core/cp: reproduce std io.Copy impl as-is

Author: ignoramous

intra/core/cp.go

@@ -14,6 +14,9 @@ import (
 var (
 	errNoPipe   = errors.New("pipe: src or dst nil")
 	errNoStream = errors.New("stream: reader or writer nil")
+
+	// errInvalidWrite means that a write returned an impossible count.
+	errInvalidWrite = errors.New("invalid write result")
 )
 
 // Pipe copies data from src to dst, and returns the number of bytes copied.
@@ -37,7 +40,7 @@ func Pipe(dst io.Writer, src io.Reader) (int64, error) {
 // Stream reads data from src in to dst until error, and returns the no. of bytes read.
 // Internally, it bypasses io.ReaderFrom and io.WriterTo but uses io.CopyBuffer,
 // recycling buffers from a global pool.
-func Stream(dst io.Writer, src io.Reader) (int64, error) {
+func Stream(dst io.Writer, src io.Reader) (written int64, err error) {
 	if IsNil(src) || IsNil(dst) {
 		return 0, errNoStream
 	}
@@ -52,13 +55,41 @@ func Stream(dst io.Writer, src io.Reader) (int64, error) {
 	}
 
 	bptr := Alloc()
-	b := *bptr
-	b = b[:cap(b)]
+	buf := *bptr
+	buf = buf[:cap(buf)]
 	defer func() {
-		*bptr = b
+		*bptr = buf
 		Recycle(bptr)
 	}()
-	return io.CopyBuffer(dst, src, b)
+	// implementation from: io.CopyBuffer
+	for {
+		nr, er := src.Read(buf)
+		if nr > 0 {
+			nw, ew := dst.Write(buf[0:nr])
+			if nw < 0 || nr < nw {
+				nw = 0
+				if ew == nil {
+					ew = errInvalidWrite
+				}
+			}
+			written += int64(nw)
+			if ew != nil {
+				err = ew
+				break
+			}
+			if nr != nw {
+				err = io.ErrShortWrite
+				break
+			}
+		}
+		if er != nil {
+			if er != io.EOF {
+				err = er
+			}
+			break
+		}
+	}
+	return written, err
 }
 
 // ref: github.com/golang/go/issues/58808