_dispatchWaiters decided whether a waiter was satisfied by clipping its
range against dls.src.Size(), the size of the fs.Object snapshot taken
when the Downloaders was created. _ensureDownloader decided whether to
start a downloader from Item.FindMissing, which clips against
item.info.Size instead.
When item.info.Size dropped below the offset a waiter was parked on while
the source object still reported the full size, the two disagreed.
_ensureDownloader found nothing missing so it started no downloader, and
_dispatchWaiters found the range absent so it never released the waiter.
Nothing was downloaded and no error was produced, so the error count never
reached maxErrorCount and the waiter was never woken. The reader blocked
forever with nothing logged at any level.
Wake a waiter when FindMissing reports nothing left to download for it as
well as when its data has arrived. Since _ensureDownloader starts a
downloader only when FindMissing is non empty, a waiter with nothing
missing has nothing that could ever wake it.
Fixes#9769
DaveandNick Craig-Wood
6e0c71bd27