db: make the ephemeral GC safe to close twice

A second Close panicked on the closed cancel channel.

hscontrol/db/ephemeral_garbage_collector_test.go

@@ -512,3 +512,16 @@ func TestEphemeralGarbageCollectorConcurrentScheduleAndClose(t *testing.T) {
 
 	t.Logf("Final number of goroutines: %d", runtime.NumGoroutine())
 }
+
+// TestEphemeralGarbageCollectorCloseTwice pins that a second Close does not
+// panic on the already closed cancel channel.
+func TestEphemeralGarbageCollectorCloseTwice(t *testing.T) {
+	gc := NewEphemeralGarbageCollector(func(types.NodeID) {})
+	go gc.Start()
+
+	gc.Schedule(types.NodeID(1), time.Hour)
+
+	gc.Close()
+
+	assert.NotPanics(t, gc.Close)
+}

hscontrol/db/node.go

@@ -377,8 +377,9 @@ type EphemeralGarbageCollector struct {
 	// was superseded by a Cancel or reschedule can be recognised and dropped.
 	gen uint64
 
-	deleteCh chan pendingDeletion
-	cancelCh chan struct{}
+	deleteCh  chan pendingDeletion
+	cancelCh  chan struct{}
+	closeOnce sync.Once
 }
 
 // ephemeralTimer pairs a node's pending-deletion timer with a done channel
@@ -410,7 +411,7 @@ func NewEphemeralGarbageCollector(deleteFunc func(types.NodeID)) *EphemeralGarba
 	}
 }
 
-// Close stops the garbage collector.
+// Close stops the garbage collector. It is safe to call more than once.
 func (e *EphemeralGarbageCollector) Close() {
 	e.mu.Lock()
 	defer e.mu.Unlock()
@@ -421,7 +422,7 @@ func (e *EphemeralGarbageCollector) Close() {
 	}
 
 	// Close the cancel channel to signal all goroutines to exit
-	close(e.cancelCh)
+	e.closeOnce.Do(func() { close(e.cancelCh) })
 }
 
 // Schedule schedules a node for deletion after the expiry duration.