Simplify plugin/pkg/fall (#1358)

* Simplify plugin/pkg/fall

* Remove unused import

* Fix fall_test

* Get fall coverage to 100% just because

* gofmt. sigh.

plugin/pkg/fall/fall.go

@@ -7,71 +7,52 @@ import (
 
 // F can be nil to allow for no fallthrough, empty allow all zones to fallthrough or
 // contain a zone list that is checked.
-type F []string
-
-// New returns a new F.
-func New() *F { return new(F) }
+type F struct {
+	Zones []string
+}
 
 // Through will check if we should fallthrough for qname. Note that we've named the
 // variable in each plugin "Fall", so this then reads Fall.Through().
-func (f *F) Through(qname string) bool {
-	if f == nil {
-		return false
-	}
-	if len(*f) == 0 {
-		return true
-	}
-	zone := plugin.Zones(*f).Matches(qname)
-	return zone != ""
+func (f F) Through(qname string) bool {
+	return plugin.Zones(f.Zones).Matches(qname) != ""
 }
 
-// SetZones will set zones in f.
-func (f *F) SetZones(zones []string) {
+// setZones will set zones in f.
+func (f *F) setZones(zones []string) {
 	for i := range zones {
 		zones[i] = plugin.Host(zones[i]).Normalize()
 	}
-	*f = zones
+	f.Zones = zones
 }
 
-// Example returns an F with example.org. as the zone name.
-var Example = func() *F {
-	f := F([]string{"example.org."})
-	return &f
-}()
-
-// Zero returns a zero valued F.
-var Zero = func() *F {
-	f := F([]string{})
-	return &f
+// SetZonesFromArgs sets zones in f to the passed value or to "." if the slice is empty.
+func (f *F) SetZonesFromArgs(zones []string) {
+	if len(zones) == 0 {
+		f.setZones(Root().Zones)
+		return
+	}
+	f.setZones(zones)
 }
 
-// IsNil returns true is f is nil.
-func (f *F) IsNil() bool { return f == nil }
-
-// IsZero returns true is f is zero (and not nil).
-func (f *F) IsZero() bool {
-	if f == nil {
+// Equal returns true if f and g are equal.
+func (f F) Equal(g F) bool {
+	if len(f.Zones) != len(g.Zones) {
 		return false
 	}
-	return len(*f) == 0
-}
-
-// Equal returns true if f and g are equal. Only useful in tests, The (possible) zones
-// are *not* checked.
-func (f *F) Equal(g *F) bool {
-	if f.IsNil() {
-		if g.IsNil() {
-			return true
+	for i := range f.Zones {
+		if f.Zones[i] != g.Zones[i] {
+			return false
 		}
-		return false
-	}
-	if f.IsZero() {
-		if g.IsZero() {
-			return true
-		}
-	}
-	if len(*f) != len(*g) {
-		return false
 	}
 	return true
 }
+
+// Zero returns a zero valued F.
+var Zero = func() F {
+	return F{[]string{}}
+}
+
+// Root returns F set to only ".".
+var Root = func() F {
+	return F{[]string{"."}}
+}