When the tomb is created with a canceled context, then the workers
started via `t.Go` exist nearly immediately. Once for the first time all
started goroutines have been stopped, it is not allowed to issue further
calls to `t.Go`. This is a problem when the started goroutines exit
immediately, as for example the first goroutine might already have
stopped before starting the second one, which is not allowed as once the
first goroutines has stopped no goroutines were running.
To fix this race condition the startup and main task of the archiver now
also run within a `t.Go` function. This also allows unifying the error
handling as it is no longer necessary to distinguish between errors
returned by the workers or the saveTree processing. The tomb now just
returns the first error encountered, which should also be the most
descriptive one.
Depending on the used backend, operations started with a canceled
context may fail or not. For example the local backend still works in
large parts when called with a canceled context. Backends transfering
data via http don't work. It is also not possible to retry failed
operations in that state as the RetryBackend will abort with a 'context
canceled' error.
Ensure uniform behavior of all backends by checking for a canceled
context by checking for a canceled context as a first step in the
RetryBackend. This ensures uniform behavior across all backends, as
backends are always wrapped in a RetryBackend.
If the context was canceled then saveTree might receive a treeID or not
depending on the timing. This could cause saveTree to incorrectly return
a nil treeID as valid. Fix this always returning an error when the
context was canceled in the meantime.
A canceled background context lets the blob/tree/fileSavers exit
without reporting an error. The error handling previously replaced
a 'context canceled' error received by the main backup method with
the error reported by the savers. However, in case of a canceled
background context that error is nil, causing restic to loose the
error and save a snapshot with a nil tree.
