plugin/file: load secondary zones lazily on startup (#2944)

This fixes a long standing bug:
fixes: #1609

Load secondary zones in a go-routine; this required another mutex to
protect some fields; I think those were needded anyway because a
transfer can also happen when we're running; we just didn't have a test
for that situation.

The test had to be changed to wait for the transfer to happen at this is
async now.

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/file/zone.go

@@ -21,7 +21,8 @@ type Zone struct {
 	origLen int
 	file    string
 	*tree.Tree
-	Apex Apex
+	Apex
+	apexMu sync.RWMutex
 
 	TransferTo   []string
 	StartupOnce  sync.Once
@@ -32,7 +33,7 @@ type Zone struct {
 	LastReloaded   time.Time
 	reloadMu       sync.RWMutex
 	reloadShutdown chan bool
-	Upstream       *upstream.Upstream // Upstream for looking up external names during the resolution process
+	Upstream       *upstream.Upstream // Upstream for looking up external names during the resolution process.
 }
 
 // Apex contains the apex records of a zone: SOA, NS and their potential signatures.
@@ -55,7 +56,6 @@ func NewZone(name, file string) *Zone {
 		LastReloaded:   time.Now(),
 	}
 	*z.Expired = false
-
 	return z
 }
 
@@ -186,11 +186,6 @@ func (z *Zone) All() []dns.RR {
 	return append([]dns.RR{z.Apex.SOA}, records...)
 }
 
-// Print prints the zone's tree to stdout.
-func (z *Zone) Print() {
-	z.Tree.Print()
-}
-
 // NameFromRight returns the labels from the right, staring with the
 // origin and then i labels extra. When we are overshooting the name
 // the returned boolean is set to true.