R3 final pass surfaced one issue, addressed:
R2#2 introduced PID-suffixed <target>.{pid}.tmp / .{pid}.partN to
prevent concurrent agnes pull invocations from yanking each other's
in-progress writes. The pre-clean inside _download_chunked /
_download_single_stream only deletes leftovers from the CURRENT
process's PID — files from a SIGKILL'd or crashed prior pull (any
other PID) are never touched and accumulate on disk forever.
Add _reap_dead_pid_leftovers(target_path) called at the start of both
download paths. Globs <target>.*.tmp / <target>.*.partN, extracts the
embedded PID, calls os.kill(pid, 0) to test liveness (POSIX standard
no-op probe), and unlinks files whose process no longer exists.
Permission-denied = process is alive but owned by another user → keep
the file (conservative). Windows users get the conservative 'keep'
default.
Two new tests pin the behavior — live-PID file preserved, dead-PID
.tmp + .partN reaped, bare-name (legacy) untouched, garbage filenames
skipped without raise.
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