# Execution: fix the kvm2 code-review findings
Plan: [docs/plans/2026-09-17-1515-kvm2-review-fixes.md](../plans/2026-09-17-1515-kvm2-review-fixes.md)

- [x] Step 0 — Save plan into the repo
- [x] Step 1 — H1: a stalled write must not delay the teardown

Remaining: the other 51 findings in `review/REPORT.md`, in severity order — H2 (the
`StorageVolUpload` stream wedge), H3 (`address()` swallowing errors), H4 (the shared base
image cache), then the mediums. Each gets its own checklist line here when it is planned,
so this list stays a record of decided work rather than a wish.

## Step 0 — plan committed

Copied out of `~/.claude/plans/` and committed alone before touching code, as the repo's
plan rules require.

## Step 1 — H1: close the SSH client, and signal dead before killing

Two edits, both small, in the order the bug unfolds.

`sshSocket.Close` was `errors.Join(s.Conn.Close(), s.client.Close())`. Go evaluates those
arguments left to right, so the blocking close ran before the unblocking one. `s.Conn` is
the forwarded libvirt socket, and closing an SSH channel writes a close message under
`ch.writeMu`, which go-libvirt already holds while streaming a base image through the same
channel. Against a host that vanished without RST or FIN, that write blocks until the
kernel stops retransmitting — `tcp_retries2`, about 15 minutes. `s.client.Close()`, a bare
`net.Conn.Close()` on the TCP socket that takes no lock and unblocks the stalled writer
through netpoll, was never reached. It now closes the client only.

**Dropping the channel close rather than reordering it** was the judgment call. Reordering
works, but afterwards the channel close returns an error — `net.ErrClosed`, a
`*net.OpError`, the transport's sticky write error, or `io.EOF` once `mux.loop` has run
`ch.close()` — and `closeErr` is memoised, so a filter that missed a case would poison
every later `Close` and break `ssh_test.go:66,69`, which assert both closes return nil.
Nothing wants the polite channel-close message: `dialSSHSocket` builds the client solely
for this one channel, the `sshSocket` owns it exclusively, and `dialSSHSocket`'s own error
paths already tear down by closing the client.

`link.markDead` then compounded it by running `kill()` before `close(l.closed)`, and doing
both inside a `sync.Once`. Reordering alone would not have been enough: `sync.Once.Do`
blocks concurrent callers until the first body returns, so every other `markDead` — including
the one in `call`'s own ctx-cancel branch — queued behind the stalled kill. **Cancelling the
reconcile did not help.** The `once` field is gone; the mutex the struct already had now
guards the "close it exactly once" invariant, `reason` is set before the signal so a
released caller always reads a populated `deadReason()`, and `kill` runs outside the lock.

**A second route to the same pin, found while planning and fixed by the same edits.**
go-libvirt's `Disconnect` closes the `net.Conn` it was handed (`libvirt.go:331` →
`socket.go:164`) — and that conn *is* our `*sshSocket`. So `libvirtBackend.Close`'s
`Disconnect` goroutine entered `sshSocket.closeOnce` and hung there; the 5 s `closeBudget`
released `Close`, which called `markDead("closed")` → `kill()` → `sshSocket.Close()` →
blocked in the other goroutine's `sync.Once`. The budget did not protect it.

The precondition was verified in the pinned x/crypto v0.54.0 rather than assumed: the
flow-control park (`remoteWin.reserve`, `channel.go:268`) happens *outside* `writeMu`, and
only `writePacket` holds it across the TCP write. So a writer parked in SSH flow control
never blocks the close; a writer inside the TCP write always does. Since the channel window
is 2 MiB (`channelWindowSize = 64 * channelMaxPacket`), reproducing it by hand needs a
kernel send buffer that fills first — shrink `SO_SNDBUF` well under 2 MiB.

The older kvm provider does not have this bug: it closes `client` only
(`internal/provider/kvm/conn.go:429,456`) and never touches the forwarded channel. kvm2's
rewrite introduced it.

### Tests: planned four, shipped one

The first draft had three hermetic tests plus a `//go:build integration && linux`
stalled-write test with a TCP proxy and shrunken socket buffers. The user pushed back —
once the bug is fixed the connection closes properly, so what is left to test? — and they
were right about most of it. `TestDial_forwardsUnixSocket` already covers "Close tears the
connection down", and after the fix `Close` is a single call with no ordering to get wrong.
The stalled-write test was the worst of the four: ~150 lines of Linux-only,
buffer-autotuning-dependent scaffolding whose own false-green guard made it *skip* whenever
the kernel did not cooperate, to demonstrate a bug that reading x/crypto settles and that
the fix deletes the code path for.

What survived is `TestMarkDead_slowKillDoesNotBlockCallers`, because the `markDead` change
*adds* a guarantee that did not exist before and nothing else touches it —
`TestCall_killedByAnotherCallerFailsTransient` passes a nil kill, so it cannot see the
property. It is also the plausible regression: restoring the `sync.Once` looks like a
correctness improvement. The call's own `fn` signals when it is in flight, so the parked
caller is genuinely parked rather than refused by the dead check on the way in.

The step that actually validates a regression test is running it against the unfixed code.
`markDead` was reverted in place, the test run, and the fixed version restored from a copy:

```bash
cp internal/provider/kvm2/backend.go "$SCRATCH/backend.go.fixed"
# ... revert markDead to the sync.Once form and re-add the once field ...
go test -race -count=1 -run 'TestMarkDead_slowKillDoesNotBlockCallers' ./internal/provider/kvm2/
#   --- FAIL: TestMarkDead_slowKillDoesNotBlockCallers (1.00s)
#       backend_test.go:186: link not signalled dead while its kill is still running
cp "$SCRATCH/backend.go.fixed" internal/provider/kvm2/backend.go
```

It fails in a second with the right diagnostic rather than hanging the suite, because the
blocked kill is released by `t.Cleanup`.

```bash
go build ./... && go vet ./... && go test -race -count=1 ./internal/provider/kvm2/
```

### Worth noting

**An accepted risk, deliberately not covered by a test.** Nothing now stops someone
re-adding `s.Conn.Close()` to `sshSocket.Close` as tidiness; it would look harmless and
every test would still pass, because in a healthy test the channel close returns instantly.
The guard is the doc comment on `sshSocket`, which says outright that closing the channel
would queue behind a stalled transfer. That is in idiom for this package — `hosts.go:25-34`
and `create.go:151-153` carry the same kind of hard-won decision — but it is a comment, not
a test, and the failure it prevents is a 15-minute operator-wide stall.

**Left for later, on purpose.** `ReconciliationTimeout` in `SetupWithManager` does not exist
in this repo at all; adding a per-reconcile deadline would change the contract for every
provider, since `hosts.go` and `image.go` both cite "no caller deadline", so it needs its own
change and its own spec entry. `sshSocket.keepalive` still has no test (sweep finding S1.2).

**Docs.** `docs/kvm2-provider.md` said the adapter "kills the connection (socket and SSH
client)", which is no longer true and was the description that made the bug look correct.
It now states the order (mark dead, then kill) and why both steps are lock-free.
`docs/kvm2-provider-spec.md` entries 18 and 19 were checked and are not falsified.

**No CHANGELOG entry yet** — the repo's rule is to add one once the user confirms the change
works, so it lands with a later "validate and record" step, as in the kvm campaign.