xio: Convert CountReader from io.Reader -> to xio.Reader

Done toprovide uniform API in xio package.
Compensate for API change in places where xio.CountReader was used.

xbufio/xbufio.go

-// Copyright (C) 2017  Nexedi SA and Contributors.
-//                     Kirill Smelkov <kirr@nexedi.com>
+// Copyright (C) 2017-2019  Nexedi SA and Contributors.
+//                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
 // it under the terms of the GNU General Public License version 3, or (at your
@@ -22,6 +22,7 @@ package xbufio
 
 import (
 	"bufio"
+	"context"
 	"io"
 
 	"lab.nexedi.com/kirr/go123/xio"
@@ -40,12 +41,13 @@ func NewReader(r io.Reader) *Reader {
 	}
 
 	// idempotent(xio.CountedReader)
-	cr, ok := r.(*xio.CountedReader)
+	xr := xio.WithCtxR(r)
+	cr, ok := xr.(*xio.CountedReader)
 	if !ok {
-		cr = xio.CountReader(r)
+		cr = xio.CountReader(xr)
 	}
 
-	return &Reader{bufio.NewReader(cr), cr}
+	return &Reader{bufio.NewReader(xio.BindCtxR(cr, context.Background())), cr}
 }
 
 // InputOffset returns current logical position in input stream.

xio/xio.go

@@ -26,6 +26,10 @@
 //	  It is the opposite operation for BindCtx, but for arbitrary io.X
 //	  returned xio.X handles context only on best-effort basis. In
 //	  particular IO cancellation is not reliably handled for os.File .
+//
+// Miscellaneous utilities:
+//
+//	- CountReader provides InputOffset for a Reader.
 package xio
 
 import (
@@ -272,14 +276,14 @@ func (s *stubCtxRWC) Close() error					{ return s.rw.Close() }
 // ----------------------------------------
 
 
-// CountedReader is an io.Reader that count total bytes read.
+// CountedReader is a Reader that count total bytes read.
 type CountedReader struct {
-	r     io.Reader
+	r     Reader
 	nread int64
 }
 
-func (cr *CountedReader) Read(p []byte) (int, error) {
-	n, err := cr.r.Read(p)
+func (cr *CountedReader) Read(ctx context.Context, p []byte) (int, error) {
+	n, err := cr.r.Read(ctx, p)
 	cr.nread += int64(n)
 	return n, err
 }
@@ -290,6 +294,6 @@ func (cr *CountedReader) InputOffset() int64 {
 }
 
 // CountReader wraps r with CountedReader.
-func CountReader(r io.Reader) *CountedReader {
+func CountReader(r Reader) *CountedReader {
 	return &CountedReader{r, 0}
 }