purgeDataObjects(), which runs at the beginning of the sync process,
uses transactions, and those can fail after the default wait timeout if
there's another active transaction. Instead, check explicitly for
another transaction and, if there is one, display a nice message and
wait for it to finish. This isn't foolproof, but it should reduce the
frequency of "operation timed out" sync errors. (Avoiding all long
transactions would be a better solution.)
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