[core] Do not start the plugin loader on non-clients (#16111)

The plugin loader loads task and device driver plugins which are not
used on server nodes.

command/agent/agent.go

@@ -142,10 +142,6 @@ func NewAgent(config *Config, logger log.InterceptLogger, logOutput io.Writer, i
 		return nil, fmt.Errorf("Failed to initialize Consul client: %v", err)
 	}
 
-	if err := a.setupPlugins(); err != nil {
-		return nil, err
-	}
-
 	if err := a.setupServer(); err != nil {
 		return nil, err
 	}
@@ -568,10 +564,6 @@ func (a *Agent) finalizeServerConfig(c *nomad.Config) {
 	// Setup the logging
 	c.Logger = a.logger
 	c.LogOutput = a.logOutput
-
-	// Setup the plugin loaders
-	c.PluginLoader = a.pluginLoader
-	c.PluginSingletonLoader = a.pluginSingletonLoader
 	c.AgentShutdown = func() error { return a.Shutdown() }
 }
 
@@ -1001,6 +993,14 @@ func (a *Agent) setupClient() error {
 	if !a.config.Client.Enabled {
 		return nil
 	}
+
+	// Plugin setup must happen before the call to clientConfig, because it
+	// copies the pointers to the plugin loaders from the Agent to the
+	// Client config.
+	if err := a.setupPlugins(); err != nil {
+		return err
+	}
+
 	// Setup the configuration
 	conf, err := a.clientConfig()
 	if err != nil {

command/agent/plugins_test.go

@@ -0,0 +1,33 @@
+package agent
+
+import (
+	"testing"
+
+	"github.com/hashicorp/nomad/api"
+	"github.com/shoenig/test/must"
+)
+
+func TestPlugins_WhenNotClientSkip(t *testing.T) {
+	s, _, _ := testServer(t, false, nil)
+	must.Nil(t, s.Agent.pluginSingletonLoader)
+}
+
+func TestPlugins_WhenClientRun(t *testing.T) {
+	s, _, _ := testServer(t, true, nil)
+	must.NotNil(t, s.Agent.pluginSingletonLoader)
+}
+
+func testServer(t *testing.T, runClient bool, cb func(*Config)) (*TestAgent, *api.Client, string) {
+	// Make a new test server
+	a := NewTestAgent(t, t.Name(), func(config *Config) {
+		config.Client.Enabled = runClient
+
+		if cb != nil {
+			cb(config)
+		}
+	})
+	t.Cleanup(a.Shutdown)
+
+	c := a.Client()
+	return a, c, a.HTTPAddr()
+}