bugfix: Fix a race condition when creating guest accounts

It was possible to both select the same next numeric ID and then both
attempt to INSERT this into the table. This would cause a UNIQUE violation
which then presented itself as an error in sqlite because it does not
implement `common.IsUniqueConstraintViolationErr`.

The fix here is NOT to implement `common.IsUniqueConstraintViolationErr`
otherwise the 2 users would get the SAME guest account. Instead, all of
these operations should be done inside a transaction. This is what this
PR does.

clientapi/auth/storage/accounts/interface.go

@@ -30,6 +30,7 @@ type Database interface {
 	SetAvatarURL(ctx context.Context, localpart string, avatarURL string) error
 	SetDisplayName(ctx context.Context, localpart string, displayName string) error
 	CreateAccount(ctx context.Context, localpart, plaintextPassword, appserviceID string) (*authtypes.Account, error)
+	CreateGuestAccount(ctx context.Context) (*authtypes.Account, error)
 	UpdateMemberships(ctx context.Context, eventsToAdd []gomatrixserverlib.Event, idsToRemove []string) error
 	GetMembershipInRoomByLocalpart(ctx context.Context, localpart, roomID string) (authtypes.Membership, error)
 	GetMembershipsByLocalpart(ctx context.Context, localpart string) (memberships []authtypes.Membership, err error)

clientapi/auth/storage/accounts/postgres/storage.go

@@ -118,6 +118,10 @@ func (d *Database) SetDisplayName(
 	return d.profiles.setDisplayName(ctx, localpart, displayName)
 }
 
+func (d *Database) CreateGuestAccount(ctx context.Context) (*authtypes.Account, error) {
+	return nil, nil
+}
+
 // CreateAccount makes a new account with the given login name and password, and creates an empty profile
 // for this account. If no password is supplied, the account will be a passwordless account. If the
 // account already exists, it will return nil, nil.

clientapi/auth/storage/accounts/sqlite3/account_data_table.go

@@ -72,10 +72,9 @@ func (s *accountDataStatements) prepare(db *sql.DB) (err error) {
 }
 
 func (s *accountDataStatements) insertAccountData(
-	ctx context.Context, localpart, roomID, dataType, content string,
+	ctx context.Context, txn *sql.Tx, localpart, roomID, dataType, content string,
 ) (err error) {
-	stmt := s.insertAccountDataStmt
+	_, err = txn.Stmt(s.insertAccountDataStmt).ExecContext(ctx, localpart, roomID, dataType, content)
-	_, err = stmt.ExecContext(ctx, localpart, roomID, dataType, content)
 	return
 }
 

clientapi/auth/storage/accounts/sqlite3/accounts_table.go

@@ -89,16 +89,16 @@ func (s *accountsStatements) prepare(db *sql.DB, server gomatrixserverlib.Server
 // this account will be passwordless. Returns an error if this account already exists. Returns the account
 // on success.
 func (s *accountsStatements) insertAccount(
-	ctx context.Context, localpart, hash, appserviceID string,
+	ctx context.Context, txn *sql.Tx, localpart, hash, appserviceID string,
 ) (*authtypes.Account, error) {
 	createdTimeMS := time.Now().UnixNano() / 1000000
 	stmt := s.insertAccountStmt
 
 	var err error
 	if appserviceID == "" {
-		_, err = stmt.ExecContext(ctx, localpart, createdTimeMS, hash, nil)
+		_, err = txn.Stmt(stmt).ExecContext(ctx, localpart, createdTimeMS, hash, nil)
 	} else {
-		_, err = stmt.ExecContext(ctx, localpart, createdTimeMS, hash, appserviceID)
+		_, err = txn.Stmt(stmt).ExecContext(ctx, localpart, createdTimeMS, hash, appserviceID)
 	}
 	if err != nil {
 		return nil, err
@@ -144,8 +144,12 @@ func (s *accountsStatements) selectAccountByLocalpart(
 }
 
 func (s *accountsStatements) selectNewNumericLocalpart(
-	ctx context.Context,
+	ctx context.Context, txn *sql.Tx,
 ) (id int64, err error) {
-	err = s.selectNewNumericLocalpartStmt.QueryRowContext(ctx).Scan(&id)
+	stmt := s.selectNewNumericLocalpartStmt
+	if txn != nil {
+		stmt = txn.Stmt(stmt)
+	}
+	err = stmt.QueryRowContext(ctx).Scan(&id)
 	return
 }

clientapi/auth/storage/accounts/sqlite3/profile_table.go

@@ -73,9 +73,9 @@ func (s *profilesStatements) prepare(db *sql.DB) (err error) {
 }
 
 func (s *profilesStatements) insertProfile(
-	ctx context.Context, localpart string,
+	ctx context.Context, txn *sql.Tx, localpart string,
 ) (err error) {
-	_, err = s.insertProfileStmt.ExecContext(ctx, localpart, "", "")
+	_, err = txn.Stmt(s.insertProfileStmt).ExecContext(ctx, localpart, "", "")
 	return
 }
 

clientapi/auth/storage/accounts/sqlite3/storage.go

@@ -18,6 +18,7 @@ import (
 	"context"
 	"database/sql"
 	"errors"
+	"strconv"
 
 	"github.com/matrix-org/dendrite/clientapi/auth/authtypes"
 	"github.com/matrix-org/dendrite/common"
@@ -118,14 +119,36 @@ func (d *Database) SetDisplayName(
 	return d.profiles.setDisplayName(ctx, localpart, displayName)
 }
 
+func (d *Database) CreateGuestAccount(ctx context.Context) (acc *authtypes.Account, err error) {
+	err = common.WithTransaction(d.db, func(txn *sql.Tx) error {
+		numLocalpart, err := d.accounts.selectNewNumericLocalpart(ctx, txn)
+		if err != nil {
+			return err
+		}
+		localpart := strconv.FormatInt(numLocalpart, 10)
+		acc, err = d.createAccount(ctx, txn, localpart, "", "")
+		return err
+	})
+	return acc, err
+}
+
 // CreateAccount makes a new account with the given login name and password, and creates an empty profile
 // for this account. If no password is supplied, the account will be a passwordless account. If the
 // account already exists, it will return nil, nil.
 func (d *Database) CreateAccount(
 	ctx context.Context, localpart, plaintextPassword, appserviceID string,
+) (acc *authtypes.Account, err error) {
+	err = common.WithTransaction(d.db, func(txn *sql.Tx) error {
+		acc, err = d.createAccount(ctx, txn, localpart, plaintextPassword, appserviceID)
+		return err
+	})
+	return
+}
+
+func (d *Database) createAccount(
+	ctx context.Context, txn *sql.Tx, localpart, plaintextPassword, appserviceID string,
 ) (*authtypes.Account, error) {
 	var err error
-
 	// Generate a password hash if this is not a password-less user
 	hash := ""
 	if plaintextPassword != "" {
@@ -134,13 +157,14 @@ func (d *Database) CreateAccount(
 			return nil, err
 		}
 	}
-	if err := d.profiles.insertProfile(ctx, localpart); err != nil {
+	if err := d.profiles.insertProfile(ctx, txn, localpart); err != nil {
 		if common.IsUniqueConstraintViolationErr(err) {
 			return nil, nil
 		}
 		return nil, err
 	}
-	if err := d.SaveAccountData(ctx, localpart, "", "m.push_rules", `{
+
+	if err := d.accountDatas.insertAccountData(ctx, txn, localpart, "", "m.push.rules", `{
 		"global": {
 			"content": [],
 			"override": [],
@@ -151,7 +175,7 @@ func (d *Database) CreateAccount(
 	}`); err != nil {
 		return nil, err
 	}
-	return d.accounts.insertAccount(ctx, localpart, hash, appserviceID)
+	return d.accounts.insertAccount(ctx, txn, localpart, hash, appserviceID)
 }
 
 // SaveMembership saves the user matching a given localpart as a member of a given
@@ -258,7 +282,9 @@ func (d *Database) newMembership(
 func (d *Database) SaveAccountData(
 	ctx context.Context, localpart, roomID, dataType, content string,
 ) error {
-	return d.accountDatas.insertAccountData(ctx, localpart, roomID, dataType, content)
+	return common.WithTransaction(d.db, func(txn *sql.Tx) error {
+		return d.accountDatas.insertAccountData(ctx, txn, localpart, roomID, dataType, content)
+	})
 }
 
 // GetAccountData returns account data related to a given localpart
@@ -288,7 +314,7 @@ func (d *Database) GetAccountDataByType(
 func (d *Database) GetNewNumericLocalpart(
 	ctx context.Context,
 ) (int64, error) {
-	return d.accounts.selectNewNumericLocalpart(ctx)
+	return d.accounts.selectNewNumericLocalpart(ctx, nil)
 }
 
 func hashPassword(plaintext string) (hash string, err error) {

clientapi/routing/register.go

@@ -516,16 +516,7 @@ func handleGuestRegistration(
 	accountDB accounts.Database,
 	deviceDB devices.Database,
 ) util.JSONResponse {
-
+	acc, err := accountDB.CreateGuestAccount(req.Context())
-	//Generate numeric local part for guest user
-	id, err := accountDB.GetNewNumericLocalpart(req.Context())
-	if err != nil {
-		util.GetLogger(req.Context()).WithError(err).Error("accountDB.GetNewNumericLocalpart failed")
-		return jsonerror.InternalServerError()
-	}
-
-	localpart := strconv.FormatInt(id, 10)
-	acc, err := accountDB.CreateAccount(req.Context(), localpart, "", "")
 	if err != nil {
 		return util.JSONResponse{
 			Code: http.StatusInternalServerError,