drop-in: add support for drop-ins

This allows a list of drop-ins for a unit to be declared inline within a
cloud-config. For example:

  #cloud-config
  coreos:
    units:
      - name: docker.service
        drop-ins:
          - name: 50-insecure-registry.conf
            content: |
              [Service]
              Environment=DOCKER_OPTS='--insecure-registry="10.0.1.0/24"'

initialize/config.go

@@ -201,6 +201,11 @@ func processUnits(units []system.Unit, root string, um system.UnitManager) error
 	actions := make([]action, 0, len(units))
 	reload := false
 	for _, unit := range units {
+		if unit.Name == "" {
+			log.Printf("Skipping unit without name")
+			continue
+		}
+
 		if unit.Content != "" {
 			log.Printf("Writing unit %q to filesystem", unit.Name)
 			if err := um.PlaceUnit(unit); err != nil {
@@ -210,6 +215,17 @@ func processUnits(units []system.Unit, root string, um system.UnitManager) error
 			reload = true
 		}
 
+		for _, dropin := range unit.DropIns {
+			if dropin.Name != "" && dropin.Content != "" {
+				log.Printf("Writing drop-in unit %q to filesystem", dropin.Name)
+				if err := um.PlaceUnitDropIn(unit, dropin); err != nil {
+					return err
+				}
+				log.Printf("Wrote drop-in unit %q", dropin.Name)
+				reload = true
+			}
+		}
+
 		if unit.Mask {
 			log.Printf("Masking unit file %q", unit.Name)
 			if err := um.MaskUnit(unit); err != nil {

initialize/config_test.go

@@ -37,6 +37,10 @@ func (tum *TestUnitManager) PlaceUnit(u system.Unit) error {
 	tum.placed = append(tum.placed, u.Name)
 	return nil
 }
+func (tum *TestUnitManager) PlaceUnitDropIn(u system.Unit, d config.UnitDropIn) error {
+	tum.placed = append(tum.placed, u.Name+".d/"+d.Name)
+	return nil
+}
 func (tum *TestUnitManager) EnableUnitFile(u system.Unit) error {
 	tum.enabled = append(tum.enabled, u.Name)
 	return nil
@@ -122,6 +126,69 @@ func TestProcessUnits(t *testing.T) {
 				enabled: []string{"woof"},
 			},
 		},
+		{
+			units: []system.Unit{
+				system.Unit{Unit: config.Unit{
+					Name:    "hi.service",
+					Runtime: true,
+					Content: "[Service]\nExecStart=/bin/echo hi",
+					DropIns: []config.UnitDropIn{
+						{
+							Name:    "lo.conf",
+							Content: "[Service]\nExecStart=/bin/echo lo",
+						},
+						{
+							Name:    "bye.conf",
+							Content: "[Service]\nExecStart=/bin/echo bye",
+						},
+					},
+				}},
+			},
+			result: TestUnitManager{
+				placed:   []string{"hi.service", "hi.service.d/lo.conf", "hi.service.d/bye.conf"},
+				unmasked: []string{"hi.service"},
+				reload:   true,
+			},
+		},
+		{
+			units: []system.Unit{
+				system.Unit{Unit: config.Unit{
+					DropIns: []config.UnitDropIn{
+						{
+							Name:    "lo.conf",
+							Content: "[Service]\nExecStart=/bin/echo lo",
+						},
+					},
+				}},
+			},
+			result: TestUnitManager{},
+		},
+		{
+			units: []system.Unit{
+				system.Unit{Unit: config.Unit{
+					Name: "hi.service",
+					DropIns: []config.UnitDropIn{
+						{
+							Content: "[Service]\nExecStart=/bin/echo lo",
+						},
+					},
+				}},
+			},
+			result: TestUnitManager{},
+		},
+		{
+			units: []system.Unit{
+				system.Unit{Unit: config.Unit{
+					Name: "hi.service",
+					DropIns: []config.UnitDropIn{
+						{
+							Name: "lo.conf",
+						},
+					},
+				}},
+			},
+			result: TestUnitManager{},
+		},
 	}
 
 	for _, tt := range tests {