bufio: make Reader.Read implement io.Reader semantics

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

src/pkg/bufio/bufio.go

@@ -128,43 +128,44 @@ 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)
-		if b.w == b.r {
-			if b.err != nil {
-				return nn, b.err
-			}
-			if len(p) >= len(b.buf) {
-				// Large read, empty buffer.
-				// Read directly into p to avoid copy.
-				n, b.err = b.rd.Read(p)
-				if n > 0 {
-					b.lastByte = int(p[n-1])
-					b.lastRuneSize = -1
-				}
-				p = p[n:]
-				nn += n
-				continue
+// 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 0, b.err
+		}
+		if len(p) >= len(b.buf) {
+			// Large read, empty buffer.
+			// Read directly into p to avoid copy.
+			n, b.err = b.rd.Read(p)
+			if n > 0 {
+				b.lastByte = int(p[n-1])
+				b.lastRuneSize = -1
 			}
-			b.fill()
-			continue
+			p = p[n:]
+			return n, b.err
 		}
-		if n > b.w-b.r {
-			n = b.w - b.r
+		b.fill()
+		if b.w == b.r {
+			return 0, b.err
 		}
-		copy(p[0:n], b.buf[b.r:])
-		p = p[n:]
-		b.r += n
-		b.lastByte = int(b.buf[b.r-1])
-		b.lastRuneSize = -1
-		nn += n
 	}
-	return nn, nil
+
+	if n > b.w-b.r {
+		n = b.w - b.r
+	}
+	copy(p[0:n], b.buf[b.r:])
+	p = p[n:]
+	b.r += n
+	b.lastByte = int(b.buf[b.r-1])
+	b.lastRuneSize = -1
+	return n, nil
 }
 
 // ReadByte reads and returns a single byte.