fix: allow only valid types of the essential field

internal/setup/setup_test.go

@@ -3659,7 +3659,7 @@ var setupTests = []setupTest{{
 	},
 	relerror: "essential loop detected: mypkg1_myslice, mypkg2_myslice",
 }, {
-	summary: "Format v3 expects a map in 'essential' (pkg)",
+	summary: "Format v3 expects a map in 'essential' (pkg), received a list",
 	input: map[string]string{
 		"chisel.yaml": strings.ReplaceAll(testutil.DefaultChiselYaml, "format: v1", "format: v3"),
 		"slices/mydir/mypkg.yaml": `
@@ -3673,7 +3673,20 @@ var setupTests = []setupTest{{
 	},
 	relerror: `cannot parse package "mypkg": essential expects a map`,
 }, {
-	summary: "Format v3 expects a map in 'essential' (slice)",
+	summary: "Format v3 expects a map in 'essential' (pkg), received a string",
+	input: map[string]string{
+		"chisel.yaml": strings.ReplaceAll(testutil.DefaultChiselYaml, "format: v1", "format: v3"),
+		"slices/mydir/mypkg.yaml": `
+			package: mypkg
+			essential: not-a-list-or-map
+			slices:
+				myslice1:
+				myslice2:
+		`,
+	},
+	relerror: `cannot parse package "mypkg": essential expects a map`,
+}, {
+	summary: "Format v3 expects a map in 'essential' (slice), received a list",
 	input: map[string]string{
 		"chisel.yaml": strings.ReplaceAll(testutil.DefaultChiselYaml, "format: v1", "format: v3"),
 		"slices/mydir/mypkg.yaml": `
@@ -3686,6 +3699,18 @@ var setupTests = []setupTest{{
 		`,
 	},
 	relerror: `cannot parse slice mypkg_myslice1: essential expects a map`,
+}, {
+	summary: "Format v3 expects a map in 'essential' (slice), received a string",
+	input: map[string]string{
+		"chisel.yaml": strings.ReplaceAll(testutil.DefaultChiselYaml, "format: v1", "format: v3"),
+		"slices/mydir/mypkg.yaml": `
+			package: mypkg
+			slices:
+				myslice:
+					essential: not-a-list-or-map
+		`,
+	},
+	relerror: `cannot parse slice mypkg_myslice: essential expects a map`,
 }, {
 	summary: "In format v3 'v3-essential' is not supported (pkg)",
 	input: map[string]string{

internal/setup/yaml.go

@@ -65,19 +65,31 @@ type yamlPackage struct {
 	V3Essential map[string]*yamlEssential `yaml:"v3-essential,omitempty"`
 }
 
+type essentialStyle int
+
+const (
+	unsetEssential essentialStyle = iota
+	// invalidEssential is set when the marshaler was called but the value was of an invalid kind.
+	invalidEssential
+	// listEssential is set when the marshaler found a list.
+	// A list is valid only in format "v1" and "v2".
+	listEssential
+	// mapEssential is set when the marshaler found a map.
+	// A map is valid only from format "v3" onwards.
+	mapEssential
+)
+
 type yamlEssentialListMap struct {
 	Values map[string]*yamlEssential
-	// isList is set to true when the marshaler found a list and false if it
-	// found a map. The former is only valid in format "v1" and "v2" while the
-	// latter is valid from "v3" onwards.
-	isList bool
+	// Style of value the marshaler found.
+	style essentialStyle
 }
 
 func (es *yamlEssentialListMap) UnmarshalYAML(value *yaml.Node) error {
 	m := map[string]*yamlEssential{}
 	switch value.Kind {
 	case yaml.SequenceNode:
-		es.isList = true
+		es.style = listEssential
 		l := []string{}
 		err := value.Decode(&l)
 		if err != nil {
@@ -90,11 +102,13 @@ func (es *yamlEssentialListMap) UnmarshalYAML(value *yaml.Node) error {
 			m[sliceName] = &yamlEssential{}
 		}
 	case yaml.MappingNode:
-		es.isList = false
+		es.style = mapEssential
 		err := value.Decode(&m)
 		if err != nil {
 			return err
 		}
+	default:
+		es.style = invalidEssential
 	}
 	es.Values = m
 	return nil
@@ -441,26 +455,26 @@ func parsePackage(format, pkgName, pkgPath string, data []byte) (*Package, error
 	}
 
 	if format == "v1" || format == "v2" {
-		if len(yamlPkg.Essential.Values) > 0 && !yamlPkg.Essential.isList {
+		if yamlPkg.Essential.style != unsetEssential && yamlPkg.Essential.style != listEssential {
 			return nil, fmt.Errorf("cannot parse package %q: essential expects a list", pkgName)
 		}
 		for sliceName, yamlSlice := range yamlPkg.Slices {
-			if len(yamlSlice.Essential.Values) > 0 && !yamlSlice.Essential.isList {
+			if yamlSlice.Essential.style != unsetEssential && yamlSlice.Essential.style != listEssential {
 				return nil, fmt.Errorf("cannot parse slice %s: essential expects a list", SliceKey{pkgName, sliceName})
 			}
 		}
 	} else {
 		if yamlPkg.V3Essential != nil {
 			return nil, fmt.Errorf("cannot parse package %q: v3-essential is obsolete since format v3", pkgName)
 		}
-		if len(yamlPkg.Essential.Values) > 0 && yamlPkg.Essential.isList {
+		if yamlPkg.Essential.style != unsetEssential && yamlPkg.Essential.style != mapEssential {
 			return nil, fmt.Errorf("cannot parse package %q: essential expects a map", pkgName)
 		}
 		for sliceName, yamlSlice := range yamlPkg.Slices {
 			if yamlSlice.V3Essential != nil {
 				return nil, fmt.Errorf("cannot parse slice %s: v3-essential is obsolete since format v3", SliceKey{pkgName, sliceName})
 			}
-			if len(yamlSlice.Essential.Values) > 0 && yamlSlice.Essential.isList {
+			if yamlSlice.Essential.style != unsetEssential && yamlSlice.Essential.style != mapEssential {
 				return nil, fmt.Errorf("cannot parse slice %s: essential expects a map", SliceKey{pkgName, sliceName})
 			}
 		}