spec, should rollback the transaction.
		t.rollback(ctx)
		return ts, err
	}
	// err == nil, return commit timestamp.
	return ts, err
}

// writeOnlyTransaction provides the most efficient way of doing write-only transactions. It essentially does blind writes to Cloud Spanner.
type writeOnlyTransaction struct {
	// sp is the session pool which writeOnlyTransaction uses to get Cloud Spanner sessions for blind writes.
	sp *sessionPool
}

// applyAtLeastOnce commits a list of mutations to Cloud Spanner for at least once, unless one of the following happends:
//     1) Context is timeout.
//     2) An unretryable error(e.g. database not found) occurs.
//     3) There is a malformed Mutation object.
func (t *writeOnlyTransaction) applyAtLeastOnce(ctx context.Context, ms ...*Mutation) (time.Time, error) {
	var (
		ts time.Time
		sh *sessionHandle
	)
	mPb, err := mutationsProto(ms)
	if err != nil {
		// Malformed mutation found, just return the error.
		return ts, err
	}
	err = runRetryable(ctx, func(ct context.Context) error {
		var e error
		var trailers metadata.MD
		if sh == nil || sh.getID() == "" || sh.getClient() == nil {
			// No usable session for doing the commit, take one from pool.
			sh, e = t.sp.take(ctx)
			if e != nil {
				// sessionPool.Take already retries for session creations/retrivals.
				return e
			}
		}
		res, e := sh.getClient().Commit(contextWithOutgoingMetadata(ctx, sh.getMetadata()), &sppb.CommitRequest{
			Session: sh.getID(),
			Transaction: &sppb.CommitRequest_SingleUseTransaction{
				SingleUseTransaction: &sppb.TransactionOptions{
					Mode: &sppb.TransactionOptions_ReadWrite_{
						ReadWrite: &sppb.TransactionOptions_ReadWrite{},
					},
				},
			},
			Mutations: mPb,
		}, grpc.Trailer(&trailers))
		if e != nil {
			if isAbortErr(e) {
				// Mask ABORT error as retryable, because aborted transactions are allowed to be retried.
				return errRetry(toSpannerErrorWithMetadata(e, trailers))
			}
			if shouldDropSession(e) {
				// Discard the bad session.
				sh.destroy()
			}
			return e
		}
		if tstamp := res.GetCommitTimestamp(); tstamp != nil {
			ts = time.Unix(tstamp.Seconds, int64(tstamp.Nanos))
		}
		return nil
	})
	if sh != nil {
		sh.recycle()
	}
	return ts, err
}

// isAbortedErr returns true if the error indicates that an gRPC call is aborted on the server side.
func isAbortErr(err error) bool {
	if err == nil {
		return false
	}
	if ErrCode(err) == codes.Aborted {
		return true
	}
	return false
}