Retry upstream connection-refused with exponential backoff

A refused dial (service down/restarting) is retried with a wait that
doubles from BACKOFF_INITIAL (1s) up to BACKOFF_MAX (60s) until the
upstream answers or the client disconnects. Handles the Windows WSA
errno (10061) as well as POSIX ECONNREFUSED. compose.yaml.example now
uses host.docker.internal like the working local deployment.

README.md

@@ -49,6 +49,8 @@ startup.
 | `FREE_TIMEOUT` | `30s` | `POST /free` call after an image job |
 | `WARM_TIMEOUT` | `2m` | Warm-model reload after an image job |
 | `SHUTDOWN_TIMEOUT` | `10s` | Graceful shutdown on SIGINT/SIGTERM |
+| `BACKOFF_INITIAL` | `1s` | First retry wait when an upstream refuses a connection |
+| `BACKOFF_MAX` | `60s` | Cap for the exponential retry backoff |
 | `PROMPT_CAPTURE_LIMIT` | `65536` | Bytes of the `/prompt` response buffered to find `prompt_id` (pass-through is unaffected) |
 
 ## Observability

SPEC.md

@@ -119,6 +119,8 @@ load time. Off by default.
 | `FREE_TIMEOUT` | `30s` | `POST /free` call after an image job |
 | `WARM_TIMEOUT` | `2m` | warm-model reload after an image job |
 | `SHUTDOWN_TIMEOUT` | `10s` | graceful shutdown on SIGINT/SIGTERM |
+| `BACKOFF_INITIAL` | `1s` | first retry wait when an upstream refuses a connection |
+| `BACKOFF_MAX` | `60s` | cap for the exponential retry backoff |
 | `PROMPT_CAPTURE_LIMIT` | `65536` | bytes of the `/prompt` response buffered to find `prompt_id` (pass-through is unaffected) |
 
 Startup fails fast on unparsable values. Both upstreams are probed once at
@@ -146,6 +148,11 @@ start (`/api/version`, `/system_stats`); failure is logged, not fatal.
   `JOB_TIMEOUT` releases the lock; log at warn.
 - Ollama unreachable during unload: continue with the image job; the whole
   point is not to block users on a misbehaving neighbour.
+- Upstream connection refused while proxying (service down or restarting):
+  retry with exponential backoff — `BACKOFF_INITIAL`, doubling per attempt,
+  capped at `BACKOFF_MAX` — until the upstream answers or the client
+  disconnects. Retrying a refused connection is safe: no request bytes were
+  sent. Other upstream errors are not retried.
 - `POST /prompt` with a body that ComfyUI rejects (400): lock released
   immediately, body passed back.
 - Websocket `/ws` connections are long-lived and never take the lock.

cmd/gpu-turnstile/main.go

@@ -40,6 +40,8 @@ type config struct {
 	freeTimeout         time.Duration
 	warmTimeout         time.Duration
 	shutdownTimeout     time.Duration
+	backoffInitial      time.Duration
+	backoffMax          time.Duration
 	promptCaptureLimit  int64
 
 	warmModel string
@@ -76,6 +78,8 @@ func loadConfig(getenv func(string) string) (config, error) {
 		freeTimeout:         30 * time.Second,
 		warmTimeout:         2 * time.Minute,
 		shutdownTimeout:     10 * time.Second,
+		backoffInitial:      time.Second,
+		backoffMax:          time.Minute,
 		promptCaptureLimit:  64 * 1024,
 
 		logLevel: slog.LevelInfo,
@@ -107,6 +111,8 @@ func loadConfig(getenv func(string) string) (config, error) {
 		{"FREE_TIMEOUT", &cfg.freeTimeout},
 		{"WARM_TIMEOUT", &cfg.warmTimeout},
 		{"SHUTDOWN_TIMEOUT", &cfg.shutdownTimeout},
+		{"BACKOFF_INITIAL", &cfg.backoffInitial},
+		{"BACKOFF_MAX", &cfg.backoffMax},
 	} {
 		if err := envDuration(getenv, e.name, e.dst); err != nil {
 			return cfg, err
@@ -185,6 +191,8 @@ func main() {
 		HistoryPollInterval: cfg.historyPollInterval,
 		FreeTimeout:         cfg.freeTimeout,
 		WarmTimeout:         cfg.warmTimeout,
+		BackoffInitial:      cfg.backoffInitial,
+		BackoffMax:          cfg.backoffMax,
 		PromptCaptureLimit:  cfg.promptCaptureLimit,
 		WarmModel:           cfg.warmModel,
 	})

compose.yaml.example

@@ -1,14 +1,20 @@
 # Example deployment for gpu-turnstile. Copy to compose.yaml and adjust.
 #
 # gpu-turnstile listens on the ports the services normally use; the actual
-# Ollama and ComfyUI instances run one port higher on the workstation.
+# Ollama and ComfyUI instances run one port higher (11435 / 8189) and must
+# bind 0.0.0.0 so the container can reach them (OLLAMA_HOST=0.0.0.0:11435,
+# ComfyUI --listen 0.0.0.0 --port 8189).
 services:
   gpu-turnstile:
     image: git.rambossek.at/public/gpu-turnstile:v0.1.1
     restart: unless-stopped
     environment:
-      OLLAMA_URL: http://<workstation-ip>:11435
-      COMFY_URL: http://<workstation-ip>:8189
+      # Services on the Docker host itself:
+      OLLAMA_URL: http://host.docker.internal:11435
+      COMFY_URL: http://host.docker.internal:8189
+      # Services on another machine: use its LAN IP instead, e.g.
+      # OLLAMA_URL: http://192.168.1.10:11435
+      # COMFY_URL: http://192.168.1.10:8189
       # UNLOAD_TIMEOUT: 60s
       # JOB_TIMEOUT: 15m
       # LLM_WAIT_TIMEOUT: 10m

internal/proxy/proxy.go

@@ -13,6 +13,7 @@ import (
 	"net/http"
 	"net/http/httputil"
 	"net/url"
+	"syscall"
 	"time"
 
 	"gpu-turnstile/internal/comfy"
@@ -41,6 +42,13 @@ type Config struct {
 	UnloadTimeout  time.Duration
 	JobTimeout     time.Duration
 
+	// BackoffInitial and BackoffMax control the exponential retry backoff
+	// when an upstream refuses a connection: the wait doubles from
+	// BackoffInitial up to BackoffMax between attempts. Zero selects the
+	// defaults (1s / 60s).
+	BackoffInitial time.Duration
+	BackoffMax     time.Duration
+
 	// UnloadPollInterval and HistoryPollInterval override the clients'
 	// /api/ps and /history poll intervals when > 0.
 	UnloadPollInterval  time.Duration
@@ -62,6 +70,8 @@ type Server struct {
 	freeTimeout    time.Duration
 	warmTimeout    time.Duration
 	captureLimit   int64
+	backoffInitial time.Duration
+	backoffMax     time.Duration
 
 	ollamaProxy *httputil.ReverseProxy
 	comfyProxy  *httputil.ReverseProxy
@@ -99,19 +109,89 @@ func New(cfg Config) (*Server, error) {
 	if captureLimit <= 0 {
 		captureLimit = defaultCaptureLimit
 	}
+	backoffInitial := cfg.BackoffInitial
+	if backoffInitial <= 0 {
+		backoffInitial = time.Second
+	}
+	backoffMax := cfg.BackoffMax
+	if backoffMax <= 0 {
+		backoffMax = time.Minute
+	}
+	retry := &retryTransport{
+		base:    http.DefaultTransport,
+		initial: backoffInitial,
+		max:     backoffMax,
+		log:     log,
+	}
 	return &Server{
 		cfg:            cfg,
 		log:            log,
 		freeTimeout:    freeTimeout,
 		warmTimeout:    warmTimeout,
 		captureLimit:   captureLimit,
-		ollamaProxy:  newReverseProxy(ollamaURL, log.With("upstream", "ollama")),
-		comfyProxy:   newReverseProxy(comfyURL, log.With("upstream", "comfy")),
+		backoffInitial: backoffInitial,
+		backoffMax:     backoffMax,
+		ollamaProxy:    newReverseProxy(ollamaURL, retry, log.With("upstream", "ollama")),
+		comfyProxy:     newReverseProxy(comfyURL, retry, log.With("upstream", "comfy")),
 	}, nil
 }
 
-func newReverseProxy(target *url.URL, log *slog.Logger) *httputil.ReverseProxy {
+// retryTransport retries requests that fail with "connection refused" —
+// typically the upstream service simply not being up yet — using an
+// exponential backoff: initial, doubling each attempt, capped at max. The
+// retry loop runs until the request succeeds, fails with a different
+// error, or the client's context is cancelled. Retrying a refused
+// connection is always safe: no request bytes were ever sent.
+type retryTransport struct {
+	base    http.RoundTripper
+	initial time.Duration
+	max     time.Duration
+	log     *slog.Logger
+}
+
+// wsaECONNREFUSED is the Windows error code for a refused connection.
+// syscall.ECONNREFUSED is the POSIX value and never matches on Windows,
+// where dial errors carry the WSA code.
+const wsaECONNREFUSED syscall.Errno = 10061
+
+func isConnRefused(err error) bool {
+	if errors.Is(err, syscall.ECONNREFUSED) {
+		return true
+	}
+	var errno syscall.Errno
+	return errors.As(err, &errno) && errno == wsaECONNREFUSED
+}
+
+func (t *retryTransport) logWarn(msg string, args ...any) {
+	if t.log != nil {
+		t.log.Warn(msg, args...)
+	}
+}
+
+func (t *retryTransport) RoundTrip(req *http.Request) (*http.Response, error) {
+	wait := t.initial
+	for {
+		resp, err := t.base.RoundTrip(req)
+		if err == nil || !isConnRefused(err) {
+			return resp, err
+		}
+		t.logWarn("upstream connection refused; backing off",
+			"path", req.URL.Path, "retry_in", wait)
+		select {
+		case <-req.Context().Done():
+			return nil, err
+		case <-time.After(wait):
+		}
+		wait *= 2
+		if wait > t.max {
+			wait = t.max
+		}
+	}
+}
+
+func newReverseProxy(target *url.URL, transport http.RoundTripper, log *slog.Logger) *httputil.ReverseProxy {
 	return &httputil.ReverseProxy{
+		Transport: transport,
 		Rewrite: func(pr *httputil.ProxyRequest) {
 			pr.SetURL(target)
 			pr.SetXForwarded()

internal/proxy/retry_test.go

@@ -0,0 +1,122 @@
+package proxy
+
+import (
+	"context"
+	"io"
+	"net"
+	"net/http"
+	"strings"
+	"syscall"
+	"testing"
+	"time"
+)
+
+// stubTransport fails with ECONNREFUSED for the first fails requests, then
+// returns a 200 response.
+type stubTransport struct {
+	fails int
+	calls int
+}
+
+func (s *stubTransport) RoundTrip(req *http.Request) (*http.Response, error) {
+	s.calls++
+	if s.calls <= s.fails {
+		return nil, &net.OpError{Op: "dial", Net: "tcp", Err: syscall.ECONNREFUSED}
+	}
+	return &http.Response{
+		StatusCode: 200,
+		Body:       io.NopCloser(strings.NewReader("ok")),
+		Header:     make(http.Header),
+	}, nil
+}
+
+func TestRetryTransportBackoff(t *testing.T) {
+	st := &stubTransport{fails: 3}
+	rt := &retryTransport{
+		base:    st,
+		initial: 10 * time.Millisecond,
+		max:     25 * time.Millisecond,
+	}
+
+	start := time.Now()
+	req, _ := http.NewRequest(http.MethodGet, "http://upstream/api/version", nil)
+	resp, err := rt.RoundTrip(req)
+	if err != nil {
+		t.Fatal(err)
+	}
+	resp.Body.Close()
+	if st.calls != 4 {
+		t.Fatalf("calls = %d, want 4", st.calls)
+	}
+	// Waits: 10ms + 20ms + 25ms (capped) = 55ms minimum.
+	elapsed := time.Since(start)
+	if elapsed < 50*time.Millisecond {
+		t.Fatalf("elapsed = %v, want >= ~55ms of backoff", elapsed)
+	}
+	if elapsed > 5*time.Second {
+		t.Fatalf("elapsed = %v, suspiciously long", elapsed)
+	}
+}
+
+func TestRetryTransportNonRefusedErrorNotRetried(t *testing.T) {
+	rt := &retryTransport{
+		base:    &stubTransport{fails: 0},
+		initial: time.Millisecond,
+		max:     time.Millisecond,
+	}
+	req, _ := http.NewRequest(http.MethodGet, "http://upstream/", nil)
+	resp, err := rt.RoundTrip(req)
+	if err != nil || resp.StatusCode != 200 {
+		t.Fatalf("resp=%v err=%v", resp, err)
+	}
+}
+
+type alwaysRefused struct{ calls int }
+
+func (a *alwaysRefused) RoundTrip(*http.Request) (*http.Response, error) {
+	a.calls++
+	return nil, &net.OpError{Op: "dial", Net: "tcp", Err: syscall.ECONNREFUSED}
+}
+
+func TestRetryTransportContextCancel(t *testing.T) {
+	ar := &alwaysRefused{}
+	rt := &retryTransport{base: ar, initial: time.Second, max: time.Second}
+
+	ctx, cancel := context.WithTimeout(context.Background(), 100*time.Millisecond)
+	defer cancel()
+	req, _ := http.NewRequestWithContext(ctx, http.MethodGet, "http://upstream/", nil)
+	start := time.Now()
+	_, err := rt.RoundTrip(req)
+	if err == nil {
+		t.Fatal("expected error after context cancellation")
+	}
+	if time.Since(start) > 2*time.Second {
+		t.Fatal("retry loop did not stop on context cancellation")
+	}
+}
+
+func TestRetryViaReverseProxy(t *testing.T) {
+	// Point the proxy at a port nothing listens on; the request should be
+	// retried (not instantly 502) until the client context ends.
+	srv, err := New(Config{
+		OllamaURL:      "http://127.0.0.1:1",
+		ComfyURL:       "http://127.0.0.1:1",
+		Lock:           nil,
+		Metrics:        nil,
+		BackoffInitial: 10 * time.Millisecond,
+		BackoffMax:     20 * time.Millisecond,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	_ = srv // construction must not panic with minimal config
+
+	rt := &retryTransport{base: http.DefaultTransport, initial: 10 * time.Millisecond, max: 20 * time.Millisecond}
+	ctx, cancel := context.WithTimeout(context.Background(), 200*time.Millisecond)
+	defer cancel()
+	req, _ := http.NewRequestWithContext(ctx, http.MethodGet, "http://127.0.0.1:1/", nil)
+	_, err = rt.RoundTrip(req)
+	if err == nil || !isConnRefused(err) {
+		t.Fatalf("err = %v, want connection refused", err)
+	}
+}