From a Rob Pike pass over the tree:
- singleOnce adds deltas to d.completed like pump does, instead of
storing absolute offsets, so the counter has one write discipline.
- one overMirrors helper replaces the three copy-pasted mirror-fallover
loops in probeRetry, fetchSeg and single.
- failedDownload becomes a pointer, the only value-type Download impl, so
Engine.untrack's == is always pointer identity and can't compare the
fields of a value (keying on Stat().ID is out — a torrent's ID changes
from source to infohash once metadata loads).
a .torrent fetched over http is added to the shared client in a second
pass (followUps); two that name the same torrent would share one torrent
on the refcount-less client, and the first to finish would Drop it out
from under the other, stranding it at 0%. dedupe the followed files by
infohash, the guard build() already applies to pass-1 sources.
an idle worker now steals the back half of the busiest in-flight segment
instead of exiting, so one slow segment no longer drains alone over a single
connection. control file persists stolen tails; resume re-tiles.
add --auto-split (got-only, opt-in): derive the connection count from file
size (one per min-split-size, up to 16), overriding -x/-s. default stays
aria2-faithful at 1 connection for a single server.
