fix: prevent sandbox halt after resume by fixing HTTP/2 HOL blocking and adding timeouts

Disable HTTP/2 on both host agent server and CP→agent transport — multiplexing
caused head-of-line blocking when a slow sandbox RPC stalled the shared connection.
Add ResponseHeaderTimeout to envd HTTP clients. Merge SetDefaults into Resume's
PostInit call to eliminate an extra round-trip that could hang on a stale connection.

cmd/host-agent/main.go

@@ -154,6 +154,11 @@ func main() {
 		Addr:              listenAddr,
 		ReadHeaderTimeout: 10 * time.Second,
 		IdleTimeout:       620 * time.Second, // > typical LB upstream timeout (600s)
+		// Disable HTTP/2: empty non-nil map prevents Go from registering
+		// the h2 ALPN token. Connect RPC works over HTTP/1.1; HTTP/2
+		// multiplexing causes HOL blocking when a slow sandbox RPC stalls
+		// the shared connection.
+		TLSNextProto: make(map[string]func(*http.Server, *tls.Conn, http.Handler)),
 	}
 
 	// mTLS is mandatory — refuse to start without a valid certificate.

internal/envdclient/dialer.go

@@ -25,6 +25,7 @@ func newHTTPClient() *http.Client {
 		Transport: &http.Transport{
 			MaxIdleConnsPerHost:   10,
 			IdleConnTimeout:       90 * time.Second,
+			ResponseHeaderTimeout: 30 * time.Second,
 			DialContext: (&net.Dialer{
 				Timeout:   10 * time.Second,
 				KeepAlive: 30 * time.Second,
@@ -40,6 +41,7 @@ func newStreamingHTTPClient() *http.Client {
 		Transport: &http.Transport{
 			MaxIdleConnsPerHost:   10,
 			IdleConnTimeout:       90 * time.Second,
+			ResponseHeaderTimeout: 30 * time.Second,
 			DialContext: (&net.Dialer{
 				Timeout:   10 * time.Second,
 				KeepAlive: 30 * time.Second,

internal/hostagent/server.go

@@ -109,18 +109,11 @@ func (s *Server) ResumeSandbox(
 	req *connect.Request[pb.ResumeSandboxRequest],
 ) (*connect.Response[pb.ResumeSandboxResponse], error) {
 	msg := req.Msg
-	sb, err := s.mgr.Resume(ctx, msg.SandboxId, int(msg.TimeoutSec), msg.KernelVersion)
+	sb, err := s.mgr.Resume(ctx, msg.SandboxId, int(msg.TimeoutSec), msg.KernelVersion, msg.DefaultUser, msg.DefaultEnv)
 	if err != nil {
 		return nil, connect.NewError(connect.CodeInternal, err)
 	}
 
-	// Apply template defaults (user, env vars) if provided.
-	if msg.DefaultUser != "" || len(msg.DefaultEnv) > 0 {
-		if err := s.mgr.SetDefaults(ctx, sb.ID, msg.DefaultUser, msg.DefaultEnv); err != nil {
-			slog.Warn("failed to set sandbox defaults on resume", "sandbox", sb.ID, "error", err)
-		}
-	}
-
 	return connect.NewResponse(&pb.ResumeSandboxResponse{
 		SandboxId: sb.ID,
 		Status:    string(sb.Status),

internal/sandbox/manager.go

@@ -626,7 +626,9 @@ func (m *Manager) Pause(ctx context.Context, sandboxID string) error {
 
 // Resume restores a paused sandbox from its snapshot using UFFD for
 // lazy memory loading. The sandbox gets a new network slot.
-func (m *Manager) Resume(ctx context.Context, sandboxID string, timeoutSec int, kernelVersion string) (*models.Sandbox, error) {
+// Optional defaultUser and defaultEnv are applied via a single PostInit
+// call so that template defaults are set without an extra round-trip.
+func (m *Manager) Resume(ctx context.Context, sandboxID string, timeoutSec int, kernelVersion string, defaultUser string, defaultEnv map[string]string) (*models.Sandbox, error) {
 	pauseDir := layout.PauseSnapshotDir(m.cfg.WrennDir, sandboxID)
 	if _, err := os.Stat(pauseDir); err != nil {
 		return nil, fmt.Errorf("no snapshot found for sandbox %s", sandboxID)
@@ -783,8 +785,8 @@ func (m *Manager) Resume(ctx context.Context, sandboxID string, timeoutSec int,
 		return nil, fmt.Errorf("wait for envd: %w", err)
 	}
 
-	// Trigger envd to re-read MMDS so it picks up the new sandbox/template IDs.
+	// Trigger envd to re-read MMDS and apply template defaults in a single call.
-	if err := client.PostInit(waitCtx); err != nil {
+	if err := client.PostInitWithDefaults(waitCtx, defaultUser, defaultEnv); err != nil {
 		slog.Warn("post-init failed after resume, metadata files may be stale", "sandbox", sandboxID, "error", err)
 	}
 

pkg/lifecycle/hostpool.go

@@ -40,6 +40,18 @@ func NewHostClientPool() *HostClientPool {
 func NewHostClientPoolTLS(tlsCfg *tls.Config) *HostClientPool {
 	transport := &http.Transport{
 		TLSClientConfig:   tlsCfg,
+		ForceAttemptHTTP2: false,
+		// Empty non-nil map disables HTTP/2 ALPN negotiation, forcing HTTP/1.1.
+		// Connect RPC works over HTTP/1.1; HTTP/2 multiplexing causes HOL
+		// blocking when a single slow sandbox RPC stalls the shared connection.
+		TLSNextProto:          make(map[string]func(authority string, c *tls.Conn) http.RoundTripper),
+		MaxIdleConnsPerHost:   20,
+		IdleConnTimeout:       90 * time.Second,
+		ResponseHeaderTimeout: 45 * time.Second,
+		DialContext: (&net.Dialer{
+			Timeout:   10 * time.Second,
+			KeepAlive: 30 * time.Second,
+		}).DialContext,
 	}
 	return &HostClientPool{
 		clients: make(map[string]hostagentv1connect.HostAgentServiceClient),