From 6144c7270e5812d9de8fb97456ee4e5ae657fcbb Mon Sep 17 00:00:00 2001
From: Alex Brainman <alex.brainman@gmail.com>
Date: Tue, 9 May 2017 16:50:41 +1000
Subject: [PATCH] os: make windows Stat as fast as Lstat for files and
 directories
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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>
---
 src/os/os_test.go      | 44 ++++++++++++++++++++++++++++++++++++++++++
 src/os/stat_windows.go | 20 +++++++++++++++++--
 2 files changed, 62 insertions(+), 2 deletions(-)

diff --git a/src/os/os_test.go b/src/os/os_test.go
index 22777aef9f..91c6be6148 100644
--- a/src/os/os_test.go
+++ b/src/os/os_test.go
@@ -398,6 +398,50 @@ func BenchmarkReaddir(b *testing.B) {
 	benchmarkReaddir(".", b)
 }
 
+func benchmarkStat(b *testing.B, path string) {
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_, err := Stat(path)
+		if err != nil {
+			b.Fatalf("Stat(%q) failed: %v", path, err)
+		}
+	}
+}
+
+func benchmarkLstat(b *testing.B, path string) {
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_, err := Lstat(path)
+		if err != nil {
+			b.Fatalf("Lstat(%q) failed: %v", path, err)
+		}
+	}
+}
+
+func BenchmarkStatDot(b *testing.B) {
+	benchmarkStat(b, ".")
+}
+
+func BenchmarkStatFile(b *testing.B) {
+	benchmarkStat(b, filepath.Join(runtime.GOROOT(), "src/os/os_test.go"))
+}
+
+func BenchmarkStatDir(b *testing.B) {
+	benchmarkStat(b, filepath.Join(runtime.GOROOT(), "src/os"))
+}
+
+func BenchmarkLstatDot(b *testing.B) {
+	benchmarkLstat(b, ".")
+}
+
+func BenchmarkLstatFile(b *testing.B) {
+	benchmarkLstat(b, filepath.Join(runtime.GOROOT(), "src/os/os_test.go"))
+}
+
+func BenchmarkLstatDir(b *testing.B) {
+	benchmarkLstat(b, filepath.Join(runtime.GOROOT(), "src/os"))
+}
+
 // Read the directory one entry at a time.
 func smallReaddirnames(file *File, length int, t *testing.T) []string {
 	names := make([]string, length)
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
-- 
2.50.0