fix(systemd): remove any existing unit when calling mask

Documentation/cloud-config.md

@@ -123,6 +123,7 @@ The `coreos.units.*` parameters define a list of arbitrary systemd units to star
 - **enable**: Boolean indicating whether or not to handle the [Install] section of the unit file. This is similar to running `systemctl enable <name>`. Default value is false.
 - **content**: Plaintext string representing entire unit file. If no value is provided, the unit is assumed to exist already.
 - **command**: Command to execute on unit: start, stop, reload, restart, try-restart, reload-or-restart, reload-or-try-restart. Default value is restart.
+- **mask**: Whether to mask the unit file by symlinking it to `/dev/null` (analogous to `systemctl mask <name>`). Note that unlike `systemctl mask`, **this will destructively remove any existing unit file** located at `/etc/systemd/system/<unit>`, to ensure that the mask succeeds. Default value is false.
 
 **NOTE:** The command field is ignored for all network, netdev, and link units. The systemd-networkd.service unit will be restarted in their place.
 

system/systemd.go

@@ -179,9 +179,17 @@ func MachineID(root string) string {
 	return id
 }
 
+// MaskUnit masks a Unit by the given name by symlinking its unit file (in
+// /etc/systemd/system) to /dev/null, analogous to `systemctl mask`
+// N.B.: Unlike `systemctl mask`, this function will *remove any existing unit
+// file* in /etc/systemd/system, to ensure that the mask will succeed.
 func MaskUnit(unit string, root string) error {
 	masked := path.Join(root, "etc", "systemd", "system", unit)
-	if err := os.MkdirAll(path.Dir(masked), os.FileMode(0755)); err != nil {
+	if _, err := os.Stat(masked); os.IsNotExist(err) {
+		if err := os.MkdirAll(path.Dir(masked), os.FileMode(0755)); err != nil {
+			return err
+		}
+	} else if err := os.Remove(masked); err != nil {
 		return err
 	}
 	return os.Symlink("/dev/null", masked)

system/systemd_test.go

@@ -154,16 +154,33 @@ func TestMaskUnit(t *testing.T) {
 		t.Fatalf("Unable to create tempdir: %v", err)
 	}
 	defer os.RemoveAll(dir)
-	if err := MaskUnit("foo.service", dir); err != nil {
-		t.Fatalf("Unable to mask unit: %v", err)
-	}
 
-	fullPath := path.Join(dir, "etc", "systemd", "system", "foo.service")
+	// Ensure mask works with units that do not currently exist
-	target, err := os.Readlink(fullPath)
+	if err := MaskUnit("foo.service", dir); err != nil {
+		t.Fatalf("Unable to mask new unit: %v", err)
+	}
+	fooPath := path.Join(dir, "etc", "systemd", "system", "foo.service")
+	fooTgt, err := os.Readlink(fooPath)
 	if err != nil {
 		t.Fatalf("Unable to read link", err)
 	}
-	if target != "/dev/null" {
+	if fooTgt != "/dev/null" {
-		t.Fatalf("unit not masked, got unit target", target)
+		t.Fatalf("unit not masked, got unit target", fooTgt)
+	}
+
+	// Ensure mask works with unit files that already exist
+	barPath := path.Join(dir, "etc", "systemd", "system", "bar.service")
+	if _, err := os.Create(barPath); err != nil {
+		t.Fatalf("Error creating new unit file: %v", err)
+	}
+	if err := MaskUnit("bar.service", dir); err != nil {
+		t.Fatalf("Unable to mask existing unit: %v", err)
+	}
+	barTgt, err := os.Readlink(barPath)
+	if err != nil {
+		t.Fatalf("Unable to read link", err)
+	}
+	if barTgt != "/dev/null" {
+		t.Fatalf("unit not masked, got unit target", barTgt)
 	}
 }