write zstd to file, fix blocking select in custom reader

* do some refactoring so we can pipe the zstd encoded snapshot to file for debugging
* select statement in custom reader was blocking without setting default

cmd/filecoin-chain-archiver/cmds/create.go

@@ -74,6 +74,7 @@ func (sr *snapshotReader) Read(p []byte) (n int, err error)  {
 				return n, err
 			}
 			return n, io.EOF
+		default:
 		}
 	}
 }
@@ -380,10 +381,46 @@ var cmdCreate = &cli.Command{
 			}
 		}()
 
+		rrPath := filepath.Join(flagExportDir, flagFileName)
+		for {
+			info, err := os.Stat(rrPath)
+			if os.IsNotExist(err) {
+				logger.Infow("waiting for snapshot car file to begin writing")
+				time.Sleep(time.Second * 15)
+				continue
+			} else if info.IsDir() {
+				return xerrors.Errorf("trying to open directory instead of car file")
+			}
+			break
+		}
+
+		rr := newSnapshotReader(rrPath, errCh)
+
+		go func() {
+			var lastSize int64
+			for {
+				select {
+				case <-time.After(flagProgressUpdate):
+					size := e.Progress(rrPath)
+					if size == 0 {
+						continue
+					}
+					logger.Infow("update", "total", size, "speed", (size-lastSize)/int64(flagProgressUpdate/time.Second))
+					lastSize = size
+                case err := <- errCh:
+                    if err != nil {
+                           break
+                    }
+                }
+			}
+		}()
+
 		if flagDiscard {
 			logger.Infow("discarding output")
-			g, _ := errgroup.WithContext(ctx)
-
+			g, ctxGroup := errgroup.WithContext(ctx)
+			g.Go(func() error {
+				return runWriteCompressed(ctxGroup, flagFileName+".zstd", rr)
+			})
 			if err := g.Wait(); err != nil {
 				return err
 			}
@@ -392,36 +429,6 @@ var cmdCreate = &cli.Command{
 				return err
 			}
 		} else {
-			rrPath := filepath.Join(flagExportDir, flagFileName)
-			for {
-				info, err := os.Stat(rrPath)
-				if os.IsNotExist(err) {
-					logger.Infow("waiting for snapshot car file to begin writing")
-					time.Sleep(time.Second * 15)
-					continue
-				} else if info.IsDir() {
-					return xerrors.Errorf("trying to open directory instead of car file")
-				}
-				break
-			}
-
-			rr := newSnapshotReader(rrPath, errCh)
-
-			go func() {
-				var lastSize int64
-				for {
-					select {
-					case <-time.After(flagProgressUpdate):
-						size := e.Progress(rrPath)
-						if size == 0 {
-							continue
-						}
-						logger.Infow("update", "total", size, "speed", (size-lastSize)/int64(flagProgressUpdate/time.Second))
-						lastSize = size
-					}
-				}
-			}()
-
 			host := u.Hostname()
 			port := u.Port()
 			if port == "" {
@@ -503,13 +510,32 @@ var cmdCreate = &cli.Command{
 	},
 }
 
-func runUploadCompressed(ctx context.Context, minioClient *minio.Client, flagBucket, flagNamePrefix, flagRetrievalEndpointPrefix, name, peerID string, bt time.Time, source io.Reader) (*snapshotInfo, error) {
-
-	r1, w1 := io.Pipe()
+func compress(source io.Reader) io.Reader {
+	r, w := io.Pipe()
 	go func() {
-		Compress(source, w1)
-		w1.Close()
+		Compress(source, w)
+		w.Close()
 	}()
+	return r
+}
+
+func runWriteCompressed(ctx context.Context, name string, source io.Reader) error {
+	file, err := os.Create(name)
+	if err != nil {
+		return err
+	}
+	r := compress(source)
+	n, err := io.Copy(file, r)
+	if err != nil {
+		return err
+	}
+	logger.Infow("data copied to file:", n)
+	return nil
+}
+
+func runUploadCompressed(ctx context.Context, minioClient *minio.Client, flagBucket, flagNamePrefix, flagRetrievalEndpointPrefix, name, peerID string, bt time.Time, source io.Reader) (*snapshotInfo, error) {
+	r1 := compress(source)
+
 	h := sha256.New()
 	r := io.TeeReader(r1, h)