Merge pull request #3411 from cheeseprocedure/f-qemu-graceful-shutdown

Qemu driver: graceful shutdown feature

client/driver/qemu.go

@@ -3,8 +3,10 @@ package driver
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"log"
+	"net"
 	"os"
 	"os/exec"
 	"path/filepath"
@@ -13,7 +15,8 @@ import (
 	"strings"
 	"time"
 
-	"github.com/hashicorp/go-plugin"
+	"github.com/coreos/go-semver/semver"
+	plugin "github.com/hashicorp/go-plugin"
 	"github.com/hashicorp/nomad/client/config"
 	"github.com/hashicorp/nomad/client/driver/executor"
 	dstructs "github.com/hashicorp/nomad/client/driver/structs"
@@ -26,12 +29,27 @@ import (
 
 var (
 	reQemuVersion = regexp.MustCompile(`version (\d[\.\d+]+)`)
+
+	// Prior to qemu 2.10.1, monitor socket paths are truncated to 108 bytes.
+	// We should consider this if driver.qemu.version is < 2.10.1 and the
+	// generated monitor path is too long.
+
+	//
+	// Relevant fix is here:
+	// https://github.com/qemu/qemu/commit/ad9579aaa16d5b385922d49edac2c96c79bcfb6
+	qemuVersionLongSocketPathFix = semver.New("2.10.1")
 )
 
 const (
-	// The key populated in Node Attributes to indicate presence of the Qemu
+	// The key populated in Node Attributes to indicate presence of the Qemu driver
-	// driver
+	qemuDriverAttr        = "driver.qemu"
-	qemuDriverAttr = "driver.qemu"
+	qemuDriverVersionAttr = "driver.qemu.version"
+	// Represents an ACPI shutdown request to the VM (emulates pressing a physical power button)
+	// Reference: https://en.wikibooks.org/wiki/QEMU/Monitor
+	qemuGracefulShutdownMsg = "system_powerdown\n"
+	qemuMonitorSocketName   = "qemu-monitor.sock"
+	// Maximum socket path length prior to qemu 2.10.1
+	qemuLegacyMaxMonitorPathLen = 108
 )
 
 // QemuDriver is a driver for running images via Qemu
@@ -45,10 +63,11 @@ type QemuDriver struct {
 }
 
 type QemuDriverConfig struct {
-	ImagePath   string           `mapstructure:"image_path"`
+	ImagePath        string           `mapstructure:"image_path"`
-	Accelerator string           `mapstructure:"accelerator"`
+	Accelerator      string           `mapstructure:"accelerator"`
-	PortMap     []map[string]int `mapstructure:"port_map"` // A map of host port labels and to guest ports.
+	GracefulShutdown bool             `mapstructure:"graceful_shutdown"`
-	Args        []string         `mapstructure:"args"`     // extra arguments to qemu executable
+	PortMap          []map[string]int `mapstructure:"port_map"` // A map of host port labels and to guest ports.
+	Args             []string         `mapstructure:"args"`     // extra arguments to qemu executable
 }
 
 // qemuHandle is returned from Start/Open as a handle to the PID
@@ -56,6 +75,7 @@ type qemuHandle struct {
 	pluginClient   *plugin.Client
 	userPid        int
 	executor       executor.Executor
+	monitorPath    string
 	killTimeout    time.Duration
 	maxKillTimeout time.Duration
 	logger         *log.Logger
@@ -64,6 +84,29 @@ type qemuHandle struct {
 	doneCh         chan struct{}
 }
 
+// getMonitorPath is used to determine whether a qemu monitor socket can be
+// safely created and accessed in the task directory by the version of qemu
+// present on the host. If it is safe to use, the socket's full path is
+// returned along with a nil error. Otherwise, an empty string is returned
+// along with a descriptive error.
+func (d *QemuDriver) getMonitorPath(dir string) (string, error) {
+	var longPathSupport bool
+	currentQemuVer := d.DriverContext.node.Attributes[qemuDriverVersionAttr]
+	currentQemuSemver := semver.New(currentQemuVer)
+	if currentQemuSemver.LessThan(*qemuVersionLongSocketPathFix) {
+		longPathSupport = false
+		d.logger.Printf("[DEBUG] driver.qemu: long socket paths are not available in this version of QEMU (%s)", currentQemuVer)
+	} else {
+		longPathSupport = true
+		d.logger.Printf("[DEBUG] driver.qemu: long socket paths available in this version of QEMU (%s)", currentQemuVer)
+	}
+	fullSocketPath := fmt.Sprintf("%s/%s", dir, qemuMonitorSocketName)
+	if len(fullSocketPath) > qemuLegacyMaxMonitorPathLen && longPathSupport == false {
+		return "", fmt.Errorf("monitor path is too long for this version of qemu")
+	}
+	return fullSocketPath, nil
+}
+
 // NewQemuDriver is used to create a new exec driver
 func NewQemuDriver(ctx *DriverContext) Driver {
 	return &QemuDriver{DriverContext: *ctx}
@@ -81,6 +124,10 @@ func (d *QemuDriver) Validate(config map[string]interface{}) error {
 			"accelerator": {
 				Type: fields.TypeString,
 			},
+			"graceful_shutdown": {
+				Type:     fields.TypeBool,
+				Required: false,
+			},
 			"port_map": {
 				Type: fields.TypeArray,
 			},
@@ -127,9 +174,11 @@ func (d *QemuDriver) Fingerprint(cfg *config.Config, node *structs.Node) (bool,
 		delete(node.Attributes, qemuDriverAttr)
 		return false, fmt.Errorf("Unable to parse Qemu version string: %#v", matches)
 	}
+	currentQemuVersion := matches[1]
 
 	node.Attributes[qemuDriverAttr] = "1"
-	node.Attributes["driver.qemu.version"] = matches[1]
+	node.Attributes[qemuDriverVersionAttr] = currentQemuVersion
+
 	return true, nil
 }
 
@@ -190,6 +239,22 @@ func (d *QemuDriver) Start(ctx *ExecContext, task *structs.Task) (*StartResponse
 		"-nographic",
 	}
 
+	var monitorPath string
+	if d.driverConfig.GracefulShutdown {
+		if runtime.GOOS == "windows" {
+			return nil, errors.New("QEMU graceful shutdown is unsupported on the Windows platform")
+		}
+		// This socket will be used to manage the virtual machine (for example,
+		// to perform graceful shutdowns)
+		monitorPath, err := d.getMonitorPath(ctx.TaskDir.Dir)
+		if err != nil {
+			d.logger.Printf("[ERR] driver.qemu: could not get qemu monitor path: %s", err)
+			return nil, err
+		}
+		d.logger.Printf("[DEBUG] driver.qemu: got monitor path OK: %s", monitorPath)
+		args = append(args, "-monitor", fmt.Sprintf("unix:%s,server,nowait", monitorPath))
+	}
+
 	// Add pass through arguments to qemu executable. A user can specify
 	// these arguments in driver task configuration. These arguments are
 	// passed directly to the qemu driver as command line options.
@@ -231,6 +296,9 @@ func (d *QemuDriver) Start(ctx *ExecContext, task *structs.Task) (*StartResponse
 
 	// If using KVM, add optimization args
 	if accelerator == "kvm" {
+		if runtime.GOOS == "windows" {
+			return nil, errors.New("KVM accelerator is unsupported on the Windows platform")
+		}
 		args = append(args,
 			"-enable-kvm",
 			"-cpu", "host",
@@ -239,7 +307,7 @@ func (d *QemuDriver) Start(ctx *ExecContext, task *structs.Task) (*StartResponse
 		)
 	}
 
-	d.logger.Printf("[DEBUG] Starting QemuVM command: %q", strings.Join(args, " "))
+	d.logger.Printf("[DEBUG] driver.qemu: starting QemuVM command: %q", strings.Join(args, " "))
 	pluginLogFile := filepath.Join(ctx.TaskDir.Dir, "executor.out")
 	executorConfig := &dstructs.ExecutorConfig{
 		LogFile:  pluginLogFile,
@@ -272,7 +340,7 @@ func (d *QemuDriver) Start(ctx *ExecContext, task *structs.Task) (*StartResponse
 		pluginClient.Kill()
 		return nil, err
 	}
-	d.logger.Printf("[INFO] Started new QemuVM: %s", vmID)
+	d.logger.Printf("[INFO] driver.qemu: started new QemuVM: %s", vmID)
 
 	// Create and Return Handle
 	maxKill := d.DriverContext.config.MaxKillTimeout
@@ -282,6 +350,7 @@ func (d *QemuDriver) Start(ctx *ExecContext, task *structs.Task) (*StartResponse
 		userPid:        ps.Pid,
 		killTimeout:    GetKillTimeout(task.KillTimeout, maxKill),
 		maxKillTimeout: maxKill,
+		monitorPath:    monitorPath,
 		version:        d.config.Version.VersionNumber(),
 		logger:         d.logger,
 		doneCh:         make(chan struct{}),
@@ -308,7 +377,7 @@ type qemuId struct {
 func (d *QemuDriver) Open(ctx *ExecContext, handleID string) (DriverHandle, error) {
 	id := &qemuId{}
 	if err := json.Unmarshal([]byte(handleID), id); err != nil {
-		return nil, fmt.Errorf("Failed to parse handle '%s': %v", handleID, err)
+		return nil, fmt.Errorf("Failed to parse handle %q: %v", handleID, err)
 	}
 
 	pluginConfig := &plugin.ClientConfig{
@@ -317,9 +386,9 @@ func (d *QemuDriver) Open(ctx *ExecContext, handleID string) (DriverHandle, erro
 
 	exec, pluginClient, err := createExecutorWithConfig(pluginConfig, d.config.LogOutput)
 	if err != nil {
-		d.logger.Println("[ERR] driver.qemu: error connecting to plugin so destroying plugin pid and user pid")
+		d.logger.Printf("[ERR] driver.qemu: error connecting to plugin so destroying plugin pid %d and user pid %d", id.PluginConfig.Pid, id.UserPid)
 		if e := destroyPlugin(id.PluginConfig.Pid, id.UserPid); e != nil {
-			d.logger.Printf("[ERR] driver.qemu: error destroying plugin and userpid: %v", e)
+			d.logger.Printf("[ERR] driver.qemu: error destroying plugin pid %d and userpid %d: %v", id.PluginConfig.Pid, id.UserPid, e)
 		}
 		return nil, fmt.Errorf("error connecting to plugin: %v", err)
 	}
@@ -381,27 +450,43 @@ func (h *qemuHandle) Signal(s os.Signal) error {
 	return fmt.Errorf("Qemu driver can't send signals")
 }
 
-// TODO: allow a 'shutdown_command' that can be executed over a ssh connection
-// to the VM
 func (h *qemuHandle) Kill() error {
-	if err := h.executor.ShutDown(); err != nil {
+	gracefulShutdownSent := false
-		if h.pluginClient.Exited() {
+	// Attempt a graceful shutdown only if it was configured in the job
-			return nil
+	if h.monitorPath != "" {
+		if err := sendQemuShutdown(h.logger, h.monitorPath, h.userPid); err == nil {
+			gracefulShutdownSent = true
+		} else {
+			h.logger.Printf("[DEBUG] driver.qemu: error sending graceful shutdown for user process pid %d: %s", h.userPid, err)
 		}
-		return fmt.Errorf("executor Shutdown failed: %v", err)
 	}
 
+	// If Nomad did not send a graceful shutdown signal, issue an interrupt to
+	// the qemu process as a last resort
+	if gracefulShutdownSent == false {
+		if err := h.executor.ShutDown(); err != nil {
+			if h.pluginClient.Exited() {
+				return nil
+			}
+			return fmt.Errorf("executor Shutdown failed: %v", err)
+		}
+	}
+
+	// If the qemu process exits before the kill timeout is reached, doneChan
+	// will close and we'll exit without an error. If it takes too long, the
+	// timer will fire and we'll attempt to kill the process.
 	select {
 	case <-h.doneCh:
 		return nil
 	case <-time.After(h.killTimeout):
+		h.logger.Printf("[DEBUG] driver.qemu: kill timeout of %s exceeded for user process pid %d", h.killTimeout.String(), h.userPid)
+
 		if h.pluginClient.Exited() {
 			return nil
 		}
 		if err := h.executor.Exit(); err != nil {
 			return fmt.Errorf("executor Exit failed: %v", err)
 		}
-
 		return nil
 	}
 }
@@ -414,7 +499,7 @@ func (h *qemuHandle) run() {
 	ps, werr := h.executor.Wait()
 	if ps.ExitCode == 0 && werr != nil {
 		if e := killProcess(h.userPid); e != nil {
-			h.logger.Printf("[ERR] driver.qemu: error killing user process: %v", e)
+			h.logger.Printf("[ERR] driver.qemu: error killing user process pid %d: %v", h.userPid, e)
 		}
 	}
 	close(h.doneCh)
@@ -427,3 +512,23 @@ func (h *qemuHandle) run() {
 	h.waitCh <- &dstructs.WaitResult{ExitCode: ps.ExitCode, Signal: ps.Signal, Err: werr}
 	close(h.waitCh)
 }
+
+// sendQemuShutdown attempts to issue an ACPI power-off command via the qemu
+// monitor
+func sendQemuShutdown(logger *log.Logger, monitorPath string, userPid int) error {
+	if monitorPath == "" {
+		return errors.New("monitorPath not set")
+	}
+	monitorSocket, err := net.Dial("unix", monitorPath)
+	if err != nil {
+		logger.Printf("[WARN] driver.qemu: could not connect to qemu monitor %q for user process pid %d: %s", monitorPath, userPid, err)
+		return err
+	}
+	defer monitorSocket.Close()
+	logger.Printf("[DEBUG] driver.qemu: sending graceful shutdown command to qemu monitor socket %q for user process pid %d", monitorPath, userPid)
+	_, err = monitorSocket.Write([]byte(qemuGracefulShutdownMsg))
+	if err != nil {
+		logger.Printf("[WARN] driver.qemu: failed to send shutdown message %q to monitor socket %q for user process pid %d: %s", qemuGracefulShutdownMsg, monitorPath, userPid, err)
+	}
+	return err
+}

client/driver/qemu_test.go

@@ -2,10 +2,12 @@ package driver
 
 import (
 	"fmt"
+	"os"
 	"path/filepath"
 	"strings"
 	"syscall"
 	"testing"
+	"time"
 
 	"github.com/hashicorp/nomad/client/config"
 	"github.com/hashicorp/nomad/nomad/structs"
@@ -39,15 +41,16 @@ func TestQemuDriver_Fingerprint(t *testing.T) {
 	if !apply {
 		t.Fatalf("should apply")
 	}
-	if node.Attributes["driver.qemu"] == "" {
+	if node.Attributes[qemuDriverAttr] == "" {
 		t.Fatalf("Missing Qemu driver")
 	}
-	if node.Attributes["driver.qemu.version"] == "" {
+	if node.Attributes[qemuDriverVersionAttr] == "" {
 		t.Fatalf("Missing Qemu driver version")
 	}
 }
 
 func TestQemuDriver_StartOpen_Wait(t *testing.T) {
+	logger := testLogger()
 	if !testutil.IsTravis() {
 		t.Parallel()
 	}
@@ -56,8 +59,9 @@ func TestQemuDriver_StartOpen_Wait(t *testing.T) {
 		Name:   "linux",
 		Driver: "qemu",
 		Config: map[string]interface{}{
-			"image_path":  "linux-0.2.img",
+			"image_path":        "linux-0.2.img",
-			"accelerator": "tcg",
+			"accelerator":       "tcg",
+			"graceful_shutdown": false,
 			"port_map": []map[string]int{{
 				"main": 22,
 				"web":  8080,
@@ -85,10 +89,11 @@ func TestQemuDriver_StartOpen_Wait(t *testing.T) {
 
 	// Copy the test image into the task's directory
 	dst := ctx.ExecCtx.TaskDir.Dir
+
 	copyFile("./test-resources/qemu/linux-0.2.img", filepath.Join(dst, "linux-0.2.img"), t)
 
 	if _, err := d.Prestart(ctx.ExecCtx, task); err != nil {
-		t.Fatalf("Prestart faild: %v", err)
+		t.Fatalf("Prestart failed: %v", err)
 	}
 
 	resp, err := d.Start(ctx.ExecCtx, task)
@@ -112,29 +117,33 @@ func TestQemuDriver_StartOpen_Wait(t *testing.T) {
 
 	// Clean up
 	if err := resp.Handle.Kill(); err != nil {
-		fmt.Printf("\nError killing Qemu test: %s", err)
+		logger.Printf("Error killing Qemu test: %s", err)
 	}
 }
 
-func TestQemuDriverUser(t *testing.T) {
+func TestQemuDriver_GracefulShutdown(t *testing.T) {
+	logger := testLogger()
 	if !testutil.IsTravis() {
 		t.Parallel()
 	}
 	ctestutils.QemuCompatible(t)
-	tasks := []*structs.Task{{
+	task := &structs.Task{
 		Name:   "linux",
 		Driver: "qemu",
-		User:   "alice",
 		Config: map[string]interface{}{
-			"image_path":  "linux-0.2.img",
+			"image_path":        "linux-0.2.img",
-			"accelerator": "tcg",
+			"accelerator":       "tcg",
+			"graceful_shutdown": true,
 			"port_map": []map[string]int{{
 				"main": 22,
 				"web":  8080,
 			}},
 			"args": []string{"-nodefconfig", "-nodefaults"},
-			"msg":  "unknown user alice",
 		},
+		// With the use of tcg acceleration, it's very unlikely a qemu instance
+		// will boot (and gracefully halt) in a reasonable amount of time, so
+		// this timeout is kept low to reduce test execution time.
+		KillTimeout: time.Duration(1 * time.Second),
 		LogConfig: &structs.LogConfig{
 			MaxFiles:      10,
 			MaxFileSizeMB: 10,
@@ -148,7 +157,106 @@ func TestQemuDriverUser(t *testing.T) {
 				},
 			},
 		},
-	},
+	}
+
+	ctx := testDriverContexts(t, task)
+	defer ctx.AllocDir.Destroy()
+	d := NewQemuDriver(ctx.DriverCtx)
+
+	apply, err := d.Fingerprint(&config.Config{}, ctx.DriverCtx.node)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+	if !apply {
+		t.Fatalf("should apply")
+	}
+
+	dst := ctx.ExecCtx.TaskDir.Dir
+
+	copyFile("./test-resources/qemu/linux-0.2.img", filepath.Join(dst, "linux-0.2.img"), t)
+
+	if _, err := d.Prestart(ctx.ExecCtx, task); err != nil {
+		t.Fatalf("Prestart failed: %v", err)
+	}
+
+	resp, err := d.Start(ctx.ExecCtx, task)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	// Clean up
+	defer func() {
+		if err := resp.Handle.Kill(); err != nil {
+			logger.Printf("Error killing Qemu test: %s", err)
+		}
+	}()
+
+	// The monitor socket will not exist immediately, so we'll wait up to
+	// 5 seconds for it to become available.
+	monitorPath := fmt.Sprintf("%s/linux/%s", ctx.AllocDir.AllocDir, qemuMonitorSocketName)
+	monitorPathExists := false
+	for i := 0; i < 100; i++ {
+		if _, err := os.Stat(monitorPath); !os.IsNotExist(err) {
+			logger.Printf("monitor socket exists at %q\n", monitorPath)
+			monitorPathExists = true
+			break
+		}
+		time.Sleep(200 * time.Millisecond)
+	}
+	if monitorPathExists == false {
+		t.Fatalf("monitor socket did not exist after waiting 20 seconds")
+	}
+
+	// userPid supplied in sendQemuShutdown calls is bogus (it's used only
+	// for log output)
+	if err := sendQemuShutdown(ctx.DriverCtx.logger, "", 0); err == nil {
+		t.Fatalf("sendQemuShutdown should return an error if monitorPath parameter is empty")
+	}
+
+	if err := sendQemuShutdown(ctx.DriverCtx.logger, "/path/that/does/not/exist", 0); err == nil {
+		t.Fatalf("sendQemuShutdown should return an error if file does not exist at monitorPath")
+	}
+
+	if err := sendQemuShutdown(ctx.DriverCtx.logger, monitorPath, 0); err != nil {
+		t.Fatalf("unexpected error from sendQemuShutdown: %s", err)
+	}
+}
+
+func TestQemuDriverUser(t *testing.T) {
+	if !testutil.IsTravis() {
+		t.Parallel()
+	}
+	ctestutils.QemuCompatible(t)
+	tasks := []*structs.Task{
+		{
+			Name:   "linux",
+			Driver: "qemu",
+			User:   "alice",
+			Config: map[string]interface{}{
+				"image_path":        "linux-0.2.img",
+				"accelerator":       "tcg",
+				"graceful_shutdown": false,
+				"port_map": []map[string]int{{
+					"main": 22,
+					"web":  8080,
+				}},
+				"args": []string{"-nodefconfig", "-nodefaults"},
+				"msg":  "unknown user alice",
+			},
+			LogConfig: &structs.LogConfig{
+				MaxFiles:      10,
+				MaxFileSizeMB: 10,
+			},
+			Resources: &structs.Resources{
+				CPU:      500,
+				MemoryMB: 512,
+				Networks: []*structs.NetworkResource{
+					{
+						ReservedPorts: []structs.Port{{Label: "main", Value: 22000}, {Label: "web", Value: 80}},
+					},
+				},
+			},
+		},
 		{
 			Name:   "linux",
 			Driver: "qemu",
@@ -193,9 +301,127 @@ func TestQemuDriverUser(t *testing.T) {
 			resp.Handle.Kill()
 			t.Fatalf("Should've failed")
 		}
+
 		msg := task.Config["msg"].(string)
 		if !strings.Contains(err.Error(), msg) {
 			t.Fatalf("Expecting '%v' in '%v'", msg, err)
 		}
 	}
 }
+
+func TestQemuDriverGetMonitorPathOldQemu(t *testing.T) {
+	task := &structs.Task{
+		Name:   "linux",
+		Driver: "qemu",
+		Config: map[string]interface{}{
+			"image_path":        "linux-0.2.img",
+			"accelerator":       "tcg",
+			"graceful_shutdown": true,
+			"port_map": []map[string]int{{
+				"main": 22,
+				"web":  8080,
+			}},
+			"args": []string{"-nodefconfig", "-nodefaults"},
+		},
+		KillTimeout: time.Duration(1 * time.Second),
+		LogConfig: &structs.LogConfig{
+			MaxFiles:      10,
+			MaxFileSizeMB: 10,
+		},
+		Resources: &structs.Resources{
+			CPU:      500,
+			MemoryMB: 512,
+			Networks: []*structs.NetworkResource{
+				{
+					ReservedPorts: []structs.Port{{Label: "main", Value: 22000}, {Label: "web", Value: 80}},
+				},
+			},
+		},
+	}
+
+	ctx := testDriverContexts(t, task)
+	defer ctx.AllocDir.Destroy()
+
+	// Simulate an older version of qemu which does not support long monitor socket paths
+	ctx.DriverCtx.node.Attributes[qemuDriverVersionAttr] = "2.0.0"
+
+	d := &QemuDriver{DriverContext: *ctx.DriverCtx}
+
+	shortPath := strings.Repeat("x", 10)
+	_, err := d.getMonitorPath(shortPath)
+	if err != nil {
+		t.Fatal("Should not have returned an error")
+	}
+
+	longPath := strings.Repeat("x", qemuLegacyMaxMonitorPathLen+100)
+	_, err = d.getMonitorPath(longPath)
+	if err == nil {
+		t.Fatal("Should have returned an error")
+	}
+
+	// Max length includes the '/' separator and socket name
+	maxLengthCount := qemuLegacyMaxMonitorPathLen - len(qemuMonitorSocketName) - 1
+	maxLengthLegacyPath := strings.Repeat("x", maxLengthCount)
+	_, err = d.getMonitorPath(maxLengthLegacyPath)
+	if err != nil {
+		t.Fatalf("Should not have returned an error: %s", err)
+	}
+}
+
+func TestQemuDriverGetMonitorPathNewQemu(t *testing.T) {
+	task := &structs.Task{
+		Name:   "linux",
+		Driver: "qemu",
+		Config: map[string]interface{}{
+			"image_path":        "linux-0.2.img",
+			"accelerator":       "tcg",
+			"graceful_shutdown": true,
+			"port_map": []map[string]int{{
+				"main": 22,
+				"web":  8080,
+			}},
+			"args": []string{"-nodefconfig", "-nodefaults"},
+		},
+		KillTimeout: time.Duration(1 * time.Second),
+		LogConfig: &structs.LogConfig{
+			MaxFiles:      10,
+			MaxFileSizeMB: 10,
+		},
+		Resources: &structs.Resources{
+			CPU:      500,
+			MemoryMB: 512,
+			Networks: []*structs.NetworkResource{
+				{
+					ReservedPorts: []structs.Port{{Label: "main", Value: 22000}, {Label: "web", Value: 80}},
+				},
+			},
+		},
+	}
+
+	ctx := testDriverContexts(t, task)
+	defer ctx.AllocDir.Destroy()
+
+	// Simulate a version of qemu which supports long monitor socket paths
+	ctx.DriverCtx.node.Attributes[qemuDriverVersionAttr] = "2.99.99"
+
+	d := &QemuDriver{DriverContext: *ctx.DriverCtx}
+
+	shortPath := strings.Repeat("x", 10)
+	_, err := d.getMonitorPath(shortPath)
+	if err != nil {
+		t.Fatal("Should not have returned an error")
+	}
+
+	longPath := strings.Repeat("x", qemuLegacyMaxMonitorPathLen+100)
+	_, err = d.getMonitorPath(longPath)
+	if err != nil {
+		t.Fatal("Should not have returned an error")
+	}
+
+	maxLengthCount := qemuLegacyMaxMonitorPathLen - len(qemuMonitorSocketName) - 1
+	maxLengthLegacyPath := strings.Repeat("x", maxLengthCount)
+	_, err = d.getMonitorPath(maxLengthLegacyPath)
+	if err != nil {
+		t.Fatal("Should not have returned an error")
+	}
+}

website/source/docs/drivers/qemu.html.md

@@ -29,9 +29,10 @@ task "webservice" {
   driver = "qemu"
 
   config {
-    image_path  = "/path/to/my/linux.img"
+    image_path        = "/path/to/my/linux.img"
-    accelerator = "kvm"
+    accelerator       = "kvm"
-    args        = ["-nodefaults", "-nodefconfig"]
+    graceful_shutdown = true
+    args              = ["-nodefaults", "-nodefconfig"]
   }
 }  
 ```
@@ -47,6 +48,21 @@ The `qemu` driver supports the following configuration in the job spec:
   If the host machine has `qemu` installed with KVM support, users can specify
   `kvm` for the `accelerator`. Default is `tcg`.
 
+* `graceful_shutdown` `(bool: false)` - Using the [qemu
+  monitor](https://en.wikibooks.org/wiki/QEMU/Monitor), send an ACPI shutdown
+  signal to virtual machines rather than simply terminating them. This emulates
+  a physical power button press, and gives instances a chance to shut down
+  cleanly. If the VM is still running after ``kill_timeout``, it will be
+  forcefully terminated. (Note that
+  [prior to qemu 2.10.1](https://github.com/qemu/qemu/commit/ad9579aaa16d5b385922d49edac2c96c79bcfb6),
+  the monitor socket path is limited to 108 characters. Graceful shutdown will
+  be disabled if qemu is < 2.10.1 and the generated monitor path exceeds this
+  length. You may encounter this issue if you set long
+  [data_dir](https://www.nomadproject.io/docs/agent/configuration/index.html#data_dir)
+  or
+  [alloc_dir](https://www.nomadproject.io/docs/agent/configuration/client.html#alloc_dir)
+  paths.)
+
 * `port_map` - (Optional) A key-value map of port labels.
 
     ```hcl