middleware/file: don't reload zone when SOA isn't changed (#707)

* middleware/file: don't reload zone when SOA isn't changed

Give Parse an extra argument which is the SOA's serial, if > 0 we check
against the just parsed SOA and then just return.

Most notable use is in reload.go which is both used in the file and auto
middleware.

Fixes #415

* PR comments

middleware/file/reload.go

@@ -0,0 +1,72 @@
+package file
+
+import (
+	"log"
+	"os"
+	"path"
+
+	"github.com/fsnotify/fsnotify"
+)
+
+// Reload reloads a zone when it is changed on disk. If z.NoRoload is true, no reloading will be done.
+func (z *Zone) Reload() error {
+	if z.NoReload {
+		return nil
+	}
+	watcher, err := fsnotify.NewWatcher()
+	if err != nil {
+		return err
+	}
+	err = watcher.Add(path.Dir(z.file))
+	if err != nil {
+		return err
+	}
+
+	go func() {
+		// TODO(miek): needs to be killed on reload.
+		for {
+			select {
+			case event := <-watcher.Events:
+				if path.Clean(event.Name) == z.file {
+
+					reader, err := os.Open(z.file)
+					if err != nil {
+						log.Printf("[ERROR] Failed to open `%s' for `%s': %v", z.file, z.origin, err)
+						continue
+					}
+
+					serial := z.SOASerialIfDefined()
+					zone, err := Parse(reader, z.origin, z.file, serial)
+					if err != nil {
+						log.Printf("[WARNING] Parsing zone `%s': %v", z.origin, err)
+						continue
+					}
+
+					// copy elements we need
+					z.reloadMu.Lock()
+					z.Apex = zone.Apex
+					z.Tree = zone.Tree
+					z.reloadMu.Unlock()
+
+					log.Printf("[INFO] Successfully reloaded zone `%s'", z.origin)
+					z.Notify()
+				}
+			case <-z.ReloadShutdown:
+				watcher.Close()
+				return
+			}
+		}
+	}()
+	return nil
+}
+
+// SOASerialIfDefind returns the SOA's serial if the zone has a SOA record in the Apex, or
+// -1 otherwise.
+func (z *Zone) SOASerialIfDefined() int64 {
+	z.reloadMu.Lock()
+	defer z.reloadMu.Unlock()
+	if z.Apex.SOA != nil {
+		return int64(z.Apex.SOA.Serial)
+	}
+	return -1
+}