bufio: make Reader.Read implement io.Reader semantics

R=rsc
CC=golang-dev
https://golang.org/cl/3395042

src/pkg/bufio/bufio.go

@@ -128,16 +128,17 @@ func (b *Reader) Peek(n int) ([]byte, os.Error) {
 
 // Read reads data into p.
 // It returns the number of bytes read into p.
-// If nn < len(p), also returns an error explaining
-// why the read is short.  At EOF, the count will be
-// zero and err will be os.EOF.
-func (b *Reader) Read(p []byte) (nn int, err os.Error) {
-	nn = 0
-	for len(p) > 0 {
-		n := len(p)
+// It calls Read at most once on the underlying Reader,
+// hence n may be less than len(p).
+// At EOF, the count will be zero and err will be os.EOF.
+func (b *Reader) Read(p []byte) (n int, err os.Error) {
+	n = len(p)
+	if n == 0 {
+		return 0, b.err
+	}
 	if b.w == b.r {
 		if b.err != nil {
-				return nn, b.err
+			return 0, b.err
 		}
 		if len(p) >= len(b.buf) {
 			// Large read, empty buffer.
@@ -148,12 +149,14 @@ func (b *Reader) Read(p []byte) (nn int, err os.Error) {
 				b.lastRuneSize = -1
 			}
 			p = p[n:]
-				nn += n
-				continue
+			return n, b.err
 		}
 		b.fill()
-			continue
+		if b.w == b.r {
+			return 0, b.err
 		}
+	}
+
 	if n > b.w-b.r {
 		n = b.w - b.r
 	}
@@ -162,9 +165,7 @@ func (b *Reader) Read(p []byte) (nn int, err os.Error) {
 	b.r += n
 	b.lastByte = int(b.buf[b.r-1])
 	b.lastRuneSize = -1
-		nn += n
-	}
-	return nn, nil
+	return n, nil
 }
 
 // ReadByte reads and returns a single byte.