fix: avoid session leak on transaction retry (#300) (#301)

When `(*readWriteTransaction).retry()` is used, there is already an existing pending (errored) transaction. It gets overwritten with the new transaction object without closing the old one.

This change causes the old transaction to be closed (rollback'd) before the new one overwrites it, thus avoiding a potentially large session leak.

aborted_transactions_test.go

@@ -19,6 +19,7 @@ import (
 	"database/sql"
 	"reflect"
 	"testing"
+	"time"
 
 	"cloud.google.com/go/spanner"
 	sppb "cloud.google.com/go/spanner/apiv1/spannerpb"
@@ -31,10 +32,11 @@ import (
 func TestCommitAborted(t *testing.T) {
 	t.Parallel()
 
-	db, server, teardown := setupTestDBConnection(t)
+	db, server, teardown := setupTestDBConnectionWithParams(t, "minSessions=1;maxSessions=1")
 	defer teardown()
 
-	ctx := context.Background()
+	ctx, cancel := context.WithTimeout(context.Background(), time.Second)
+	defer cancel()
 	tx, err := db.BeginTx(ctx, &sql.TxOptions{})
 	if err != nil {
 		t.Fatalf("begin failed: %v", err)
@@ -56,10 +58,11 @@ func TestCommitAborted(t *testing.T) {
 func TestCommitAbortedWithInternalRetriesDisabled(t *testing.T) {
 	t.Parallel()
 
-	db, server, teardown := setupTestDBConnectionWithParams(t, "retryAbortsInternally=false")
+	db, server, teardown := setupTestDBConnectionWithParams(t, "retryAbortsInternally=false;minSessions=1;maxSessions=1")
 	defer teardown()
 
-	ctx := context.Background()
+	ctx, cancel := context.WithTimeout(context.Background(), time.Second)
+	defer cancel()
 	tx, err := db.BeginTx(ctx, &sql.TxOptions{})
 	if err != nil {
 		t.Fatalf("begin failed: %v", err)
@@ -82,10 +85,11 @@ func TestCommitAbortedWithInternalRetriesDisabled(t *testing.T) {
 func TestUpdateAborted(t *testing.T) {
 	t.Parallel()
 
-	db, server, teardown := setupTestDBConnection(t)
+	db, server, teardown := setupTestDBConnectionWithParams(t, "minSessions=1;maxSessions=1")
 	defer teardown()
 
-	ctx := context.Background()
+	ctx, cancel := context.WithTimeout(context.Background(), time.Second)
+	defer cancel()
 	tx, err := db.BeginTx(ctx, &sql.TxOptions{})
 	if err != nil {
 		t.Fatalf("begin failed: %v", err)
@@ -122,10 +126,11 @@ func TestUpdateAborted(t *testing.T) {
 func TestBatchUpdateAborted(t *testing.T) {
 	t.Parallel()
 
-	db, server, teardown := setupTestDBConnection(t)
+	db, server, teardown := setupTestDBConnectionWithParams(t, "minSessions=1;maxSessions=1")
 	defer teardown()
 
-	ctx := context.Background()
+	ctx, cancel := context.WithTimeout(context.Background(), time.Second)
+	defer cancel()
 	tx, err := db.BeginTx(ctx, &sql.TxOptions{})
 	if err != nil {
 		t.Fatalf("begin failed: %v", err)
@@ -359,13 +364,14 @@ func testRetryReadWriteTransactionWithQuery(t *testing.T, setupServer func(serve
 
 	t.Parallel()
 
-	db, server, teardown := setupTestDBConnection(t)
+	db, server, teardown := setupTestDBConnectionWithParams(t, "minSessions=1;maxSessions=1")
 	defer teardown()
 
 	if setupServer != nil {
 		setupServer(server.TestSpanner)
 	}
-	ctx := context.Background()
+	ctx, cancel := context.WithTimeout(context.Background(), time.Second)
+	defer cancel()
 	tx, err := db.BeginTx(ctx, &sql.TxOptions{})
 	if err != nil {
 		t.Fatalf("begin failed: %v", err)

transaction.go

@@ -246,6 +246,10 @@ func (tx *readWriteTransaction) runWithRetry(ctx context.Context, f func(ctx con
 // retry retries the entire read/write transaction on a new Spanner transaction.
 // It will return ErrAbortedDueToConcurrentModification if the retry fails.
 func (tx *readWriteTransaction) retry(ctx context.Context) (err error) {
+	// TODO: This should use t.ResetForRetry(ctx) instead when that function is available.
+	if tx.rwTx != nil {
+		tx.rwTx.Rollback(tx.ctx)
+	}
 	tx.rwTx, err = spanner.NewReadWriteStmtBasedTransaction(ctx, tx.client)
 	if err != nil {
 		return err