Before this change backends which supported more than one hash (eg
pcloud) or backends which wrapped backends supporting more than one
hash (combine) would fail the TestMultithreadCopy and
TestMultithreadCopyAbort with an error like
Failed to make new multi hasher: requested set 000001ff contains unknown hash types
This was caused by the tests limiting the globally available hashes to
the first hash supplied by the backend.
This was added in this commit
d5d28a7513 operations: fix overwrite of destination when multi-thread transfer fails
to overcome the tests taking >100s on the local backend because they
made every single hash that the local backend. It brought this time
down to 20s.
This commit fixes the problem and retains the CPU speedup by only
applying the fix from the original commit if the destination backend
is the local backend. This fixes the common case (testing on the local
backend). This does not fix the problem for a backend which wraps the
local backend (eg combine) but this is run only on the integration
test machine and not on all the CI.
d068e0b1a9