file: close correctlty after AXFR (#2943)

* file: close correctlty after AXFR

Don't hijack, but wait for the writes to be done and then savely close
the connection.

Fixes: #2929

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

* Update comment

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

* file: close correctlty after AXFR (#2943)

apply

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

plugin/file/xfr.go

@@ -3,6 +3,7 @@ package file
 import (
 	"context"
 	"fmt"
+	"sync"
 
 	"github.com/coredns/coredns/plugin"
 	"github.com/coredns/coredns/request"
@@ -31,9 +32,13 @@ func (x Xfr) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (in
 	}
 
 	ch := make(chan *dns.Envelope)
-	defer close(ch)
 	tr := new(dns.Transfer)
-	go tr.Out(w, r, ch)
+	wg := new(sync.WaitGroup)
+	go func() {
+		wg.Add(1)
+		tr.Out(w, r, ch)
+		wg.Done()
+	}()
 
 	j, l := 0, 0
 	records = append(records, records[0]) // add closing SOA to the end
@@ -49,9 +54,9 @@ func (x Xfr) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (in
 	if j < len(records) {
 		ch <- &dns.Envelope{RR: records[j:]}
 	}
+	close(ch) // Even though we close the channel here, we still have
+	wg.Wait() // to wait before we can return and close the connection.
 
-	w.Hijack()
-	// w.Close() // Client closes connection
 	return dns.RcodeSuccess, nil
 }