added a test for transferring a large zone in file plugin (#2905)

* added a test for transferring a large zone in file plugin

* avoid hardcoding number of RRs multiple times

* don't do testing.Parallel as suggested.

* applied gofmt -s

test/file_xfr_test.go

+package test
+
+import (
+	"fmt"
+	"io"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/coredns/coredns/plugin/test"
+
+	"github.com/miekg/dns"
+)
+
+func TestLargeAXFR(t *testing.T) {
+	// Build a large zone in text format.  It contains 64K AAAA RRs.
+	var sb strings.Builder
+	const numAAAAs = 65536
+	sb.WriteString("example.com. IN SOA . . 1 60 60 60 60\n")
+	sb.WriteString("example.com. IN NS ns.example.\n")
+	for i := 0; i < numAAAAs; i++ {
+		sb.WriteString(fmt.Sprintf("%d.example.com. IN AAAA 2001:db8::1\n", i))
+	}
+
+	// Setup the zone file and CoreDNS to serve the zone, allowing zone transfer
+	name, rm, err := test.TempFile(".", sb.String())
+	if err != nil {
+		t.Fatalf("Failed to create zone: %s", err)
+	}
+	defer rm()
+
+	corefile := `example.com:0 {
+       file ` + name + ` {
+           transfer to *
+       }
+}
+`
+
+	// Start server, and send an AXFR query to the TCP port.  We set the deadline to prevent the test from hanging.
+	i, _, tcp, err := CoreDNSServerAndPorts(corefile)
+	if err != nil {
+		t.Fatalf("Could not get CoreDNS serving instance: %s", err)
+	}
+	defer i.Stop()
+
+	m := new(dns.Msg)
+	m.SetQuestion("example.com.", dns.TypeAXFR)
+	co, err := dns.DialTimeout("tcp", tcp, 5*time.Second)
+	if err != nil {
+		t.Fatalf("Expected to establish TCP connection, but didn't: %s", err)
+	}
+	defer co.Close()
+	co.SetWriteDeadline(time.Now().Add(5 * time.Second))
+	err = co.WriteMsg(m)
+	if err != nil {
+		t.Fatalf("Unable to send TCP query: %s", err)
+	}
+
+	nrr := 0                                             // total number of transferred RRs
+	co.SetReadDeadline(time.Now().Add(60 * time.Second)) // use a longer timeout as it involves transferring a non-trivial amount of data.
+	for {
+		resp, err := co.ReadMsg()
+		if err != nil {
+			t.Fatalf("Expected to receive reply, but didn't: %s", err)
+		}
+		if len(resp.Answer) == 0 {
+			continue
+		}
+		// First RR should be SOA.
+		if nrr == 0 && resp.Answer[0].Header().Rrtype != dns.TypeSOA {
+			t.Fatalf("Expected SOA, but got type %d", resp.Answer[0].Header().Rrtype)
+		}
+		nrr += len(resp.Answer)
+		// If we see another SOA at the end of the message, we are done.
+		// Note that this check is not enough to detect all invalid responses, but checking those is not the purpose of this test.
+		if nrr > 1 && resp.Answer[len(resp.Answer)-1].Header().Rrtype == dns.TypeSOA {
+			break
+		}
+	}
+	// On successful completion, 2 SOA, 1 NS, and all AAAAs should have been transferred.
+	if nrr != numAAAAs+3 {
+		t.Fatalf("Got an unexpected number of RRs: %d", nrr)
+	}
+
+	// At the time of the initial implementation of this test, the remaining check would fail due to the problem described in PR #2866, so we return here.
+	// Once the issue is resolved this workaround should be removed to enable the check.
+	return
+
+	// Once xfr is completed the server should close the connection, so a further read should result in an EOF error.
+	// This time we use a short timeout to make it faster in case the server doesn't behave as expected.
+	co.SetReadDeadline(time.Now().Add(time.Second))
+	_, err = co.ReadMsg()
+	if err == nil {
+		t.Fatalf("Expected failure on further read, but it succeeded")
+	}
+	if err != io.EOF {
+		t.Fatalf("Expected EOF on further read, but got a different error: %v", err)
+	}
+}