Wire wake and identity into crossbar; v2 smoke and README

Implemented-By: OpenCode session (model recorded in docs/implementer-log.md)

internal/limiter/limiter_test.go

@@ -32,10 +32,10 @@ func TestParallelAndQueue(t *testing.T) {
 	go func() {
 		rel, waited, err := l.Acquire(ctx, "alpha", "m")
 		if err == nil {
-			defer rel()
 			if waited < 40*time.Millisecond {
 				err = errors.New("third acquire did not wait")
 			}
+			rel() // release before reporting, so the final count check cannot race it
 		}
 		got3 <- err
 	}()

internal/proxy/ctxguard.go

@@ -69,6 +69,35 @@ func (p *Handler) guard(w http.ResponseWriter, r *http.Request, hosts []string,
 		return newHost, movedHeader(host, newHost), estimate, false
 	}
 
+	// Rule 4a: no host fits. Before refusing, ask a waker to rouse a candidate
+	// whose context may grow when it comes up; a woken host takes the lease.
+	if p.waker != nil {
+		for _, name := range hosts {
+			s, ok := p.health.Get(name)
+			if !ok {
+				continue
+			}
+			// A healthy host does not need waking; only a down host might grow
+			// a larger context when it comes up.
+			if s.Healthy {
+				continue
+			}
+			// A host with a known per-slot context smaller than the estimate
+			// cannot serve it no matter how it wakes.
+			if psc := s.PerSlotCtx(); psc != 0 && psc < estimate {
+				continue
+			}
+			if p.cfg.Hosts[name].Wake == nil || !p.waker.Wake(r.Context(), name) {
+				continue
+			}
+			if err := p.leases.Move(lease.Key{Route: route, FP: fp, Model: model}, name, time.Now()); err != nil {
+				p.writeError(w, http.StatusBadGateway, "upstream failed")
+				return host, "", estimate, true
+			}
+			return name, movedHeader(host, name), estimate, false
+		}
+	}
+
 	// Rule 4: no host fits. Answer 400 with the estimate and the largest
 	// available per-slot context, and record the row.
 	p.refuseCtx(w, host, route, model, fp, started, estimate, largestSlotCtx(hosts, p.health))

internal/proxy/forward.go

@@ -152,6 +152,30 @@ func (p *Handler) writeError(w http.ResponseWriter, status int, msg string) {
 	_ = json.NewEncoder(w).Encode(map[string]string{"error": msg})
 }
 
+// writeNoHealthyHost answers the 503 when no candidate woke. The body names the
+// hosts that were asked to wake (an empty list, never null), and the row
+// records the miss.
+func (p *Handler) writeNoHealthyHost(w http.ResponseWriter, route, model, fp string, started time.Time, tried []string) {
+	if tried == nil {
+		tried = []string{}
+	}
+	p.writeRecord(store.Request{
+		Route:   route,
+		FP:      fp,
+		Model:   model,
+		Started: started,
+		TotalMs: time.Since(started).Milliseconds(),
+		Status:  http.StatusServiceUnavailable,
+		Err:     "no healthy host",
+	})
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(http.StatusServiceUnavailable)
+	_ = json.NewEncoder(w).Encode(map[string]any{
+		"error": "no healthy host",
+		"woke":  tried,
+	})
+}
+
 // writeRecord writes one accounting row, logging (never returning) a recorder error.
 func (p *Handler) writeRecord(req store.Request) {
 	if p.rec == nil {

internal/proxy/proxy.go

@@ -7,6 +7,7 @@ package proxy
 
 import (
 	"bytes"
+	"context"
 	"encoding/json"
 	"errors"
 	"io"
@@ -45,6 +46,11 @@ type Recorder interface {
 	RecordRequest(store.Request) error
 }
 
+// Waker rouses a sleeping host. *wake.Waker satisfies it.
+type Waker interface {
+	Wake(ctx context.Context, host string) bool
+}
+
 // Handler forwards requests for a route to one of the route's healthy hosts, choosing by lease when
 // one is configured and by health alone otherwise.
 type Handler struct {
@@ -54,6 +60,13 @@ type Handler struct {
 	lim    *limiter.Limiter
 	rec    Recorder
 	log    *slog.Logger
+	waker  Waker
+}
+
+// SetWaker installs the waker the consults when a route has no healthy host left. A nil waker
+// (the default) leaves the ErrNoHost answer as it was in v0: a plain 503.
+func (p *Handler) SetWaker(w Waker) {
+	p.waker = w
 }
 
 // New builds a Handler. A nil logger becomes slog.Default(). With a nil lease table it behaves like
@@ -223,6 +236,12 @@ func (p *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	if err != nil {
 		switch {
 		case errors.Is(err, lease.ErrNoHost):
+			// No host healthy. Ask a waker to rouse a sleeping one; it answers
+			// (served or 503) when it has had a turn, else falls through to the
+			// plain 503.
+			if p.waker != nil && p.wakeOnErrNoHost(w, r, route, routeCfg, rest, model, fp, started, lease.Key{Route: route, FP: fp, Model: model}) {
+				return
+			}
 			p.writeError(w, http.StatusServiceUnavailable, "no healthy host")
 		case errors.Is(err, lease.ErrPinnedDown):
 			p.writeError(w, http.StatusServiceUnavailable, "pinned host down")
@@ -232,6 +251,14 @@ func (p *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Slot, context guard and forward, holding the slot for the leased host.
+	p.serveLeased(w, r, route, routeCfg, rest, model, fp, started, host, reused)
+}
+
+// serveLeased queues the request against the leased host's limiter, runs the
+// context guard, and forwards. The slot is held for the originally leased host
+// even if the guard relocates the lease: the guard already moved it.
+func (p *Handler) serveLeased(w http.ResponseWriter, r *http.Request, route string, routeCfg config.Route, rest, model, fp string, started time.Time, host string, reused bool) {
 	// Slot. A full queue is a 503; a context done while waiting means the client left.
 	release, waited, err := p.lim.Acquire(r.Context(), host, model)
 	if err != nil {
@@ -273,3 +300,26 @@ func (p *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	}
 	p.forward(w, r, route, host, leaseState(reused), rest, fp, model, now, waited, 0, header)
 }
+
+// wakeOnErrNoHost answers the request when no host was healthy. It asks, in
+// route order, each candidate with a wake target to rouse itself; a host that
+// wakes is leased once more and then served. When none wakes, it answers 503
+// with the hosts it tried. It returns true when the request has been answered.
+func (p *Handler) wakeOnErrNoHost(w http.ResponseWriter, r *http.Request, route string, routeCfg config.Route, rest, model, fp string, started time.Time, key lease.Key) bool {
+	var tried []string
+	for _, name := range routeCfg.Hosts {
+		if p.cfg.Hosts[name].Wake == nil {
+			continue
+		}
+		tried = append(tried, name)
+		if !p.waker.Wake(r.Context(), name) {
+			continue
+		}
+		if newHost, _, err := p.leases.Acquire(key, routeCfg.Hosts, time.Now()); err == nil {
+			p.serveLeased(w, r, route, routeCfg, rest, model, fp, started, newHost, true)
+			return true
+		}
+	}
+	p.writeNoHealthyHost(w, route, model, fp, started, tried)
+	return true
+}

internal/proxy/proxy_test.go

@@ -85,6 +85,19 @@ func TestDifferentConversationsSpreadByFreeSlots(t *testing.T) {
 	}
 }
 
+// waitUntil polls cond every 5 ms for up to two seconds and fails the test if it never holds.
+func waitUntil(t *testing.T, cond func() bool) {
+	t.Helper()
+	deadline := time.Now().Add(2 * time.Second)
+	for time.Now().Before(deadline) {
+		if cond() {
+			return
+		}
+		time.Sleep(5 * time.Millisecond)
+	}
+	t.Fatal("condition not reached within two seconds")
+}
+
 func TestQueueFullIs503(t *testing.T) {
 	alpha := newUpstream(t, "alpha")
 	alpha.delay = 400 * time.Millisecond
@@ -99,14 +112,20 @@ hosts = ["alpha"]
 default_model = "shared"
 `, alpha)
 	codes := make(chan int, 3)
-	for i := 1; i <= 3; i++ {
-		go func(i int) {
+	fire := func(i int) {
+		go func() {
 			resp := r.post("/r/v1/chat/completions", conversation(i, 1))
 			drain(resp)
 			codes <- resp.StatusCode
-		}(i)
-		time.Sleep(30 * time.Millisecond) // arrival order: 1 runs, 2 queues, 3 finds the queue full
+		}()
 	}
+	// Arrival order is enforced by watching the limiter, not by sleeping: 1 runs, 2 queues,
+	// 3 finds the queue full.
+	fire(1)
+	waitUntil(t, func() bool { return r.lim.InFlight("alpha", "shared") == 1 })
+	fire(2)
+	waitUntil(t, func() bool { return r.lim.Queued("alpha", "shared") == 1 })
+	fire(3)
 	got := map[int]int{}
 	for i := 0; i < 3; i++ {
 		got[<-codes]++