if a copyNAR generator was not drained to completion it would not read
the full nar data from its source. this could happen if the copier was
passed to parseAndDump wrapped as a source because copyNAR would yield
nar metadata *before* it had read it, and GeneratorSource will drain a
generator fully *only* if the source is allowed to throw EndOfFile. in
the parseAndDump case this never happened because parseAndDump expects
to be given an unterminated stream, and thus the combination left some
nar metadata in the input Source, breaking the remote store protocols.
fixes#732
Change-Id: Ia59a53375992bfcdb7bc6b37764ca779622bc8f7
eldritch horrors
27d5209f4d