When the receiver cancelled a transfer partway through, the sender kept
uploading the remaining files of that batch (the loop continued past the
failure), and a newly started transfer never stopped the old goroutine —
so the old, cancelled transfer appeared to "carry on".
Fixes:
- Each send now runs under a cancellable context. A new transfer to a peer
supersedes (cancels) any still-running send to that same peer.
- A network/session error mid-batch aborts the rest of the batch, emitting
a clean Cancel for the un-sent files instead of pushing them anyway. A
local file-open error still skips only that file.
- Bounded connection timeouts (dial 10s, TLS 10s, response-header 30s) so a
vanished peer fails fast; the overall timeout stays off for large files.
Test: a receiver whose session is gone (returns 403) causes the sender to
make exactly one upload attempt and then abort, reporting the rest cancelled.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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