io: prevent ReadAtLeast spinloop if min > len(buf)

R=r, heresy.mc
CC=golang-dev
https://golang.org/cl/2017042

src/pkg/io/io.go

@@ -19,6 +19,9 @@ type Error struct {
 // but failed to return an explicit error.
 var ErrShortWrite os.Error = &Error{"short write"}
 
+// ErrShortBuffer means that a read required a longer buffer than was provided.
+var ErrShortBuffer os.Error = &Error{"short buffer"}
+
 // ErrUnexpectedEOF means that os.EOF was encountered in the
 // middle of reading a fixed-size block or data structure.
 var ErrUnexpectedEOF os.Error = &Error{"unexpected EOF"}
@@ -165,8 +168,11 @@ func WriteString(w Writer, s string) (n int, err os.Error) {
 // The error is os.EOF only if no bytes were read.
 // If an EOF happens after reading fewer than min bytes,
 // ReadAtLeast returns ErrUnexpectedEOF.
+// If min is greater than the length of buf, ReadAtLeast returns ErrShortBuffer.
 func ReadAtLeast(r Reader, buf []byte, min int) (n int, err os.Error) {
-	n = 0
+	if len(buf) < min {
+		return 0, ErrShortBuffer
+	}
 	for n < min {
 		nn, e := r.Read(buf[n:])
 		if nn > 0 {
@@ -179,7 +185,7 @@ func ReadAtLeast(r Reader, buf []byte, min int) (n int, err os.Error) {
 			return n, e
 		}
 	}
-	return n, nil
+	return
 }
 
 // ReadFull reads exactly len(buf) bytes from r into buf.

src/pkg/io/io_test.go

@@ -7,6 +7,7 @@ package io_test
 import (
 	"bytes"
 	. "io"
+	"os"
 	"testing"
 )
 
@@ -78,3 +79,42 @@ func TestCopynWriteTo(t *testing.T) {
 		t.Errorf("Copyn did not work properly")
 	}
 }
+
+func TestReadAtLeast(t *testing.T) {
+	var rb bytes.Buffer
+	rb.Write([]byte("0123"))
+	buf := make([]byte, 2)
+	n, err := ReadAtLeast(&rb, buf, 2)
+	if err != nil {
+		t.Error(err)
+	}
+	n, err = ReadAtLeast(&rb, buf, 4)
+	if err != ErrShortBuffer {
+		t.Errorf("expected ErrShortBuffer got %v", err)
+	}
+	if n != 0 {
+		t.Errorf("expected to have read 0 bytes, got %v", n)
+	}
+	n, err = ReadAtLeast(&rb, buf, 1)
+	if err != nil {
+		t.Error(err)
+	}
+	if n != 2 {
+		t.Errorf("expected to have read 2 bytes, got %v", n)
+	}
+	n, err = ReadAtLeast(&rb, buf, 2)
+	if err != os.EOF {
+		t.Errorf("expected EOF, got %v", err)
+	}
+	if n != 0 {
+		t.Errorf("expected to have read 0 bytes, got %v", n)
+	}
+	rb.Write([]byte("4"))
+	n, err = ReadAtLeast(&rb, buf, 2)
+	if err != ErrUnexpectedEOF {
+		t.Errorf("expected ErrUnexpectedEOF, got %v", err)
+	}
+	if n != 1 {
+		t.Errorf("expected to have read 1 bytes, got %v", n)
+	}
+}