implement a custom io.Reader for snapshot cars

the snapshot car files are continually appended to while the ExportRangeInternal process executes.
if one uses the os.File io.Reader, it will encounter an EOF before the snapshot is complete.
our custom io.Reader returns an io.EOF when the ExportRangeInternal method completes

Author: ognots

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

@@ -43,43 +43,46 @@ func Compress(in io.Reader, out io.Writer) error {
 	return enc.Close()
 }
 
-type autocloser struct {
-	rc io.ReadCloser
-}
-
-func (ac autocloser) Read(p []byte) (n int, err error) {
-	n, err = ac.rc.Read(p)
-	if err != nil {
-		_ = ac.rc.Close()
-	}
-	return
-}
-
-func AutoCloser(rc io.ReadCloser) io.Reader {
-	return &autocloser{rc}
+type snapshotInfo struct {
+	digest         string
+	size           int64
+	filename       string
+	latestIndex    string
+	latestLocation string
 }
 
-type multi struct {
-	io.Writer
-	cs []io.Closer
+type snapshotReader struct {
+    data []byte
+    fileLocation string
+    errCh chan error
 }
 
-func (m *multi) Close() error {
-	var first error
-	for _, c := range m.cs {
-		if err := c.Close(); err != nil && first == nil {
-			first = err
+func (sr *snapshotReader) Read(p []byte) (n int, err error)  {
+	r, err := os.OpenFile(sr.fileLocation, os.O_RDONLY, 444)
+	if err != nil {
+		return n, err
+	}
+	defer r.Close()
+	for {
+		n, err := r.Read(p)
+		if err != nil && err != io.EOF {
+			return n, err
+		}
+		select {
+		case err  := <- sr.errCh:
+			if err != nil {
+				return n, err
+			}
+			return n, io.EOF
 		}
 	}
-	return first
 }
 
-type snapshotInfo struct {
-	digest         string
-	size           int64
-	filename       string
-	latestIndex    string
-	latestLocation string
+func newSnapshotReader(f string, errChan chan error) *snapshotReader {
+	return &snapshotReader{
+		fileLocation: f,
+		errCh: errChan,
+	}
 }
 
 var cmdCreate = &cli.Command{
@@ -401,15 +404,11 @@ var cmdCreate = &cli.Command{
 				}
 				break
 			}
-			rr, err := os.OpenFile(rrPath, os.O_RDONLY, 444)
-			if err != nil {
-				return err
-			}
-			defer rr.Close()
+
+			rr := newSnapshotReader(rrPath, errCh)
 
 			go func() {
 				var lastSize int64
-				fmt.Printf("rrPath: %v", rrPath)
 				for {
 					select {
 					case <-time.After(flagProgressUpdate):