outlet/clickhouse: ensure we don't cancel flushing when we don't need

We really want to use max(current deadline, grace period timeout), but
this is not easy with contexts. So, we spawn a goroutine for that. We
need some extra care to ensure it does not leak. Maybe it would be
easier to just use `context.Background()`?

outlet/clickhouse/worker.go

@@ -136,9 +136,32 @@ func (w *realWorker) Flush(ctx context.Context) {
 			return err
 		}
 
-		// Don't use the parent context, it may be too short.
-		chCtx, cancel := context.WithTimeout(context.Background(), w.c.config.GracePeriod)
-		defer cancel()
+		// Ensure the context lives for at least GracePeriod.
+		chCtx, cancel := context.WithCancel(context.Background())
+		defer cancel() // needed in case the operation completes before grace period and parent context
+		go func() {
+			gracePeriodTimer := time.NewTimer(w.c.config.GracePeriod)
+			defer gracePeriodTimer.Stop()
+
+			select {
+			case <-gracePeriodTimer.C:
+				// Grace period elapsed, now wait for parent or end of operation.
+				select {
+				case <-ctx.Done():
+				case <-chCtx.Done():
+				}
+			case <-ctx.Done():
+				// Parent done before grace period, wait for grace period or end of operation.
+				select {
+				case <-gracePeriodTimer.C:
+					w.logger.Info().Msg("grace period to flush batch expired")
+				case <-chCtx.Done():
+				}
+			case <-chCtx.Done():
+				// Operation done!
+			}
+			cancel()
+		}()
 
 		// Send to ClickHouse in flows_XXXXX_raw.
 		start := time.Now()