fix: recover send-on-closed-channel panic in byteChannel sink

sink.go

@@ -1,6 +1,7 @@
 package keepcurrent
 
 import (
+	"fmt"
 	"io"
 	"io/ioutil"
 	"os"
@@ -72,7 +73,22 @@ func ToChannel(ch chan []byte) Sink {
 	return &byteChannel{ch}
 }
 
-func (s *byteChannel) UpdateFrom(r io.Reader) error {
+func (s *byteChannel) UpdateFrom(r io.Reader) (err error) {
+	// The channel is owned by the caller (see ToChannel) and may be closed
+	// concurrently — e.g. on shutdown or config reload — while a Runner is
+	// mid-sync. A send on a closed channel panics, which would crash the whole
+	// process; recover *only that specific panic* and surface it as a sink
+	// error instead (Runner reports sink errors via OnSinkError rather than
+	// dying). Any other panic is re-raised so genuine bugs still surface.
+	defer func() {
+		if rec := recover(); rec != nil {
+			if e, ok := rec.(error); ok && e.Error() == "send on closed channel" {
+				err = fmt.Errorf("byte channel sink: %w", e)
+				return
+			}
+			panic(rec)
+		}
+	}()
 	b, err := ioutil.ReadAll(r)
 	if err != nil {
 		return err

sink_test.go

@@ -0,0 +1,32 @@
+package keepcurrent
+
+import (
+	"bytes"
+	"testing"
+)
+
+// Regression test for the "send on closed channel" crash: the channel is owned
+// by the caller and may be closed (shutdown / config reload) while a Runner is
+// mid-sync. UpdateFrom must surface that as an error rather than panicking and
+// crashing the whole process.
+func TestByteChannelClosedDoesNotPanic(t *testing.T) {
+	ch := make(chan []byte)
+	close(ch)
+	s := ToChannel(ch)
+	err := s.UpdateFrom(bytes.NewReader([]byte("hello")))
+	if err == nil {
+		t.Fatal("expected an error sending to a closed channel, got nil")
+	}
+}
+
+// A healthy send still delivers the data unchanged.
+func TestByteChannelDelivers(t *testing.T) {
+	ch := make(chan []byte, 1)
+	s := ToChannel(ch)
+	if err := s.UpdateFrom(bytes.NewReader([]byte("hello"))); err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if got := <-ch; string(got) != "hello" {
+		t.Fatalf("got %q, want %q", got, "hello")
+	}
+}