middleware/auto: add (#333)

Add auto-load middleware that automatically picks up zones.

Every X seconds it will scan for new zones.
Add tests and documentation.

Make 'make test' use -race.

middleware/auto/walk_test.go

@@ -0,0 +1,72 @@
+package auto
+
+import (
+	"io/ioutil"
+	"log"
+	"os"
+	"path"
+	"regexp"
+	"testing"
+)
+
+var dbFiles = []string{"db.example.org", "aa.example.org"}
+
+const zoneContent = `; testzone
+@	IN	SOA	sns.dns.icann.org. noc.dns.icann.org. 2016082534 7200 3600 1209600 3600
+		NS	a.iana-servers.net.
+		NS	b.iana-servers.net.
+
+www IN A 127.0.0.1
+`
+
+func TestWalk(t *testing.T) {
+	log.SetOutput(ioutil.Discard)
+
+	tempdir, err := createFiles()
+	if err != nil {
+		if tempdir != "" {
+			os.RemoveAll(tempdir)
+		}
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tempdir)
+
+	ldr := loader{
+		directory: tempdir,
+		re:        regexp.MustCompile(`db\.(.*)`),
+		template:  `${1}`,
+	}
+
+	z := &Zones{}
+
+	z.Walk(ldr)
+
+	// db.example.org and db.example.com should be here (created in createFiles)
+	for _, name := range []string{"example.com.", "example.org."} {
+		if _, ok := z.Z[name]; !ok {
+			t.Errorf("%s should have been added", name)
+		}
+	}
+}
+
+func createFiles() (string, error) {
+	dir, err := ioutil.TempDir(os.TempDir(), "coredns")
+	if err != nil {
+		return dir, err
+	}
+
+	for _, name := range dbFiles {
+		if err := ioutil.WriteFile(path.Join(dir, name), []byte(zoneContent), 0644); err != nil {
+			return dir, err
+		}
+	}
+	// symlinks
+	if err = os.Symlink(path.Join(dir, "db.example.org"), path.Join(dir, "db.example.com")); err != nil {
+		return dir, err
+	}
+	if err = os.Symlink(path.Join(dir, "db.example.org"), path.Join(dir, "aa.example.com")); err != nil {
+		return dir, err
+	}
+
+	return dir, nil
+}