Switch to pre-0.9 behaviour for handling volumes

In Nomad 0.9, we made volume driver handling the same for `""`, and
`"local"` volumes. Prior to Nomad 0.9 however these had slightly different
behaviour for relative paths and named volumes.

Prior to 0.9 the empty string would expand relative paths within the task
dir, and `"local"` volumes that are not absolute paths would be treated
as docker named volumes.

This commit reverts to the previous behaviour as follows:

| Nomad Version | Driver  |   Volume Spec    | Behaviour                 |
|-------------------------------------------------------------------------
| all           | ""      | testing:/testing | allocdir/testing          |
| 0.8.7         | "local" | testing:/testing | "testing" as named volume |
| 0.9.0         | "local" | testing:/testing | allocdir/testing          |
| 0.9.1         | "local" | testing:/testing | "testing" as named volume |

drivers/docker/driver.go

@@ -603,9 +603,9 @@ func (d *Driver) containerBinds(task *drivers.TaskConfig, driverConfig *TaskConf
 	secretDirBind := fmt.Sprintf("%s:%s", task.TaskDir().SecretsDir, task.Env[taskenv.SecretsDir])
 	binds := []string{allocDirBind, taskLocalBind, secretDirBind}
 
-	localBindVolume := driverConfig.VolumeDriver == "" || driverConfig.VolumeDriver == "local"
+	taskLocalBindVolume := driverConfig.VolumeDriver == ""
 
-	if !d.config.Volumes.Enabled && !localBindVolume {
+	if !d.config.Volumes.Enabled && !taskLocalBindVolume {
 		return nil, fmt.Errorf("volumes are not enabled; cannot use volume driver %q", driverConfig.VolumeDriver)
 	}
 
@@ -618,12 +618,15 @@ func (d *Driver) containerBinds(task *drivers.TaskConfig, driverConfig *TaskConf
 		// Paths inside task dir are always allowed, Relative paths are always allowed as they mount within a container
 		// When a VolumeDriver is set, we assume we receive a binding in the format volume-name:container-dest
 		// Otherwise, we assume we receive a relative path binding in the format relative/to/task:/also/in/container
-		if localBindVolume {
+		if taskLocalBindVolume {
 			parts[0] = expandPath(task.TaskDir().Dir, parts[0])
+		} else {
+			// Resolve dotted path segments
+			parts[0] = filepath.Clean(parts[0])
+		}
 
-			if !d.config.Volumes.Enabled && !isParentPath(task.AllocDir, parts[0]) {
-				return nil, fmt.Errorf("volumes are not enabled; cannot mount host paths: %+q", userbind)
-			}
+		if !d.config.Volumes.Enabled && !isParentPath(task.AllocDir, parts[0]) {
+			return nil, fmt.Errorf("volumes are not enabled; cannot mount host paths: %+q", userbind)
 		}
 
 		binds = append(binds, strings.Join(parts, ":"))

drivers/docker/driver_unix_test.go

@@ -299,10 +299,10 @@ func TestDockerDriver_BindMountsHonorVolumesEnabledFlag(t *testing.T) {
 		},
 		{
 			name:            "relative local driver",
-			requiresVolumes: false,
+			requiresVolumes: true,
 			volumeDriver:    "local",
 			volumes:         []string{"test-path:/tmp/taskpath"},
-			expectedVolumes: []string{"/tmp/nomad/alloc-dir/demo/test-path:/tmp/taskpath"},
+			expectedVolumes: []string{"test-path:/tmp/taskpath"},
 		},
 		{
 			name:            "relative outside task-dir default driver",
@@ -311,27 +311,13 @@ func TestDockerDriver_BindMountsHonorVolumesEnabledFlag(t *testing.T) {
 			volumes:         []string{"../test-path:/tmp/taskpath"},
 			expectedVolumes: []string{"/tmp/nomad/alloc-dir/test-path:/tmp/taskpath"},
 		},
-		{
-			name:            "relative outside task-dir local driver",
-			requiresVolumes: false,
-			volumeDriver:    "local",
-			volumes:         []string{"../test-path:/tmp/taskpath"},
-			expectedVolumes: []string{"/tmp/nomad/alloc-dir/test-path:/tmp/taskpath"},
-		},
 		{
 			name:            "relative outside alloc-dir default driver",
 			requiresVolumes: true,
 			volumeDriver:    "",
 			volumes:         []string{"../../test-path:/tmp/taskpath"},
 			expectedVolumes: []string{"/tmp/nomad/test-path:/tmp/taskpath"},
 		},
-		{
-			name:            "relative outside task-dir local driver",
-			requiresVolumes: true,
-			volumeDriver:    "local",
-			volumes:         []string{"../../test-path:/tmp/taskpath"},
-			expectedVolumes: []string{"/tmp/nomad/test-path:/tmp/taskpath"},
-		},
 	}
 
 	t.Run("with volumes enabled", func(t *testing.T) {