no purge (#111)

providers/diff/diff.go

@@ -2,6 +2,7 @@ package diff
 
 import (
 	"fmt"
+	"log"
 	"sort"
 
 	"github.com/StackExchange/dnscontrol/models"
@@ -66,6 +67,15 @@ func (d *differ) IncrementalDiff(existing []*models.RecordConfig) (unchanged, cr
 		k := key{d.NameFQDN, d.Type}
 		desiredByNameAndType[k] = append(desiredByNameAndType[k], d)
 	}
+	//if NO_PURGE is set, just remove anything that is only in existing.
+	if d.dc.KeepUnknown {
+		for k := range existingByNameAndType {
+			if _, ok := desiredByNameAndType[k]; !ok {
+				log.Printf("Ignoring record set %s %s due to NO_PURGE", k.rType, k.name)
+				delete(existingByNameAndType, k)
+			}
+		}
+	}
 	// Look through existing records. This will give us changes and deletions and some additions.
 	// Each iteration is only for a single type/name record set
 	for key, existingRecords := range existingByNameAndType {