transactions: Split out Transaction interface

The Transaction type is currently implemented as a structure, which
makes it impossible to mock it in other packages. This commit thus
splits it up into a private `transaction` implementation which is the
same as the current `Transaction` implementation, and a new public
`Transaction` interface.

3 files changed, 27 insertions, 17 deletions

diff --git a/internal/praefect/coordinator.go b/internal/praefect/coordinator.go
index 738bf8e0b..b5f8aaefd 100644
--- a/internal/praefect/coordinator.go
+++ b/internal/praefect/coordinator.go
@@ -322,7 +322,7 @@ func (c *Coordinator) accessorStreamParameters(ctx context.Context, call grpcCal
 	}, nil, nil, nil), nil
 }
 
-func (c *Coordinator) registerTransaction(ctx context.Context, primary RouterNode, secondaries []RouterNode) (*transactions.Transaction, transactions.CancelFunc, error) {
+func (c *Coordinator) registerTransaction(ctx context.Context, primary RouterNode, secondaries []RouterNode) (transactions.Transaction, transactions.CancelFunc, error) {
 	var voters []transactions.Voter
 	var threshold uint
 
@@ -649,7 +649,7 @@ func protoMessage(mi protoregistry.MethodInfo, frame []byte) (proto.Message, err
 
 func (c *Coordinator) createTransactionFinalizer(
 	ctx context.Context,
-	transaction *transactions.Transaction,
+	transaction transactions.Transaction,
 	route RepositoryMutatorRoute,
 	virtualStorage string,
 	targetRepo *gitalypb.Repository,
diff --git a/internal/praefect/transactions/manager.go b/internal/praefect/transactions/manager.go
index 75e73a76a..abfbe8fdc 100644
--- a/internal/praefect/transactions/manager.go
+++ b/internal/praefect/transactions/manager.go
@@ -25,7 +25,7 @@ var ErrNotFound = errors.New("transaction not found")
 type Manager struct {
 	txIDGenerator         TransactionIDGenerator
 	lock                  sync.Mutex
-	transactions          map[uint64]*Transaction
+	transactions          map[uint64]*transaction
 	counterMetric         *prometheus.CounterVec
 	delayMetric           *prometheus.HistogramVec
 	subtransactionsMetric prometheus.Histogram
@@ -72,7 +72,7 @@ func WithTransactionIDGenerator(generator TransactionIDGenerator) ManagerOpt {
 func NewManager(cfg config.Config, opts ...ManagerOpt) *Manager {
 	mgr := &Manager{
 		txIDGenerator: newTransactionIDGenerator(),
-		transactions:  make(map[uint64]*Transaction),
+		transactions:  make(map[uint64]*transaction),
 		counterMetric: prometheus.NewCounterVec(
 			prometheus.CounterOpts{
 				Namespace: "gitaly",
@@ -131,7 +131,7 @@ type CancelFunc func() error
 // taking part in the transaction. `threshold` is the threshold at which an
 // election will succeed. It needs to be in the range `weight(voters)/2 <
 // threshold <= weight(voters) to avoid indecidable votes.
-func (mgr *Manager) RegisterTransaction(ctx context.Context, voters []Voter, threshold uint) (*Transaction, CancelFunc, error) {
+func (mgr *Manager) RegisterTransaction(ctx context.Context, voters []Voter, threshold uint) (Transaction, CancelFunc, error) {
 	mgr.lock.Lock()
 	defer mgr.lock.Unlock()
 
@@ -163,7 +163,7 @@ func (mgr *Manager) RegisterTransaction(ctx context.Context, voters []Voter, thr
 	}, nil
 }
 
-func (mgr *Manager) cancelTransaction(ctx context.Context, transaction *Transaction) error {
+func (mgr *Manager) cancelTransaction(ctx context.Context, transaction *transaction) error {
 	mgr.lock.Lock()
 	defer mgr.lock.Unlock()
 
diff --git a/internal/praefect/transactions/transaction.go b/internal/praefect/transactions/transaction.go
index 0c871858d..2545ed4f6 100644
--- a/internal/praefect/transactions/transaction.go
+++ b/internal/praefect/transactions/transaction.go
@@ -47,11 +47,21 @@ type Voter struct {
 	result VoteResult
 }
 
-// Transaction is a session where a set of voters votes on one or more
+// Transaction is an interface for transactions.
+type Transaction interface {
+	// ID returns the ID of the transaction which uniquely identifies the transaction.
+	ID() uint64
+	// CountSubtransactions counts the number of subtransactions.
+	CountSubtransactions() int
+	// State returns the state of each voter part of the transaction.
+	State() (map[string]VoteResult, error)
+}
+
+// transaction is a session where a set of voters votes on one or more
 // subtransactions. Subtransactions are a sequence of sessions, where each node
 // needs to go through the same sequence and agree on the same thing in the end
 // in order to have the complete transaction succeed.
-type Transaction struct {
+type transaction struct {
 	id        uint64
 	threshold uint
 	voters    []Voter
@@ -61,7 +71,7 @@ type Transaction struct {
 	subtransactions []*subtransaction
 }
 
-func newTransaction(id uint64, voters []Voter, threshold uint) (*Transaction, error) {
+func newTransaction(id uint64, voters []Voter, threshold uint) (*transaction, error) {
 	if len(voters) == 0 {
 		return nil, ErrMissingNodes
 	}
@@ -89,7 +99,7 @@ func newTransaction(id uint64, voters []Voter, threshold uint) (*Transaction, er
 		return nil, ErrInvalidThreshold
 	}
 
-	return &Transaction{
+	return &transaction{
 		id:        id,
 		threshold: threshold,
 		voters:    voters,
@@ -97,7 +107,7 @@ func newTransaction(id uint64, voters []Voter, threshold uint) (*Transaction, er
 	}, nil
 }
 
-func (t *Transaction) cancel() {
+func (t *transaction) cancel() {
 	t.lock.Lock()
 	defer t.lock.Unlock()
 
@@ -108,7 +118,7 @@ func (t *Transaction) cancel() {
 	t.state = transactionCanceled
 }
 
-func (t *Transaction) stop() error {
+func (t *transaction) stop() error {
 	t.lock.Lock()
 	defer t.lock.Unlock()
 
@@ -123,14 +133,14 @@ func (t *Transaction) stop() error {
 }
 
 // ID returns the identifier used to uniquely identify a transaction.
-func (t *Transaction) ID() uint64 {
+func (t *transaction) ID() uint64 {
 	return t.id
 }
 
 // State returns the voting state mapped by voters. A voting state of `true`
 // means all subtransactions were successful, a voting state of `false` means
 // either no subtransactions were created or any of the subtransactions failed.
-func (t *Transaction) State() (map[string]VoteResult, error) {
+func (t *transaction) State() (map[string]VoteResult, error) {
 	t.lock.Lock()
 	defer t.lock.Unlock()
 
@@ -165,7 +175,7 @@ func (t *Transaction) State() (map[string]VoteResult, error) {
 
 // CountSubtransactions counts the number of subtransactions created as part of
 // the transaction.
-func (t *Transaction) CountSubtransactions() int {
+func (t *transaction) CountSubtransactions() int {
 	t.lock.Lock()
 	defer t.lock.Unlock()
 
@@ -176,7 +186,7 @@ func (t *Transaction) CountSubtransactions() int {
 // node hasn't yet voted on or creates a new one if the node has succeeded on
 // all subtransactions. In case the node has failed on any of the
 // subtransactions, an error will be returned.
-func (t *Transaction) getOrCreateSubtransaction(node string) (*subtransaction, error) {
+func (t *transaction) getOrCreateSubtransaction(node string) (*subtransaction, error) {
 	t.lock.Lock()
 	defer t.lock.Unlock()
 
@@ -234,7 +244,7 @@ func (t *Transaction) getOrCreateSubtransaction(node string) (*subtransaction, e
 	return subtransaction, nil
 }
 
-func (t *Transaction) vote(ctx context.Context, node string, hash []byte) error {
+func (t *transaction) vote(ctx context.Context, node string, hash []byte) error {
 	subtransaction, err := t.getOrCreateSubtransaction(node)
 	if err != nil {
 		return err