plugin/file: Rename do to walk and cleanup and document (#2987)

* plugin/file: Rename do to walk, cleanup and document

* This renames Do to Walk to be more inline with Go standards. Also make
  it return an error instead of a bool.
  Also give give walk access to rrs. Alternatively e.m could be
  exported, but just access the map of rrs should work as well. Another
  alternative would be adding a whole bunch of helper functions, but
  those need grab and return the data. Just having access to the rrs
  should be easiest for most Walks.
* It adds Type and TypeForWildcard to show the different functions
* *Removes* the identical RR check when inserting; this was only done
  for A, AAAA and MX and not finished; removed under the mantra garbage
  in garbage out.
* Reuses Types to return all the types in an *tree.Elem

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

* better comments

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

plugin/file/delete_test.go

@@ -0,0 +1,65 @@
+package file
+
+import (
+	"bytes"
+	"fmt"
+	"testing"
+
+	"github.com/coredns/coredns/plugin/file/tree"
+	"github.com/coredns/coredns/plugin/test"
+
+	"github.com/miekg/dns"
+)
+
+/*
+Create a zone with:
+
+      apex
+    /
+   a MX
+   a A
+
+Test that: we create the proper tree and that delete
+deletes the correct elements
+*/
+
+var tz = NewZone("example.org.", "db.example.org.")
+
+type treebuf struct {
+	*bytes.Buffer
+}
+
+func (t *treebuf) printFunc(e *tree.Elem, rrs map[uint16][]dns.RR) error {
+	fmt.Fprintf(t.Buffer, "%v\n", rrs) // should be fixed order in new go versions.
+	return nil
+}
+
+func TestZoneInsertAndDelete(t *testing.T) {
+	tz.Insert(test.SOA("example.org. IN SOA 1 2 3 4 5"))
+
+	if x := tz.Apex.SOA.Header().Name; x != "example.org." {
+		t.Errorf("Failed to insert SOA, expected %s, git %s", "example.org.", x)
+	}
+
+	// Insert two RRs and then remove one.
+	tz.Insert(test.A("a.example.org. IN A 127.0.0.1"))
+	tz.Insert(test.MX("a.example.org. IN MX 10 mx.example.org."))
+
+	tz.Delete(test.MX("a.example.org. IN MX 10 mx.example.org."))
+
+	tb := treebuf{new(bytes.Buffer)}
+
+	tz.Walk(tb.printFunc)
+	if tb.String() != "map[1:[a.example.org.\t3600\tIN\tA\t127.0.0.1]]\n" {
+		t.Errorf("Expected 1 A record in tree, got %s", tb.String())
+	}
+
+	tz.Delete(test.A("a.example.org. IN A 127.0.0.1"))
+
+	tb.Reset()
+
+	tz.Walk(tb.printFunc)
+	if tb.String() != "" {
+		t.Errorf("Expected no record in tree, got %s", tb.String())
+	}
+}