os: use handle based APIs to read directories on windows

This CL updates File.readdir() on windows so it uses
GetFileInformationByHandleEx with FILE_ID_BOTH_DIR_INFO
instead of Find* APIs. The former is more performant because
it allows us to buffer IO calls and reduces the number of system calls,
passing from 1 per file to 1 every ~100 files
(depending on the size of the file name and the size of the buffer).

This change improve performance of File.ReadDir by 20-30%.

name        old time/op    new time/op    delta
ReadDir-12     562µs ±14%     385µs ± 9%  -31.60%  (p=0.000 n=9+9)

name        old alloc/op   new alloc/op   delta
ReadDir-12    29.7kB ± 0%    29.5kB ± 0%   -0.88%  (p=0.000 n=8+10)

name        old allocs/op  new allocs/op  delta
ReadDir-12       399 ± 0%       397 ± 0%   -0.50%  (p=0.000 n=10+10)

This change also speeds up calls to os.SameFile when using FileStats
returned from File.readdir(), as their file ID can be inferred while
reading the directory.

Change-Id: Id56a338ee66c39656b564105cac131099218fb5d
Reviewed-on: https://go-review.googlesource.com/c/go/+/452995
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>

1 files changed, 0 insertions, 69 deletions

diff --git a/src/os/file_windows.go b/src/os/file_windows.go
index d94b78f524..a48feca855 100644
--- a/src/os/file_windows.go
+++ b/src/os/file_windows.go
@@ -84,18 +84,6 @@ func NewFile(fd uintptr, name string) *File {
 	return newFile(h, name, "file")
 }
 
-// Auxiliary information if the File describes a directory
-type dirInfo struct {
-	h       syscall.Handle // search handle created with FindFirstFile
-	data    syscall.Win32finddata
-	path    string
-	isempty bool // set if FindFirstFile returns ERROR_FILE_NOT_FOUND
-}
-
-func (d *dirInfo) close() error {
-	return syscall.FindClose(d.h)
-}
-
 func epipecheck(file *File, e error) {
 }
 
@@ -103,63 +91,6 @@ func epipecheck(file *File, e error) {
 // On Unix-like systems, it is "/dev/null"; on Windows, "NUL".
 const DevNull = "NUL"
 
-func openDir(name string) (d *dirInfo, e error) {
-	var mask string
-
-	path := fixLongPath(name)
-
-	if len(path) == 2 && path[1] == ':' { // it is a drive letter, like C:
-		mask = path + `*`
-	} else if len(path) > 0 {
-		lc := path[len(path)-1]
-		if lc == '/' || lc == '\\' {
-			mask = path + `*`
-		} else {
-			mask = path + `\*`
-		}
-	} else {
-		mask = `\*`
-	}
-	maskp, e := syscall.UTF16PtrFromString(mask)
-	if e != nil {
-		return nil, e
-	}
-	d = new(dirInfo)
-	d.h, e = syscall.FindFirstFile(maskp, &d.data)
-	if e != nil {
-		// FindFirstFile returns ERROR_FILE_NOT_FOUND when
-		// no matching files can be found. Then, if directory
-		// exists, we should proceed.
-		// If FindFirstFile failed because name does not point
-		// to a directory, we should return ENOTDIR.
-		var fa syscall.Win32FileAttributeData
-		pathp, e1 := syscall.UTF16PtrFromString(path)
-		if e1 != nil {
-			return nil, e
-		}
-		e1 = syscall.GetFileAttributesEx(pathp, syscall.GetFileExInfoStandard, (*byte)(unsafe.Pointer(&fa)))
-		if e1 != nil {
-			return nil, e
-		}
-		if fa.FileAttributes&syscall.FILE_ATTRIBUTE_DIRECTORY == 0 {
-			return nil, syscall.ENOTDIR
-		}
-		if e != syscall.ERROR_FILE_NOT_FOUND {
-			return nil, e
-		}
-		d.isempty = true
-	}
-	d.path = path
-	if !isAbs(d.path) {
-		d.path, e = syscall.FullPath(d.path)
-		if e != nil {
-			d.close()
-			return nil, e
-		}
-	}
-	return d, nil
-}
-
 // openFileNolog is the Windows implementation of OpenFile.
 func openFileNolog(name string, flag int, perm FileMode) (*File, error) {
 	if name == "" {