bugfix: Fix a race condition when creating guest accounts (#882)

* 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.

* Update postgres

* Typo

* Actually use the txn when creating accounts

* bugfix for database is locked on guest reg

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

@@ -91,10 +91,10 @@ 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
+	stmt := txn.Stmt(s.insertAccountStmt)
 
 	var err error
 	if appserviceID == "" {
@@ -146,8 +146,12 @@ 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
 }