bugfix for database is locked on guest reg

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

@@ -19,6 +19,7 @@ import (
 	"database/sql"
 	"errors"
 	"strconv"
+	"sync"
 
 	"github.com/matrix-org/dendrite/clientapi/auth/authtypes"
 	"github.com/matrix-org/dendrite/common"
@@ -40,6 +41,8 @@ type Database struct {
 	threepids    threepidStatements
 	filter       filterStatements
 	serverName   gomatrixserverlib.ServerName
+
+	createGuestAccountMu sync.Mutex
 }
 
 // NewDatabase creates a new accounts and profiles database
@@ -77,7 +80,7 @@ func NewDatabase(dataSourceName string, serverName gomatrixserverlib.ServerName)
 	if err = f.prepare(db); err != nil {
 		return nil, err
 	}
-	return &Database{db, partitions, a, p, m, ac, t, f, serverName}, nil
+	return &Database{db, partitions, a, p, m, ac, t, f, serverName, sync.Mutex{}}, nil
 }
 
 // GetAccountByPassword returns the account associated with the given localpart and password.
@@ -123,6 +126,13 @@ func (d *Database) SetDisplayName(
 // for this account.
 func (d *Database) CreateGuestAccount(ctx context.Context) (acc *authtypes.Account, err error) {
 	err = common.WithTransaction(d.db, func(txn *sql.Tx) error {
+		// We need to lock so we sequentially create numeric localparts. If we don't, two calls to
+		// this function will cause the same number to be selected and one will fail with 'database is locked'
+		// when the first txn upgrades to a write txn.
+		// We know we'll be the only process since this is sqlite ;) so a lock here will be all that is needed.
+		d.createGuestAccountMu.Lock()
+		defer d.createGuestAccountMu.Unlock()
+
 		var numLocalpart int64
 		numLocalpart, err = d.accounts.selectNewNumericLocalpart(ctx, txn)
 		if err != nil {