os: make windows Stat as fast as Lstat for files and directories

Recent CL 41834 made windows Stat work for all symlinks.
But CL 41834 also made Stat slow.

John Starks sugested
(see https://github.com/golang/go/issues/19922#issuecomment-300031421)
to use GetFileAttributesEx for files and directories instead.
This makes Stat as fast as at go1.9.

I see these improvements on my Windows 7

name       old time/op  new time/op  delta
StatDot    26.5µs ± 1%  20.6µs ± 2%  -22.37%  (p=0.000 n=9+10)
StatFile   22.8µs ± 2%   6.2µs ± 1%  -72.69%  (p=0.000 n=10+10)
StatDir    21.0µs ± 2%   6.1µs ± 3%  -71.12%  (p=0.000 n=10+9)
LstatDot   20.1µs ± 1%  20.7µs ± 6%   +3.37%  (p=0.000 n=9+10)
LstatFile  6.23µs ± 1%  6.36µs ± 8%     ~     (p=0.587 n=9+10)
LstatDir   6.10µs ± 0%  6.14µs ± 4%     ~     (p=0.590 n=9+10)

and on my Windows XP

name         old time/op  new time/op  delta
StatDot-2    20.6µs ± 0%  10.8µs ± 0%  -47.44%  (p=0.000 n=10+10)
StatFile-2   20.2µs ± 0%   7.9µs ± 0%  -60.91%  (p=0.000 n=8+10)
StatDir-2    19.3µs ± 0%   7.6µs ± 0%  -60.51%  (p=0.000 n=10+9)
LstatDot-2   10.8µs ± 0%  10.8µs ± 0%   -0.48%  (p=0.000 n=10+8)
LstatFile-2  7.83µs ± 0%  7.83µs ± 0%     ~     (p=0.844 n=10+8)
LstatDir-2   7.59µs ± 0%  7.56µs ± 0%   -0.46%  (p=0.000 n=10+10)

Updates #19922

Change-Id: Ice1fb5825defb05c79bab4dec0692e0fd1bcfcd5
Reviewed-on: https://go-review.googlesource.com/43071
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

1 files changed, 18 insertions, 2 deletions

diff --git a/src/os/stat_windows.go b/src/os/stat_windows.go
index a7220041cd..667b99905d 100644
--- a/src/os/stat_windows.go
+++ b/src/os/stat_windows.go
@@ -71,7 +71,23 @@ func Stat(name string) (FileInfo, error) {
 	if err != nil {
 		return nil, &PathError{"Stat", name, err}
 	}
-
+	// Apparently (see https://github.com/golang/go/issues/19922#issuecomment-300031421)
+	// GetFileAttributesEx is fastest approach to get file info.
+	// It does not work for symlinks. But symlinks are rare,
+	// so try GetFileAttributesEx first.
+	var fs fileStat
+	err = syscall.GetFileAttributesEx(namep, syscall.GetFileExInfoStandard, (*byte)(unsafe.Pointer(&fs.sys)))
+	if err == nil && fs.sys.FileAttributes&syscall.FILE_ATTRIBUTE_REPARSE_POINT == 0 {
+		fs.path = name
+		if !isAbs(fs.path) {
+			fs.path, err = syscall.FullPath(fs.path)
+			if err != nil {
+				return nil, &PathError{"FullPath", name, err}
+			}
+		}
+		fs.name = basename(name)
+		return &fs, nil
+	}
 	// Use Windows I/O manager to dereference the symbolic link, as per
 	// https://blogs.msdn.microsoft.com/oldnewthing/20100212-00/?p=14963/
 	h, err := syscall.CreateFile(namep, 0, 0, nil,
@@ -170,7 +186,7 @@ func Lstat(name string) (FileInfo, error) {
 	if !isAbs(fs.path) {
 		fs.path, e = syscall.FullPath(fs.path)
 		if e != nil {
-			return nil, e
+			return nil, &PathError{"FullPath", name, e}
 		}
 	}
 	return fs, nil