src/os: loop in File.ReadAt to handle short reads

This matches what upstream Go does.  This also means len(b) == 0 successfully
reads 0 bytes without any extra logic.  The tests in archive/zip test for this
behaviour.

src/os/file.go

@@ -10,6 +10,7 @@
 package os
 
 import (
+	"errors"
 	"io"
 	"syscall"
 )
@@ -106,14 +107,29 @@ func (f *File) Read(b []byte) (n int, err error) {
 	return
 }
 
+var errNegativeOffset = errors.New("negative offset")
+
 // ReadAt reads up to len(b) bytes from the File at the given absolute offset.
 // It returns the number of bytes read and any error encountered, possible io.EOF.
 // At end of file, Read returns 0, io.EOF.
 func (f *File) ReadAt(b []byte, offset int64) (n int, err error) {
-	n, err = f.handle.ReadAt(b, offset)
-	if err != nil && err != io.EOF {
-		err = &PathError{"readat", f.name, err}
+	if offset < 0 {
+		return 0, &PathError{Op: "readat", Path: f.name, Err: errNegativeOffset}
 	}
+
+	for len(b) > 0 {
+		m, e := f.handle.ReadAt(b, offset)
+		if e != nil {
+			if err != io.EOF {
+				err = &PathError{"readat", f.name, err}
+			}
+			break
+		}
+		n += m
+		b = b[m:]
+		offset += int64(m)
+	}
+
 	return
 }