fix(query): allow multiple calls to Release proxy bridge iterator

The asyncStatsResultIterator used inside QueryServiceProxyBridge assumed
that Release would only be called once. The godoc for ResultIterator
specifies that it is safe to call Release multiple times. Now, you can
do that without causing Release to block indefinitely.

query/bridges.go

@@ -36,34 +36,38 @@ func (b QueryServiceProxyBridge) Query(ctx context.Context, req *Request) (flux.
 	}
 
 	r, w := io.Pipe()
-	statsChan := make(chan flux.Statistics, 1)
+	asri := &asyncStatsResultIterator{statsReady: make(chan struct{})}
 
 	go func() {
 		stats, err := b.ProxyQueryService.Query(ctx, w, preq)
 		_ = w.CloseWithError(err)
-		statsChan <- stats
+		asri.stats = stats
+		close(asri.statsReady)
 	}()
 
 	dec := csv.NewMultiResultDecoder(csv.ResultDecoderConfig{})
 	ri, err := dec.Decode(r)
-	return asyncStatsResultIterator{
-		ResultIterator: ri,
-		statsChan:      statsChan,
-	}, err
+	asri.ResultIterator = ri
+	return asri, err
 }
 
 type asyncStatsResultIterator struct {
 	flux.ResultIterator
-	statsChan chan flux.Statistics
-	stats     flux.Statistics
+
+	// Channel that is closed when stats have been written.
+	statsReady chan struct{}
+
+	// Statistics gathered from calling the proxy query service.
+	// This field must not be read until statsReady is closed.
+	stats flux.Statistics
 }
 
-func (i asyncStatsResultIterator) Release() {
+func (i *asyncStatsResultIterator) Release() {
 	i.ResultIterator.Release()
-	i.stats = <-i.statsChan
 }
 
-func (i asyncStatsResultIterator) Statistics() flux.Statistics {
+func (i *asyncStatsResultIterator) Statistics() flux.Statistics {
+	<-i.statsReady
 	return i.stats
 }