The receiver blocked in read until a peer sent a header, and a stalled
btrfs receive was never reaped, so a silent or stuck connection held a
root service instance indefinitely. The receiver now gives the header 30 s
and the unit has RuntimeMaxSec=6h.
The sender had no deadline on the TCP connect or the header write, so an
unreachable mesh address held the admin-socket handler until the kernel
gave up. Both now time out after 15 s and report it like any other
failure. The stream body stays unbounded on the sender: the receiver's
RuntimeMaxSec already caps it, and btrfs send writes it outside this
process.
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