Pre-pause snapshot signal to prevent Go runtime crash on restore

envd crashes with "fatal error: bad summary data" after Firecracker
snapshot/restore because the page allocator radix tree is inconsistent
when vCPUs are frozen mid-allocation. The port scanner goroutine
allocates heavily every second, making it the primary trigger.

Add POST /snapshot/prepare to envd — the host agent calls it before
vm.Pause to quiesce continuous goroutines and force GC. On restore,
PostInit restarts the port subsystem via the existing /init endpoint.

- New PortSubsystem abstraction with Start/Stop/Restart lifecycle
- Context-based goroutine cancellation (replaces irreversible channel close)
- Context-aware Signal to prevent scanner/forwarder deadlock
- Fix forwarder goroutine leak (was spinning forever on closed channel)
- Kill socat children on stop to prevent orphans across snapshots
- Fix double cmd.Wait panic (exec.Command instead of CommandContext)

envd/main.go

@@ -190,7 +190,14 @@ func main() {
 	processLogger := l.With().Str("logger", "process").Logger()
 	processService := processRpc.Handle(m, &processLogger, defaults, cgroupManager)
 
-	service := api.New(&envLogger, defaults, mmdsChan, isNotFC)
+	// Port scanner and forwarder are managed by PortSubsystem, which
+	// supports stop/restart across Firecracker snapshot/restore cycles.
+	portLogger := l.With().Str("logger", "port-forwarder").Logger()
+	portSubsystem := publicport.NewPortSubsystem(&portLogger, cgroupManager, portScannerInterval)
+	portSubsystem.Start(ctx)
+	defer portSubsystem.Stop()
+
+	service := api.New(&envLogger, defaults, mmdsChan, isNotFC, ctx, portSubsystem)
 	handler := api.HandlerFromMux(service, m)
 	middleware := authn.NewMiddleware(permissions.AuthenticateUsername)
 
@@ -229,16 +236,6 @@ func main() {
 		}
 	}
 
-	// Bind all open ports on 127.0.0.1 and localhost to the eth0 interface
-	portScanner := publicport.NewScanner(portScannerInterval)
-	defer portScanner.Destroy()
-
-	portLogger := l.With().Str("logger", "port-forwarder").Logger()
-	portForwarder := publicport.NewForwarder(&portLogger, portScanner, cgroupManager)
-	go portForwarder.StartForwarding(ctx)
-
-	go portScanner.ScanAndBroadcast()
-
 	err := s.ListenAndServe()
 	if err != nil {
 		log.Fatalf("error starting server: %v", err)