Backport of template: allow change_mode script to run after client restart into release/1.8.x

.changelog/23663.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+template: Fixed a bug where change_mode = "script" would not execute after a client restart
+```

client/allocrunner/taskrunner/task_runner_hooks.go

@@ -126,6 +126,7 @@ func (tr *TaskRunner) initHooks() {
 			consulNamespace:     consulNamespace,
 			nomadNamespace:      tr.alloc.Job.Namespace,
 			renderOnTaskRestart: task.RestartPolicy.RenderTemplates,
+			driverHandle:        tr.handle,
 		}))
 	}
 

client/allocrunner/taskrunner/template/template.go

@@ -585,7 +585,7 @@ func (tm *TaskTemplateManager) processScript(script *structs.ChangeScript, wg *s
 
 	if tm.handle == nil {
 		failureMsg := fmt.Sprintf(
-			"Template failed to run script %v with arguments %v because task driver doesn't support the exec operation",
+			"Template failed to run script %v with arguments %v because task driver handle is not available",
 			script.Command,
 			script.Args,
 		)

client/allocrunner/taskrunner/template_hook.go

@@ -55,6 +55,11 @@ type templateHookConfig struct {
 
 	// hookResources are used to fetch Consul tokens
 	hookResources *cstructs.AllocHookResources
+
+	// driverHandle is the task driver executor used to run scripts when the
+	// template change mode is set to script. Typically this will be nil in this
+	// config struct, unless we're restoring a task after a client restart.
+	driverHandle ti.ScriptExecutor
 }
 
 type templateHook struct {
@@ -68,7 +73,10 @@ type templateHook struct {
 	managerLock     sync.Mutex
 
 	// driverHandle is the task driver executor used by the template manager to
-	// run scripts when the template change mode is set to script.
+	// run scripts when the template change mode is set to script. This value is
+	// set in the Poststart hook after the task has run, or passed in as
+	// configuration if this is a task that's being restored after a client
+	// restart.
 	//
 	// Must obtain a managerLock before changing. It may be nil.
 	driverHandle ti.ScriptExecutor
@@ -105,6 +113,7 @@ func newTemplateHook(config *templateHookConfig) *templateHook {
 		config:          config,
 		consulNamespace: config.consulNamespace,
 		logger:          config.logger.Named(templateHookName),
+		driverHandle:    config.driverHandle,
 	}
 }
 
@@ -206,7 +215,7 @@ func (h *templateHook) Poststart(_ context.Context, req *interfaces.TaskPoststar
 	} else {
 		for _, tmpl := range h.config.templates {
 			if tmpl.ChangeMode == structs.TemplateChangeModeScript {
-				return fmt.Errorf("template has change mode set to 'script' but the driver it uses does not provide exec capability")
+				return fmt.Errorf("template has change mode set to 'script' but task driver handle is not available")
 			}
 		}
 	}

client/allocrunner/taskrunner/template_hook_test.go

@@ -8,7 +8,9 @@ import (
 	"fmt"
 	"net/http"
 	"net/http/httptest"
+	"os"
 	"path"
+	"path/filepath"
 	"sync"
 	"testing"
 	"time"
@@ -269,3 +271,70 @@ func Test_templateHook_Prestart_Vault(t *testing.T) {
 		})
 	}
 }
+
+// TestTemplateHook_RestoreChangeModeScript exercises change_mode=script
+// behavior for a task restored after a client restart
+func TestTemplateHook_RestoreChangeModeScript(t *testing.T) {
+
+	logger := testlog.HCLogger(t)
+	tmpDir := t.TempDir()
+	destPath := filepath.Join(tmpDir, "foo.txt")
+	must.NoError(t, os.WriteFile(destPath, []byte("original-content"), 0755))
+
+	clientConfig := config.DefaultConfig()
+	clientConfig.TemplateConfig.DisableSandbox = true
+
+	alloc := mock.BatchAlloc()
+	task := alloc.Job.TaskGroups[0].Tasks[0]
+	envBuilder := taskenv.NewBuilder(mock.Node(), alloc, task, clientConfig.Region)
+
+	lifecycle := trtesting.NewMockTaskHooks()
+	lifecycle.HasHandle = true
+
+	events := &trtesting.MockEmitter{}
+
+	executor := &simpleExec{
+		code: 117,
+		err:  fmt.Errorf("oh no"),
+	}
+
+	hook := newTemplateHook(&templateHookConfig{
+		alloc:     alloc,
+		logger:    logger,
+		lifecycle: lifecycle,
+		events:    events,
+		templates: []*structs.Template{{
+			DestPath:     destPath,
+			EmbeddedTmpl: "changed-content",
+			ChangeMode:   structs.TemplateChangeModeScript,
+			ChangeScript: &structs.ChangeScript{
+				Command: "echo",
+				Args:    []string{"foo"},
+			},
+		}},
+		clientConfig:  clientConfig,
+		envBuilder:    envBuilder,
+		hookResources: &cstructs.AllocHookResources{},
+		driverHandle:  executor,
+	})
+	req := &interfaces.TaskPrestartRequest{
+		Alloc:   alloc,
+		Task:    task,
+		TaskDir: &allocdir.TaskDir{Dir: tmpDir},
+	}
+
+	must.NoError(t, hook.Prestart(context.TODO(), req, nil))
+
+	// self-test the test by making sure we really changed the template file
+	out, err := os.ReadFile(destPath)
+	must.NoError(t, err)
+	must.Eq(t, "changed-content", string(out))
+
+	// verify our change script executed
+	gotEvents := events.Events()
+	must.Len(t, 1, gotEvents)
+	must.Eq(t, structs.TaskHookFailed, gotEvents[0].Type)
+	must.Eq(t, "Template failed to run script echo with arguments [foo] on change: oh no Exit code: 117",
+		gotEvents[0].DisplayMessage)
+
+}