runtime,os,syscall,internal/poll: replace getdirentries on iOS

The getdirentries syscall is considered private API on iOS and is
rejected by the App Store submission checks. Replace it with the
fdopendir/readdir_r/closedir syscalls.

Fixes #28984

Change-Id: I73341b124310e9cb34834a95f946769f337ec5b7
Reviewed-on: https://go-review.googlesource.com/c/153338
Reviewed-by: Keith Randall <khr@golang.org>

src/os/dir_unix.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build aix darwin dragonfly freebsd js,wasm linux nacl netbsd openbsd solaris
+// +build aix darwin,!arm,!arm64 dragonfly freebsd js,wasm linux nacl netbsd openbsd solaris
 
 package os
 
@@ -12,37 +12,19 @@ import (
 	"syscall"
 )
 
+// Auxiliary information if the File describes a directory
+type dirInfo struct {
+	buf  []byte // buffer for directory I/O
+	nbuf int    // length of buf; return value from Getdirentries
+	bufp int    // location of next record in buf.
+}
+
 const (
 	// More than 5760 to work around https://golang.org/issue/24015.
 	blockSize = 8192
 )
 
-func (f *File) readdir(n int) (fi []FileInfo, err error) {
-	dirname := f.name
-	if dirname == "" {
-		dirname = "."
-	}
-	names, err := f.Readdirnames(n)
-	fi = make([]FileInfo, 0, len(names))
-	for _, filename := range names {
-		fip, lerr := lstat(dirname + "/" + filename)
-		if IsNotExist(lerr) {
-			// File disappeared between readdir + stat.
-			// Just treat it as if it didn't exist.
-			continue
-		}
-		if lerr != nil {
-			return fi, lerr
-		}
-		fi = append(fi, fip)
-	}
-	if len(fi) == 0 && err == nil && n > 0 {
-		// Per File.Readdir, the slice must be non-empty or err
-		// must be non-nil if n > 0.
-		err = io.EOF
-	}
-	return fi, err
-}
+func (d *dirInfo) close() {}
 
 func (f *File) readdirnames(n int) (names []string, err error) {
 	// If this file has no dirinfo, create one.