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plugin/transfer: fix goroutine leak on axfr err #7516

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Merged
yongtang merged 1 commit into from
Sep 5, 2025
Merged

plugin/transfer: fix goroutine leak on axfr err #7516

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11 changes: 6 additions & 5 deletions plugin/transfer/transfer.go
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Original file line number Diff line number Diff line change
Expand Up @@ -134,6 +134,11 @@ func (t *Transfer) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Ms
select {
case ch <- &dns.Envelope{RR: rrs}:
case err := <-errCh:
// Client errored; drain pchan to avoid blocking the producer goroutine.
go func() {
for range pchan {
}
}()
return dns.RcodeServerFailure, err
}
l += len(rrs)
Expand Down Expand Up @@ -161,11 +166,7 @@ func (t *Transfer) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Ms
}

if len(rrs) > 0 {
select {
case ch <- &dns.Envelope{RR: rrs}:
case err := <-errCh:
return dns.RcodeServerFailure, err
}
ch <- &dns.Envelope{RR: rrs}
l += len(rrs)
}

Expand Down
62 changes: 62 additions & 0 deletions plugin/transfer/transfer_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -4,6 +4,7 @@ import (
"context"
"fmt"
"testing"
"time"

"github.com/coredns/coredns/plugin"
"github.com/coredns/coredns/plugin/pkg/dnstest"
Expand Down Expand Up @@ -277,3 +278,64 @@ func TestTransferNotAllowed(t *testing.T) {
t.Errorf("Expected REFUSED response code, got %s", dns.RcodeToString[w.Msg.Rcode])
}
}

// errWriter is a dns.ResponseWriter that simulates a client error on write.
type errWriter struct {
test.ResponseWriter
}

func (e *errWriter) WriteMsg(m *dns.Msg) error { return fmt.Errorf("write error") }

// blockingTransferer produces many records into the channel and signals when done.
type blockingTransferer struct {
Zone string
done chan struct{}
}

func (b *blockingTransferer) Name() string { return "blockingtransferer" }
func (b *blockingTransferer) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
return 0, nil
}
func (b *blockingTransferer) Transfer(zone string, serial uint32) (<-chan []dns.RR, error) {
if zone != b.Zone {
return nil, ErrNotAuthoritative
}
ch := make(chan []dns.RR, 2)
go func() {
defer close(ch)
defer close(b.done)
soa := test.SOA(fmt.Sprintf("%s 100 IN SOA ns.dns.%s hostmaster.%s %d 7200 1800 86400 100", b.Zone, b.Zone, b.Zone, 1))
ch <- []dns.RR{soa}
for range 2000 {
ch <- []dns.RR{test.A("ns.dns." + b.Zone + " 100 IN A 1.2.3.4")}
}
ch <- []dns.RR{soa}
}()
return ch, nil
}

// Test that when the client errors mid-transfer, the server drains the producer channel
// so the producer goroutine can complete (no leak/block on small buffer).
func TestTransferDrainsProducerOnClientError(t *testing.T) {
b := &blockingTransferer{Zone: "example.org.", done: make(chan struct{})}

transfer := &Transfer{
Transferers: []Transferer{b},
xfrs: []*xfr{{Zones: []string{"example.org."}, to: []string{"*"}}},
Next: b,
}

ctx := context.TODO()
w := &errWriter{ResponseWriter: test.ResponseWriter{TCP: true}}
m := &dns.Msg{}
m.SetAxfr("example.org.")

_, _ = transfer.ServeDNS(ctx, w, m)

select {
case <-b.done:
// success
case <-time.After(2 * time.Second):
t.Fatal("producer goroutine did not finish; channel likely not drained on client error")
}
}