path/filepath: use RtlIsDosDeviceName_U to detect Windows devices

RtlIsDosDeviceName_U is specifically designed to detect Windows devices.
We were using GetFullPathName to do this, but it's not the right API
for the job, as it is slower and allocates more memory.

goos: windows
goarch: amd64
pkg: path/filepath
cpu: Intel(R) Core(TM) i7-10850H CPU @ 2.70GHz
           │   old.txt    │               new.txt                │
           │    sec/op    │    sec/op     vs base                │
IsLocal-12   5.685µ ± 59%   1.853µ ± 12%  -67.41% (p=0.000 n=10)

           │   old.txt   │              new.txt               │
           │    B/op     │    B/op     vs base                │
IsLocal-12   496.00 ± 0%   48.00 ± 0%  -90.32% (p=0.000 n=10)

           │   old.txt   │              new.txt               │
           │  allocs/op  │ allocs/op   vs base                │
IsLocal-12   10.000 ± 0%   6.000 ± 0%  -40.00% (p=0.000 n=10)

Change-Id: Ib40ad7a90ab93cf7051c8d6becbce4d287f10f4e
Reviewed-on: https://go-review.googlesource.com/c/go/+/650578
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
Reviewed-by: Damien Neil <[email protected]>

Author: qmuntal

Diff for: src/internal/filepathlite/path_windows.go

@@ -7,6 +7,7 @@ package filepathlite
 import (
 	"internal/bytealg"
 	"internal/stringslite"
+	"internal/syscall/windows"
 	"syscall"
 )
 
@@ -114,13 +115,14 @@ func isReservedName(name string) bool {
 		return true
 	}
 	// The path element is a reserved name with an extension.
-	// Some Windows versions consider this a reserved name,
-	// while others do not. Use FullPath to see if the name is
-	// reserved.
-	if p, _ := syscall.FullPath(name); len(p) >= 4 && p[:4] == `\\.\` {
-		return true
+	// Since Windows 11, reserved names with extensions are no
+	// longer reserved. For example, "CON.txt" is a valid file
+	// name. Use RtlIsDosDeviceName_U to see if the name is reserved.
+	p, err := syscall.UTF16PtrFromString(name)
+	if err != nil {
+		return false
 	}
-	return false
+	return windows.RtlIsDosDeviceName_U(p) > 0
 }
 
 func isReservedBaseName(name string) bool {
@@ -297,11 +299,6 @@ func cutPath(path string) (before, after string, found bool) {
 	return path, "", false
 }
 
-// isUNC reports whether path is a UNC path.
-func isUNC(path string) bool {
-	return len(path) > 1 && IsPathSeparator(path[0]) && IsPathSeparator(path[1])
-}
-
 // postClean adjusts the results of Clean to avoid turning a relative path
 // into an absolute or rooted one.
 func postClean(out *lazybuf) {

Diff for: src/internal/syscall/windows/syscall_windows.go

@@ -535,3 +535,4 @@ const (
 //sys   NtOpenFile(handle *syscall.Handle, access uint32, oa *OBJECT_ATTRIBUTES, iosb *IO_STATUS_BLOCK, share uint32, options uint32) (ntstatus error) = ntdll.NtOpenFile
 //sys   rtlNtStatusToDosErrorNoTeb(ntstatus NTStatus) (ret syscall.Errno) = ntdll.RtlNtStatusToDosErrorNoTeb
 //sys   NtSetInformationFile(handle syscall.Handle, iosb *IO_STATUS_BLOCK, inBuffer uintptr, inBufferLen uint32, class uint32) (ntstatus error) = ntdll.NtSetInformationFile
+//sys	RtlIsDosDeviceName_U(name *uint16) (ret uint32) = ntdll.RtlIsDosDeviceName_U

Diff for: src/internal/syscall/windows/zsyscall_windows.go

@@ -1889,3 +1889,18 @@ func TestEvalSymlinksTooManyLinks(t *testing.T) {
 		t.Fatal("expected error, got nil")
 	}
 }
+
+func BenchmarkIsLocal(b *testing.B) {
+	tests := islocaltests
+	if runtime.GOOS == "windows" {
+		tests = append(tests, winislocaltests...)
+	}
+	if runtime.GOOS == "plan9" {
+		tests = append(tests, plan9islocaltests...)
+	}
+	for b.Loop() {
+		for _, test := range tests {
+			filepath.IsLocal(test.path)
+		}
+	}
+}