disallow services starting with "-"

- consolidate svc / -svc validation in CombineLayers

internals/plan/plan.go

@@ -769,10 +769,7 @@ func CombineLayers(layers ...*Layer) (*Layer, error) {
 			if serviceName == "all" || serviceName == "-all" {
 				continue
 			}
-			if _, ok := combined.Services[serviceName]; ok {
-				continue
-			}
-			// Check if this is `-svc`
+			// This could be `-svc` - try to trim a preceding
 			serviceName = strings.TrimPrefix(serviceName, "-")
 			if _, ok := combined.Services[serviceName]; ok {
 				continue
@@ -932,6 +929,11 @@ func ParseLayer(order int, label string, data []byte) (*Layer, error) {
 		if name == "all" || name == "default" || name == "none" {
 			logger.Noticef("WARNING: using %q as service name is deprecated", name)
 		}
+		if strings.HasPrefix(name, "-") {
+			return nil, &FormatError{
+				Message: fmt.Sprintf(`cannot use service name %q: starting with "-" not allowed`, name),
+			}
+		}
 		if service == nil {
 			return nil, &FormatError{
 				Message: fmt.Sprintf("service object cannot be null for service %q", name),

internals/plan/plan_test.go

@@ -1102,6 +1102,15 @@ var planTests = []planTest{{
 				services: [nonexistent]
 				override: merge
 `},
+}, {
+	summary: `Service name can't start with "-"`,
+	error:   `cannot use service name "-svc1": starting with "-" not allowed`,
+	input: []string{`
+		services:
+			-svc1:
+				command: foo
+				override: merge
+`},
 }}
 
 func (s *S) TestParseLayer(c *C) {