The retry code path did not filter `ERROR_NOT_SUPPORTED`. Just call the
original function a second time to correctly follow the low privilege
code path.
Calling `Load()` twice for an atomic variable can return different
values each time. This resulted in trying to read the security
descriptor with high privileges, but then not entering the code path to
switch to low privileges when another thread has already done so
concurrently.
Failed locking attempts were immediately retried up to three times
without any delay between the retries. If a lock file is not found while
checking for other locks, with the reworked backend retries there is no
delay between those retries. This is a problem if a backend requires a
few seconds to reflect file deletions in the file listings. To work
around this problem, introduce a short exponentially increasing delay
between the retries. The number of retries is now increased to 4. This
results in delays of 5, 10 and 20 seconds between the retries.
When the context used for a load operation is canceled, then the result
is always an error independent of whether the file could be retrieved
from the backend. Do not false positively trip the circuit breaker in
this case.
The old behavior was problematic when trying to lock a repository. When
`Lock.checkForOtherLocks` listed multiple lock files in parallel and one
of them fails to load, then all other loads were canceled. This
cancelation was remembered by the circuit breaker, such that locking
retries would fail.
* removes files which are no longer in the repository, including index files, snapshot files and pack files from the cache.
cache: fix ids set initialisation with NewIDSet()
Files were not included in the backup if the extended metadata for the
file could not be read. This is rather drastic. Instead settle on
returning a warning but still including the file in the backup.
This keeps backwards compatibility with the previous empty structs.
And maybe we'd want to put other fields into the inner struct later,
rather than the outer message.
Previously, an error JSON fragment would look like:
{"message_type": "error", "error": {}}
This is because encoding/json cannot marshal an error interface.
Instead, we now call .Error() to get the string value.
