os, syscall: refactor Unix directory parsing

Moved the details of how to read a directory
and how to parse the results behind the new
syscall functions ReadDirent and ParseDirent.

Now os needs just one copy of Readdirnames
for the three Unix variants, and it no longer
imports "unsafe".

R=r, r2
CC=golang-dev
https://golang.org/cl/4368048

src/pkg/os/dir_unix.go

@@ -0,0 +1,54 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package os
+
+import (
+	"syscall"
+)
+
+const (
+	blockSize = 4096
+)
+
+// Readdirnames reads the contents of the directory associated with file and
+// returns an array of up to count names, in directory order.  Subsequent
+// calls on the same file will yield further names.
+// A negative count means to read until EOF.
+// Readdirnames returns the array and an Error, if any.
+func (file *File) Readdirnames(count int) (names []string, err Error) {
+	// If this file has no dirinfo, create one.
+	if file.dirinfo == nil {
+		file.dirinfo = new(dirInfo)
+		// The buffer must be at least a block long.
+		file.dirinfo.buf = make([]byte, blockSize)
+	}
+	d := file.dirinfo
+	size := count
+	if size < 0 {
+		size = 100
+	}
+	names = make([]string, 0, size) // Empty with room to grow.
+	for count != 0 {
+		// Refill the buffer if necessary
+		if d.bufp >= d.nbuf {
+			d.bufp = 0
+			var errno int
+			d.nbuf, errno = syscall.ReadDirent(file.fd, d.buf)
+			if errno != 0 {
+				return names, NewSyscallError("readdirent", errno)
+			}
+			if d.nbuf <= 0 {
+				break // EOF
+			}
+		}
+
+		// Drain the buffer
+		var nb, nc int
+		nb, nc, names = syscall.ParseDirent(d.buf[d.bufp:d.nbuf], count, names)
+		d.bufp += nb
+		count -= nc
+	}
+	return names, nil
+}