When EXECUTE STATEMENT ... ON EXTERNAL DATA SOURCE was introiduced in v2.5.0 there was informal agreement that calls of
isc_attach_database should be serialized to avoid deadlocks in fbclient (reported by users from time to time, but never reproduced).
Such serialization is almost not visible by end users when all works as expected, in particular when remote hosts can be reached via network.
But when there is some network issues, or when remote address is not available there could be significant delays before isc_attach_database
returns corresponding error. Due to serialization of call of isc_attach_database by the engine all other attachments running EXECUTE STATEMENT
and connecting to the their remote databases should wait for the unlucky one.
Starting from v3 it seems we can remove this artificial limitation in the engine and allow all attachments to call isc_attach_database
in parallel without serialization.