cmd/cli: use better approach for detecting NetworkManager

Currently, ctrld assumes that NetworkManager is not available if writing
to /etc/NetworkManager/conf.d return directory not exist error. That
would work on most Linux distros. However, cloud provider may do some
hacks, causing ctrld confusion and think that NetworkManager is
available.

Fixing this by checking whether NetworkManager binary presents first.

While at it, also fixing a bug when restarting NetworkManager failed
causing ctrld hangs. The go-systemd library is not clear about this, but
the waitCh channel won't never be closed if error occurred, so we must
return immediately instead of receiving from it blindly.

cmd/cli/network_manager_linux.go

@@ -3,6 +3,7 @@ package cli
 import (
 	"context"
 	"os"
+	"os/exec"
 	"path/filepath"
 	"time"
 
@@ -16,13 +17,21 @@ const (
 dns=none
 systemd-resolved=false
 `
-	nmSystemdUnitName   = "NetworkManager.service"
-	systemdEnabledState = "enabled"
+	nmSystemdUnitName = "NetworkManager.service"
 )
 
 var networkManagerCtrldConfFile = filepath.Join(nmConfDir, nmCtrldConfFilename)
 
+// hasNetworkManager reports whether NetworkManager executable found.
+func hasNetworkManager() bool {
+	exe, _ := exec.LookPath("NetworkManager")
+	return exe != ""
+}
+
 func setupNetworkManager() error {
+	if !hasNetworkManager() {
+		return nil
+	}
 	if content, _ := os.ReadFile(nmCtrldConfContent); string(content) == nmCtrldConfContent {
 		mainLog.Load().Debug().Msg("NetworkManager already setup, nothing to do")
 		return nil
@@ -43,6 +52,9 @@ func setupNetworkManager() error {
 }
 
 func restoreNetworkManager() error {
+	if !hasNetworkManager() {
+		return nil
+	}
 	err := os.Remove(networkManagerCtrldConfFile)
 	if os.IsNotExist(err) {
 		mainLog.Load().Debug().Msg("NetworkManager is not available")
@@ -71,6 +83,7 @@ func reloadNetworkManager() {
 	waitCh := make(chan string)
 	if _, err := conn.ReloadUnitContext(ctx, nmSystemdUnitName, "ignore-dependencies", waitCh); err != nil {
 		mainLog.Load().Debug().Err(err).Msg("could not reload NetworkManager")
+		return
 	}
 	<-waitCh
 }