cmd/main.go

@@ -146,8 +146,9 @@ func main() {
 		"Interval between provider health checks (can the cluster reach each hypervisor, API "+
 			"and account), published as ProviderHealth objects and metrics. 0 turns them off; "+
 			"otherwise at least 10s.")
-	flag.DurationVar(&providerCheckTimeout, "provider-check-timeout", 15*time.Second,
-		"Time limit for one round of provider health checks; at most the interval.")
+	flag.DurationVar(&providerCheckTimeout, "provider-check-timeout", 0,
+		"Time limit for one round of provider health checks: 15s, or the interval when that is "+
+			"shorter. If set, at most the interval.")
 
 	opts := zap.Options{
 		Development: true,

config/manager/manager.yaml

@@ -68,7 +68,8 @@ spec:
           # (high volume: ~1/s per proxy).
           # Provider health checks (kubectl get providerhealth) run every
           # minute; tune with --provider-check-interval (0 turns them off,
-          # otherwise at least 10s) and --provider-check-timeout (15s).
+          # otherwise at least 10s) and --provider-check-timeout (15s, or
+          # the interval if that is shorter).
         env:
           # Bearer token for the discovery API. Optional: without the
           # Secret the API serves unauthenticated (with a loud warning).

docs/architecture.md

@@ -378,7 +378,8 @@ target and never fails as a whole. All three providers do:
 
 ```text
 Monitor.Start(ctx)   ── leader-only; --provider-check-interval (1m; 0 = off,
-  │                     otherwise ≥ 10s), --provider-check-timeout (15s)
+  │                     otherwise ≥ 10s), --provider-check-timeout (15s, or
+  │                     the interval if that is shorter)
   ├─ once: delete managed-by-labelled ProviderHealth objects whose provider
   │   is not configured (all of them when checks are off) — the providers
   │   config is read once per process
@@ -598,8 +599,13 @@ they read as choices, not accidents. Chronological by build step.
   fresh dial would not pick up a rotated Secret anyway, since the SSH
   material is cached. It would also open an sshd session per host per
   minute. And a working cached connection is exactly what reconciles use.
-  The cost is that a check against a hung host drops the shared
-  connection, the same rule every cancelled reconcile already follows.
+  Sharing must not cost the reconciles anything, so the check calls
+  through `hostPool.probe`. A check that runs out of time abandons its
+  RPCs instead of closing the connection, which would kill an upload or a
+  Create running on it. At most one abandoned check per host is left
+  waiting, and until it returns, the next check reports `Timeout` at once.
+  Waiting for a host's connection slot also ends with the caller's
+  deadline, so a reconcile redialing a dead host cannot stall the round.
 - **Provider transitions are logs, not Events**, in line with the
   EventRecorder decision above. They log at Info with a `WARNING:` prefix,
   and each target's `lastTransitionTime` in the object records when it

docs/kvm-provider.md

@@ -292,7 +292,7 @@ shows its SSH `user`. A failing target says where the check broke:
 | `Unreachable` | the TCP connect failed, or the link died mid-check | DNS, Tailscale or a firewall between the cluster and `address` |
 | `SSHHandshakeFailed` | the host key is missing from known_hosts or has changed, or `user` rejected the key | the known_hosts entry; `user`'s `authorized_keys` |
 | `LibvirtConnectFailed` | SSH worked, the libvirt socket did not | `user` in the `libvirt` group, `libvirtd` running, the `socket` path |
-| `Timeout` | no answer within `--provider-check-timeout` (default 15s) | a hung `libvirtd` or a saturated link |
+| `Timeout` | no answer within `--provider-check-timeout` (default 15s), or the previous check of the host is still waiting | a hung `libvirtd`, a saturated link, or a reconcile stuck redialing the host |
 | `PoolNotFound` | there is no pool named `pool` | `virsh pool-list --all` |
 | `PoolNotActive` | the pool exists but is not running | `virsh pool-start <pool>` and `virsh pool-autostart <pool>` |
 | `LibvirtError` | any other libvirt error | the message carries libvirt's own text |
@@ -302,9 +302,14 @@ Sharing the connection has two consequences:
 - The leader now keeps one SSH connection per host open from startup, with a
   keepalive every 15s. Before, the first connection opened with the first KVM
   reconcile.
-- A check that hits its timeout drops the host's connection, just as a
-  cancelled reconcile already does. Calls in flight on a hung link then fail
-  fast, and the next call redials.
+- A check that runs out of time gives up on its RPCs but leaves the
+  connection alone, so an image upload or a Create running on it is not cut
+  off. At most one abandoned check per host keeps waiting; until it
+  returns, the next check reports `Timeout` straight away. A link that is
+  really dead is closed by the keepalive within about two keepalive
+  intervals.
+- A check never waits longer than its timeout for the host's connection,
+  even while a reconcile holds it to redial a host that is down.
 
 ## Known gaps
 

docs/plans-executions/2026-09-11-1526-provider-health-checks.md

@@ -312,3 +312,54 @@ contained `proxy_operator_provider_target_up{provider="kubernetes",target="apise
 That exercises the generated RBAC, the API-reader path and the `main.go` gauge wiring
 end to end. The two skipped specs are the Tempo-gated tracing specs; `TEMPO_URL` was not
 set.
+
+### Follow-up — the check must not cost the reconciles their connection
+
+Reviewing the shared-connection decision after the automated validation turned up two
+ways the health check could hurt the work it shares the link with, both of which the
+plan's "a check against a hung host drops the shared connection" line accepted too
+easily:
+
+- A check that timed out went through `hostPool.call`, whose cancel path drops the
+  connection. A `libvirtd` slow to answer one `ConnectGetLibVersion` would therefore
+  cut off an image upload or a `Create` running on the same link.
+- `hostEntry` was guarded by a `sync.Mutex`, so waiting for a host's slot could not be
+  interrupted. A reconcile redialing a host that is down holds the slot for as long as
+  the SSH dial takes — minutes on a blackholed address — and the whole check round
+  would wait behind it, past its own timeout.
+
+So the check now calls `hostPool.probe` instead of `call`: on `ctx` ending it abandons
+the RPC and leaves the connection alone (a link that is really dead is closed by the
+keepalive anyway, which ends the abandoned `fn` too). One abandoned probe per host is
+allowed to stay waiting — `hostEntry.probing` is an `atomic.Bool`, not a counter, so a
+daemon that never answers cannot pile up goroutines; further probes get the new
+`errProbeBusy`, which `checkReason` maps to `Timeout`. `hostEntry.mu` became a one-slot
+channel with an `acquire(ctx)` that only bounds *waiting*: an uncontended caller takes a
+free slot even with an expired context, so `call`'s behaviour is unchanged where nothing
+is contended.
+
+The second change came out of the same pass: `--provider-check-timeout` defaulted to a
+flat 15s while the flag validation rejected a timeout longer than the interval, so
+`--provider-check-interval=12s` alone failed to start. The flag now defaults to 0 and
+`Monitor.setDefaults` derives `min(15s, Interval)`; `ValidateSchedule` accepts 0 and
+rejects only negatives and timeouts past the interval.
+
+Re-verified with the same commands as above:
+
+```bash
+go vet ./...
+go test -race -count=1 ./internal/provider/kvm/ ./internal/providerhealth/
+make test
+# every package ok; internal/provider/kvm 92.2 %, internal/providerhealth 93.7 % coverage
+```
+
+Worth noting:
+- `make lint` still cannot run, for the toolchain reason recorded above; unchanged by
+  this follow-up.
+- The two new `kvm` tests are timing-shaped by necessity — they assert that a caller
+  with a 50ms deadline does not wait out a dial that is blocked on a channel. The
+  assertions are one-sided (`> 1s` fails) so a slow machine does not make them flaky.
+- `docs/architecture.md`, `docs/kvm-provider.md` and `docs/testing.md` were corrected in
+  the same commit: the old text promised the drop-on-timeout behaviour that is now gone.
+- The real-hypervisor commands from the plan's Verification section are still the user's
+  to run; nothing here replaces them.

docs/testing.md

@@ -60,7 +60,7 @@ manual runs too.
 | | `errors_test.go`, `conn_test.go` | libvirt-code classification table; connection pool (reuse, redial, drop on transport error, cancel releases a blocked RPC, one dial per host under concurrency); SSH dial, host-key policy, keepalive | fake dialer; a **real in-process `x/crypto/ssh` server** forwarding a unix socket |
 | | `image_test.go`, `kvm_test.go` | image cache (fresh fetch, ready, marker missing, checksum change/mismatch, 4xx/5xx, concurrent callers); Create resume after every partial step, leftover truncated seed, state/IP mapping, every Delete already-gone case, ListByTag across hosts / one down / unreadable domain; logging tiers (V0 silent, V1 one line per RPC, V2 XML) with cloud-init never logged | `httptest` image server + in-memory `fakeAPI` behind the `libvirtAPI` seam |
 | | `trace_test.go`, `conn_test.go`, `errors_test.go` | every `libvirtAPI` method traced exactly once (guarded by reflection on the interface), results and errors pass through, libvirt error fields, detail keys only at V2, upload content never logged; pool connect/drop lines, no wrapping below V1 | `fakeAPI` + `funcr` capture |
-| | `check_test.go`, `conn_test.go` | health check: details, per-host verdicts in config order, hosts checked concurrently, timeout, the reason table; an inactive or missing pool and daemon errors keep the connection; every dial stage tagged through the real SSH server, with classification unchanged | `fakeAPI`, fake dialers, in-process SSH server |
+| | `check_test.go`, `conn_test.go` | health check: details, per-host verdicts in config order, hosts checked concurrently, a timeout that keeps the connection and refuses a second check while the first still waits, no waiting behind a reconcile's dial, the reason table; `probe` abandons without closing, but still drops on a transport error; waiting for a host's slot ends with the caller's deadline; an inactive or missing pool and daemon errors keep the connection; every dial stage tagged through the real SSH server, with classification unchanged | `fakeAPI`, fake dialers, in-process SSH server |
 | | `kvm_integration_test.go` (`-tags=integration`) | one real VM: image fetch, Create, idempotent re-Create, Running with IP, ListByTag, Delete, pool state afterwards; `TestKVMCheck_realHost` runs the health check against the host and creates nothing | a **real hypervisor** — see below |
 | `internal/health` | `probe_test.go` | probes through a **real CONNECT-capable proxy stub** to a real TLS server: tunnel success, refused CONNECT, wrong status, dead proxy, plain-http forward | `httptest` + hijacked bidirectional tunnel |
 | | `engine_test.go` | thresholds, first-verdict, latency suppression matrix, dropped-event retry, stale-probe UID guard, tick scheduling/seeding/pruning, `Start` end-to-end | fake `client.Reader` + injected `probeFn` |

internal/provider/kvm/check.go

@@ -48,10 +48,12 @@ func (p *Provider) Check(ctx context.Context) []provider.TargetStatus {
 	return out
 }
 
-// checkHost returns the RPC errors from fn untouched: call drops the
-// shared connection on anything that is not a libvirt error, and a pool
-// that exists but is stopped says nothing about the link. facts is read
-// only after call returned nil, since an abandoned fn may still write it.
+// checkHost goes through probe, so a check that runs out of time abandons
+// its RPCs rather than closing the connection the reconciles share. fn
+// returns the RPC errors untouched, since a transport error still drops the
+// link; a pool that exists but is stopped says nothing about the link, so
+// that verdict is reached afterwards. facts is read only after probe
+// returned nil, since an abandoned fn may still write it.
 func (p *Provider) checkHost(ctx context.Context, host provider.KVMHost) provider.TargetStatus {
 	ts := provider.TargetStatus{
 		Name:    host.Name,
@@ -60,7 +62,7 @@ func (p *Provider) checkHost(ctx context.Context, host provider.KVMHost) provide
 	}
 	start := time.Now()
 	var facts hostFacts
-	err := p.hosts.call(ctx, p.logger(ctx), host, func(api libvirtAPI) error {
+	err := p.hosts.probe(ctx, p.logger(ctx), host, func(api libvirtAPI) error {
 		version, err := api.ConnectGetLibVersion()
 		if err != nil {
 			return err
@@ -125,7 +127,7 @@ func checkReason(err error) string {
 		}
 	}
 	switch {
-	case errors.Is(err, context.DeadlineExceeded), errors.Is(err, context.Canceled):
+	case errors.Is(err, context.DeadlineExceeded), errors.Is(err, context.Canceled), errors.Is(err, errProbeBusy):
 		return provider.ReasonTimeout
 	case hasCode(err, libvirt.ErrNoStoragePool):
 		return reasonPoolNotFound

internal/provider/kvm/check_test.go

@@ -7,6 +7,7 @@ import (
 	"io"
 	"maps"
 	"strings"
+	"sync"
 	"sync/atomic"
 	"testing"
 	"time"
@@ -174,14 +175,18 @@ func TestCheck_hostsCheckedConcurrently(t *testing.T) {
 	}
 }
 
-func TestCheck_timeout(t *testing.T) {
+// A daemon that stopped answering times the check out, but the connection
+// the reconciles share stays up, and the next check does not stack a
+// second hung RPC on top of the first.
+func TestCheck_timeoutKeepsTheConnection(t *testing.T) {
 	t.Parallel()
 	api := newFakeAPI("default")
 	block := make(chan struct{})
 	t.Cleanup(func() { close(block) })
 	blocked := &blockingAPI{fakeAPI: api, block: block}
+	var closed atomic.Bool
 	dial := func(context.Context, provider.KVMHost) (*hostClient, error) {
-		return &hostClient{api: blocked, close: func() {}}, nil
+		return &hostClient{api: blocked, close: func() { closed.Store(true) }}, nil
 	}
 	pc := testConfig("nas1")
 	p := newWithDialer(pc.Name, *pc.KVM, dial, nil)
@@ -193,6 +198,49 @@ func TestCheck_timeout(t *testing.T) {
 	if got[0].Ready || got[0].Reason != provider.ReasonTimeout {
 		t.Errorf("Check() = %+v, want Timeout", got[0])
 	}
+	if closed.Load() {
+		t.Error("the timed-out check closed the connection the reconciles share")
+	}
+	again := p.Check(context.Background())
+	if again[0].Reason != provider.ReasonTimeout || !strings.Contains(again[0].Message, "still waiting") {
+		t.Errorf("second Check() = %+v, want an immediate Timeout while the first still waits", again[0])
+	}
+}
+
+// A reconcile redialing a dead host holds the host's slot for minutes; the
+// check must give up at its own deadline rather than wait that out.
+func TestCheck_doesNotWaitBehindAReconcileDial(t *testing.T) {
+	t.Parallel()
+	release := make(chan struct{})
+	dialing := make(chan struct{})
+	var once sync.Once
+	dial := func(context.Context, provider.KVMHost) (*hostClient, error) {
+		once.Do(func() { close(dialing) })
+		<-release
+		return nil, errHostOff
+	}
+	pc := testConfig("nas1")
+	p := newWithDialer(pc.Name, *pc.KVM, dial, nil)
+	reconcile := make(chan error, 1)
+	go func() {
+		_, err := p.Get(context.Background(), formatProviderID("nas1", testName))
+		reconcile <- err
+	}()
+	<-dialing
+	ctx, cancel := context.WithTimeout(context.Background(), 50*time.Millisecond)
+	defer cancel()
+	start := time.Now()
+
+	got := p.Check(ctx)
+
+	if waited := time.Since(start); waited > time.Second {
+		t.Errorf("Check() took %s behind the reconcile's dial, want about 50ms", waited)
+	}
+	if got[0].Reason != provider.ReasonTimeout {
+		t.Errorf("Check() = %+v, want Timeout", got[0])
+	}
+	close(release)
+	<-reconcile
 }
 
 // blockingAPI hangs in ConnectGetLibVersion until block closes, like a
@@ -222,6 +270,7 @@ func TestCheckReason(t *testing.T) {
 			&dialError{stage: stageSSHHandshake, err: context.DeadlineExceeded}), reasonSSHHandshakeFailed},
 		{"deadline", context.DeadlineExceeded, provider.ReasonTimeout},
 		{"cancelled", context.Canceled, provider.ReasonTimeout},
+		{"previous check still waiting", errProbeBusy, provider.ReasonTimeout},
 		{"no pool", lvError(libvirt.ErrNoStoragePool, "no pool"), reasonPoolNotFound},
 		{"other libvirt error", lvError(libvirt.ErrInternalError, "boom"), reasonLibvirtError},
 		{"link died mid-call", io.EOF, provider.ReasonUnreachable},

internal/provider/kvm/conn.go

@@ -9,6 +9,7 @@ import (
 	"net"
 	"os"
 	"sync"
+	"sync/atomic"
 	"time"
 
 	libvirt "github.com/digitalocean/go-libvirt"
@@ -102,9 +103,16 @@ type hostPool struct {
 	entries map[string]*hostEntry
 }
 
+// hostEntry is one host's slot in the pool. lock is a one-slot semaphore
+// rather than a mutex so that waiting for it can end with the caller's
+// context: whoever holds it may be dialing a dead host for as long as the
+// dial takes.
 type hostEntry struct {
-	mu     sync.Mutex
+	lock   chan struct{}
 	client *hostClient
+	// probing is set while a probe's fn runs, including one whose caller
+	// has already given up on it.
+	probing atomic.Bool
 }
 
 func newHostPool(dial hostDialer) *hostPool {
@@ -116,16 +124,37 @@ func (p *hostPool) entry(name string) *hostEntry {
 	defer p.mu.Unlock()
 	e, ok := p.entries[name]
 	if !ok {
-		e = &hostEntry{}
+		e = &hostEntry{lock: make(chan struct{}, 1)}
 		p.entries[name] = e
 	}
 	return e
 }
 
+// acquire takes e.lock. Only waiting is bounded by ctx: a free slot is
+// taken even when ctx has already ended, so an uncontended caller behaves
+// as it did with a mutex, and a contended one gives up with ctx's error.
+func (e *hostEntry) acquire(ctx context.Context) error {
+	select {
+	case e.lock <- struct{}{}:
+		return nil
+	default:
+	}
+	select {
+	case e.lock <- struct{}{}:
+		return nil
+	case <-ctx.Done():
+		return ctx.Err()
+	}
+}
+
+func (e *hostEntry) release() { <-e.lock }
+
 func (p *hostPool) get(ctx context.Context, log logr.Logger, host provider.KVMHost) (*hostClient, error) {
 	e := p.entry(host.Name)
-	e.mu.Lock()
-	defer e.mu.Unlock()
+	if err := e.acquire(ctx); err != nil {
+		return nil, fmt.Errorf("waiting for the connection to host %s (%s): %w", host.Name, host.Address, err)
+	}
+	defer e.release()
 	if e.client != nil {
 		if e.client.alive() {
 			return e.client, nil
@@ -149,8 +178,8 @@ func (p *hostPool) get(ctx context.Context, log logr.Logger, host provider.KVMHo
 // connection that was already replaced by a redial is left alone.
 func (p *hostPool) drop(host string, c *hostClient) {
 	e := p.entry(host)
-	e.mu.Lock()
-	defer e.mu.Unlock()
+	e.lock <- struct{}{}
+	defer e.release()
 	if e.client == c {
 		e.client = nil
 	}
@@ -173,19 +202,61 @@ func (p *hostPool) call(ctx context.Context, log logr.Logger, host provider.KVMH
 	if err != nil {
 		return err
 	}
-	api := c.api
-	if log.V(1).Enabled() {
-		api = &tracingAPI{inner: c.api, log: log}
-	}
-	err = awaitOrCancel(ctx, func() error { return fn(api) }, func() {
+	err = awaitOrCancel(ctx, func() error { return fn(traced(c, log)) }, func() {
 		log.V(1).Info("libvirt connection dropped", "reason", "context cancelled", "error", ctx.Err().Error())
 		p.drop(host.Name, c)
 	})
+	p.dropOnTransportError(ctx, log, host, c, err)
+	return err
+}
+
+// errProbeBusy refuses a probe while the previous one on the same host has
+// not come back yet.
+var errProbeBusy = errors.New("the previous check of this host is still waiting for libvirt to answer")
+
+// probe is call for the health check, which must never cost the reconciles
+// their connection: when ctx ends first, fn is abandoned and the connection
+// left alone. A link that is really dead is closed by the keepalive, which
+// ends fn too. At most one probe per host runs at a time, an abandoned one
+// included, so a daemon that stopped answering cannot pile up goroutines;
+// until it answers, further probes get errProbeBusy.
+func (p *hostPool) probe(ctx context.Context, log logr.Logger, host provider.KVMHost, fn func(api libvirtAPI) error) error {
+	log = log.WithName("libvirt")
+	e := p.entry(host.Name)
+	if !e.probing.CompareAndSwap(false, true) {
+		return errProbeBusy
+	}
+	c, err := p.get(ctx, log, host)
+	if err != nil {
+		e.probing.Store(false)
+		return err
+	}
+	err = awaitOrCancel(ctx, func() error {
+		defer e.probing.Store(false)
+		return fn(traced(c, log))
+	}, func() {
+		log.V(1).Info("libvirt probe abandoned, connection kept", "error", ctx.Err().Error())
+	})
+	p.dropOnTransportError(ctx, log, host, c, err)
+	return err
+}
+
+// traced returns c's API, logging every RPC when log has V(1) enabled.
+func traced(c *hostClient, log logr.Logger) libvirtAPI {
+	if log.V(1).Enabled() {
+		return &tracingAPI{inner: c.api, log: log}
+	}
+	return c.api
+}
+
+// dropOnTransportError drops c when err is evidence against the link:
+// neither a libvirt error, nor a verdict already classified, nor the
+// caller's own context ending.
+func (p *hostPool) dropOnTransportError(ctx context.Context, log logr.Logger, host provider.KVMHost, c *hostClient, err error) {
 	if err != nil && !isLibvirtError(err) && !isClassified(err) && !errors.Is(err, ctx.Err()) {
 		log.V(1).Info("libvirt connection dropped", "reason", "transport error", "error", err.Error())
 		p.drop(host.Name, c)
 	}
-	return err
 }
 
 // awaitOrCancel runs fn in the background and returns its result, unless ctx

internal/provider/kvm/conn_test.go

@@ -232,6 +232,97 @@ func TestHostPool_call_hostsAreIndependent(t *testing.T) {
 	}
 }
 
+// A dial to a dead host holds the host's slot for as long as it takes; a
+// caller with a deadline must not wait past it.
+func TestHostPool_get_waitingRespectsContext(t *testing.T) {
+	t.Parallel()
+	release := make(chan struct{})
+	dialing := make(chan struct{})
+	var once sync.Once
+	dial := func(context.Context, provider.KVMHost) (*hostClient, error) {
+		once.Do(func() { close(dialing) })
+		<-release
+		return nil, errors.New("ssh dial: connection timed out")
+	}
+	p := newHostPool(dial)
+	slow := make(chan error, 1)
+	go func() { slow <- p.call(context.Background(), discard, testHost, noop) }()
+	<-dialing
+	ctx, cancel := context.WithTimeout(context.Background(), 50*time.Millisecond)
+	defer cancel()
+	start := time.Now()
+
+	err := p.call(ctx, discard, testHost, noop)
+
+	if !errors.Is(err, context.DeadlineExceeded) {
+		t.Errorf("call() error = %v, want the caller's deadline", err)
+	}
+	if waited := time.Since(start); waited > time.Second {
+		t.Errorf("call() waited %s behind another caller's dial, want about 50ms", waited)
+	}
+	close(release)
+	if err := <-slow; err == nil {
+		t.Error("the blocked dial succeeded, want its error")
+	}
+}
+
+func TestHostPool_probe_abandonsWithoutClosing(t *testing.T) {
+	t.Parallel()
+	d := &fakeDialer{}
+	p := newHostPool(d.dial)
+	unblock := make(chan struct{})
+	ctx, cancel := context.WithTimeout(context.Background(), 20*time.Millisecond)
+	defer cancel()
+
+	err := p.probe(ctx, discard, testHost, func(libvirtAPI) error {
+		<-unblock // a daemon that stopped answering
+		return nil
+	})
+
+	if !errors.Is(err, context.DeadlineExceeded) {
+		t.Fatalf("probe() error = %v, want DeadlineExceeded", err)
+	}
+	if d.clients[0].closed.Load() {
+		t.Error("probe closed the shared connection when it gave up")
+	}
+	if err := p.probe(context.Background(), discard, testHost, noop); !errors.Is(err, errProbeBusy) {
+		t.Errorf("probe() while the abandoned one still waits = %v, want errProbeBusy", err)
+	}
+	if err := p.call(context.Background(), discard, testHost, noop); err != nil {
+		t.Errorf("call() on the kept connection = %v", err)
+	}
+	close(unblock)
+	deadline := time.Now().Add(time.Second)
+	for {
+		err := p.probe(context.Background(), discard, testHost, noop)
+		if err == nil {
+			break
+		}
+		if !errors.Is(err, errProbeBusy) || time.Now().After(deadline) {
+			t.Fatalf("probe() after the abandoned one returned = %v, want nil", err)
+		}
+		time.Sleep(time.Millisecond)
+	}
+	if d.dialCount() != 1 {
+		t.Errorf("dials = %d, want 1: nothing should have redialed", d.dialCount())
+	}
+}
+
+func TestHostPool_probe_dropsConnectionOnTransportError(t *testing.T) {
+	t.Parallel()
+	d := &fakeDialer{}
+	p := newHostPool(d.dial)
+
+	err := p.probe(context.Background(), discard, testHost, func(libvirtAPI) error { return io.ErrUnexpectedEOF })
+
+	if !errors.Is(err, io.ErrUnexpectedEOF) {
+		t.Fatalf("probe() error = %v, want io.ErrUnexpectedEOF passed through", err)
+	}
+	if !d.clients[0].closed.Load() {
+		t.Error("a transport error during a probe left the connection cached")
+	}
+}
+
 // --- SSH dial against an in-process server ---
 
 func newKeyPair(t *testing.T) (ssh.Signer, []byte) {

internal/providerhealth/monitor.go

@@ -73,7 +73,8 @@ type Monitor struct {
 	// round runs at start: nothing races it, and early visibility is the
 	// point.
 	Interval time.Duration
-	// Timeout bounds each round (default 15s).
+	// Timeout bounds each round; 0 means 15s, or the interval when that is
+	// shorter.
 	Timeout time.Duration
 
 	now func() time.Time
@@ -92,31 +93,36 @@ func (m *Monitor) NeedLeaderElection() bool { return true }
 
 // ValidateSchedule checks --provider-check-interval and
 // --provider-check-timeout. The interval is 0 (off) or at least 10s, which
-// is what keeps one status write per provider per round cheap; the timeout
-// is positive and no longer than the interval.
+// is what keeps one status write per provider per round cheap. The timeout
+// is 0 (derived from the interval, see Monitor.Timeout) or positive and no
+// longer than the interval.
 func ValidateSchedule(interval, timeout time.Duration) error {
 	switch {
 	case interval == 0:
 		return nil
 	case interval < minInterval:
 		return fmt.Errorf("--provider-check-interval must be 0 (off) or at least %s, got %s", minInterval, interval)
-	case timeout <= 0 || timeout > interval:
-		return fmt.Errorf("--provider-check-timeout must be positive and at most the interval (%s), got %s", interval, timeout)
+	case timeout < 0 || timeout > interval:
+		return fmt.Errorf("--provider-check-timeout must be at most the interval (%s), got %s", interval, timeout)
 	}
 	return nil
 }
 
+func (m *Monitor) setDefaults() {
+	if m.Timeout == 0 {
+		m.Timeout = min(defaultTimeout, m.Interval)
+	}
+	if m.now == nil {
+		m.now = time.Now
+	}
+}
+
 // Start deletes the objects of providers that are no longer configured,
 // then checks every Interval until ctx ends. The providers config is read
 // once per process, so cleaning up once is enough. With checking off,
 // every object goes, so none is left claiming a stale Ready.
 func (m *Monitor) Start(ctx context.Context) error {
-	if m.Timeout == 0 {
-		m.Timeout = defaultTimeout
-	}
-	if m.now == nil {
-		m.now = time.Now
-	}
+	m.setDefaults()
 	log := logf.FromContext(ctx).WithName("provider-health")
 	ctx = logf.IntoContext(ctx, log)
 

internal/providerhealth/monitor_test.go

@@ -340,7 +340,8 @@ func TestValidateSchedule(t *testing.T) {
 		{"shortest interval", 10 * time.Second, 10 * time.Second, false},
 		{"too frequent", 5 * time.Second, time.Second, true},
 		{"negative interval", -time.Minute, time.Second, true},
-		{"no timeout", time.Minute, 0, true},
+		{"timeout left to derive, short interval", 12 * time.Second, 0, false},
+		{"negative timeout", time.Minute, -time.Second, true},
 		{"timeout longer than interval", time.Minute, 2 * time.Minute, true},
 	}
 	for _, tc := range tests {
@@ -352,3 +353,25 @@ func TestValidateSchedule(t *testing.T) {
 		})
 	}
 }
+
+func TestMonitor_setDefaults_timeout(t *testing.T) {
+	t.Parallel()
+	tests := []struct {
+		name                    string
+		interval, timeout, want time.Duration
+	}{
+		{"default", time.Minute, 0, 15 * time.Second},
+		{"interval shorter than the default", 12 * time.Second, 0, 12 * time.Second},
+		{"explicit", time.Minute, 20 * time.Second, 20 * time.Second},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			m := &Monitor{Interval: tc.interval, Timeout: tc.timeout}
+			m.setDefaults()
+			if m.Timeout != tc.want {
+				t.Errorf("Timeout = %s, want %s", m.Timeout, tc.want)
+			}
+		})
+	}
+}