Open() touched several pieces of host state before hitting the step
that returned the error:
* SQLite handle (store.Open)
* managed SSH client config block (ensureVMSSHClientConfig)
* vm-DNS UDP listener goroutine (startVMDNS)
* systemd-resolved per-interface routing (ensureVMDNSResolverRouting)
The only deferred cleanup guarded stopVMDNS. A reconcile() or
initializeTapPool() failure therefore left the listener running, the
resolver wiring in place, and the SQLite handle open. A subsequent
startup attempt ran into "port 42069 already in use" or silently
published stale state.
Fix: once `d` exists, defer `d.Close()` on `err != nil`. Close is
idempotent (sync.Once) and every teardown step (listener close, DNS
listener close, resolver revert, session registry close, store close)
is nil-guarded, so calling it on a daemon that never got past the
first startup step is safe.
Tests (internal/daemon/open_close_test.go):
- TestCloseOnPartiallyInitialisedDaemon: Close survives a daemon
with only store + closing channel, and with a vmDNS listener but
nothing else. Catches regressions where a teardown step forgets
to nil-check.
- TestCloseIdempotentUnderConcurrency: 5 goroutines racing on
Close() never panic (sync.Once + close(d.closing) survive).
- TestOpenFailureRunsCloseCleanup: structural check that the
`defer cleanup() if err != nil` pattern actually fires.
Live: `banger daemon stop` cleanly, `banger vm ls` restarts daemon
without a residual listener on port 42069.
