Allow overriding proxy target independently of the powered-on machine

pkg/config/config.go

@@ -20,9 +20,21 @@ type Config struct {
 	PowerOnCooldown   int            `yaml:"powerOnCooldown"` // seconds
 	ProxyTimeouts     ProxyTimeouts  `yaml:"proxyTimeouts"`
 	MachineMetadata   map[string]any `yaml:"machineMetadata"`
+	ProxyTarget       *ProxyTarget   `yaml:"proxyTarget"`
 	Machine           *machine.GoogleComputeEngine
 }
 
+// ProxyTarget optionally overrides where requests are proxied to.
+// When set, the machine referenced by MachineMetadata is still powered on and
+// pinged, but HTTP traffic is forwarded to ProxyTarget instead. This supports
+// topologies where a sidecar (e.g. a Cloud Run frontend container) serves
+// requests while still depending on the remote machine being up.
+type ProxyTarget struct {
+	Scheme string `yaml:"scheme"`
+	Host   string `yaml:"host"`
+	Port   int    `yaml:"port"`
+}
+
 type ProxyTimeouts struct {
 	DialTimeout           int `yaml:"dialTimeout"`           // seconds, default: 120
 	KeepAlive             int `yaml:"keepAlive"`             // seconds, default: 120

pkg/proxy/proxy_test.go

@@ -156,3 +156,26 @@ func TestReverseProxy_SetHost(t *testing.T) {
 		t.Error("Target is nil after SetHost()")
 	}
 }
+
+func TestReverseProxy_SetHost_ProxyTargetOverride(t *testing.T) {
+	config := &config.Config{
+		Scheme: "http",
+		Port:   8080,
+		ProxyTarget: &config.ProxyTarget{
+			Scheme: "http",
+			Host:   "localhost",
+			Port:   9000,
+		},
+		Machine: machine.NewGceMachine(),
+	}
+
+	proxy := New(config)
+	proxy.SetHost()
+
+	if proxy.Target.Host != "localhost:9000" {
+		t.Errorf("Target.Host = %q, want localhost:9000", proxy.Target.Host)
+	}
+	if proxy.Target.Scheme != "http" {
+		t.Errorf("Target.Scheme = %q, want http", proxy.Target.Scheme)
+	}
+}

pkg/proxy/tls.go

@@ -31,9 +31,13 @@ func New(c *config.Config) *ReverseProxy {
 	tlsHandshakeTimeout := time.Duration(c.ProxyTimeouts.TLSHandshakeTimeout) * time.Second
 	expectContinueTimeout := time.Duration(c.ProxyTimeouts.ExpectContinueTimeout) * time.Second
 
+	scheme := c.Scheme
+	if c.ProxyTarget != nil && c.ProxyTarget.Scheme != "" {
+		scheme = c.ProxyTarget.Scheme
+	}
 	return &ReverseProxy{
 		Target: &url.URL{
-			Scheme: c.Scheme,
+			Scheme: scheme,
 		},
 		Config: c,
 		Transport: &http.Transport{
@@ -59,6 +63,15 @@ func New(c *config.Config) *ReverseProxy {
 }
 
 func (p *ReverseProxy) SetHost() {
+	if p.Config.ProxyTarget != nil && p.Config.ProxyTarget.Host != "" {
+		port := p.Config.ProxyTarget.Port
+		if port == 0 {
+			port = p.Config.Port
+		}
+		p.Target.Host = net.JoinHostPort(p.Config.ProxyTarget.Host, strconv.Itoa(port))
+		slog.Debug("Set proxy target host", "host", p.Target.Host)
+		return
+	}
 	p.Target.Host = net.JoinHostPort(
 		p.Config.Machine.Host(),
 		strconv.Itoa(p.Config.Port),