Stop trying to use mount for image based drivers

Fixes #2178 and allows using Docker and other image based drivers even
when nomad is run as a non-root user.

`client/allocdir` tests can be run as a non-root user to ensure this
behavior and tests that rely on root or non-root users properly detect
their effective user and skip instead of fail.

client/allocdir/alloc_dir_test.go

@@ -150,11 +150,11 @@ func TestAllocDir_Snapshot(t *testing.T) {
 
 	// Build 2 task dirs
 	td1 := d.NewTaskDir(t1.Name)
-	if err := td1.Build(nil, cstructs.FSIsolationNone); err != nil {
+	if err := td1.Build(nil, cstructs.FSIsolationImage); err != nil {
 		t.Fatalf("error build task=%q dir: %v", t1.Name, err)
 	}
 	td2 := d.NewTaskDir(t2.Name)
-	if err := td2.Build(nil, cstructs.FSIsolationNone); err != nil {
+	if err := td2.Build(nil, cstructs.FSIsolationImage); err != nil {
 		t.Fatalf("error build task=%q dir: %v", t2.Name, err)
 	}
 
@@ -224,12 +224,12 @@ func TestAllocDir_Move(t *testing.T) {
 	defer d2.Destroy()
 
 	td1 := d1.NewTaskDir(t1.Name)
-	if err := td1.Build(nil, cstructs.FSIsolationNone); err != nil {
+	if err := td1.Build(nil, cstructs.FSIsolationImage); err != nil {
 		t.Fatalf("TaskDir.Build() faild: %v", err)
 	}
 
 	td2 := d2.NewTaskDir(t1.Name)
-	if err := td2.Build(nil, cstructs.FSIsolationNone); err != nil {
+	if err := td2.Build(nil, cstructs.FSIsolationImage); err != nil {
 		t.Fatalf("TaskDir.Build() faild: %v", err)
 	}
 
@@ -322,7 +322,7 @@ func TestAllocDir_ReadAt_SecretDir(t *testing.T) {
 	defer d.Destroy()
 
 	td := d.NewTaskDir(t1.Name)
-	if err := td.Build(nil, cstructs.FSIsolationNone); err != nil {
+	if err := td.Build(nil, cstructs.FSIsolationImage); err != nil {
 		t.Fatalf("TaskDir.Build() failed: %v", err)
 	}
 

client/allocdir/task_dir.go

@@ -89,11 +89,12 @@ func (t *TaskDir) Build(chroot map[string]string, fsi cstructs.FSIsolation) erro
 		}
 	}
 
-	// Always link the shared task directory even though image based
-	// filesystem isolalation doesn't require it. This way we have a
-	// consistent task dir.
-	if err := linkDir(t.SharedAllocDir, t.SharedTaskDir); err != nil {
-		return fmt.Errorf("Failed to mount shared directory for task: %v", err)
+	// Only link alloc dir into task dir for no and chroot fs isolation.
+	// Image based isolation will bind the shared alloc dir in the driver.
+	if fsi == cstructs.FSIsolationNone || fsi == cstructs.FSIsolationChroot {
+		if err := linkDir(t.SharedAllocDir, t.SharedTaskDir); err != nil {
+			return fmt.Errorf("Failed to mount shared directory for task: %v", err)
+		}
 	}
 
 	// Create the secret directory

client/allocdir/task_dir_test.go

@@ -5,6 +5,8 @@ import (
 	"os"
 	"path/filepath"
 	"testing"
+
+	cstructs "github.com/hashicorp/nomad/client/structs"
 )
 
 // Test that building a chroot will skip nonexistent directories.
@@ -82,3 +84,26 @@ func TestTaskDir_EmbedDirs(t *testing.T) {
 		}
 	}
 }
+
+// Test that task dirs for image based isolation don't require root.
+func TestTaskDir_NonRoot(t *testing.T) {
+	if os.Geteuid() == 0 {
+		t.Skip("test should be run as non-root user")
+	}
+	tmp, err := ioutil.TempDir("", "AllocDir")
+	if err != nil {
+		t.Fatalf("Couldn't create temp dir: %v", err)
+	}
+	defer os.RemoveAll(tmp)
+
+	d := NewAllocDir(testLogger(), tmp)
+	defer d.Destroy()
+	td := d.NewTaskDir(t1.Name)
+	if err := d.Build(); err != nil {
+		t.Fatalf("Build() failed: %v", err)
+	}
+
+	if err := td.Build(nil, cstructs.FSIsolationImage); err != nil {
+		t.Fatalf("TaskDir.Build failed: %v", err)
+	}
+}