os: Windows: Stat fails on special files like 'con'

[lifenjoiner]

What version of Go are you using (go version)?

$ go version
go version go1.13.1 windows/386

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
set GOHOSTARCH=386
set GOHOSTOS=windows

What did you do?

try test_Stat-con.go to reproduce the issue

package main

import (
	"os"
)

func main() {
	_, err := os.Stat("con")
	if err != nil {
		panic(err)
	}
}

I have debugged and located that GetFileAttributesEx fails on special files like 'con':

go/src/os/stat_windows.go

Lines 61 to 76 in 902d5aa

	err = syscall.GetFileAttributesEx(namep, syscall.GetFileExInfoStandard, (*byte)(unsafe.Pointer(&fa)))
	if err == nil && fa.FileAttributes&syscall.FILE_ATTRIBUTE_REPARSE_POINT == 0 {
	// Not a symlink.
	fs := &fileStat{
	FileAttributes: fa.FileAttributes,
	CreationTime: fa.CreationTime,
	LastAccessTime: fa.LastAccessTime,
	LastWriteTime: fa.LastWriteTime,
	FileSizeHigh: fa.FileSizeHigh,
	FileSizeLow: fa.FileSizeLow,
	}
	if err := fs.saveInfoFromPath(name); err != nil {
	return nil, err
	}
	return fs, nil
	}

and only 'nul':

go/src/os/stat_windows.go

Lines 24 to 26 in 902d5aa

	if isWindowsNulName(file.name) {
	return &devNullStat, nil
	}

go/src/os/stat_windows.go

Lines 24 to 26 in 902d5aa

	if isWindowsNulName(file.name) {
	return &devNullStat, nil
	}

reserved names
CON, PRN, AUX, NUL, COM1, COM2, COM3, COM4, COM5, COM6, COM7, COM8, COM9, LPT1, LPT2, LPT3, LPT4, LPT5, LPT6, LPT7, LPT8, and LPT9:
https://docs.microsoft.com/zh-cn/windows/win32/fileio/naming-a-file#naming-conventions
CONIN$ and CONOUT$:
https://docs.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-createfilea#consoles

ref:
https://github.com/gcc-mirror/gcc/blob/bbcdad768752f5cab6727242515bcd15706acbea/gcc/ada/adaint.c#L1606-L1618

What did you expect to see?

We can access these special Windows files, especially con for printing to stdout.

What did you see instead?

I found sometimes we need to use stdout as a file:
https://github.com/DNSCrypt/dnscrypt-proxy/blob/858957ce91015057a1f6c5a9a24f3ea2beb48537/dnscrypt-proxy/example-dnscrypt-proxy.toml#L339-L341
But con doesn't work.
By doing some research, I located the code here as described.
GetFileAttributesEx and CreateFile of Stat failed on the reserved file name con, and only 'nul' is considered as the special case.

[ianlancetaylor]

Not quite sure what to do here if GetFileAttribuesEx fails on CON. What should os.Stat return?

CC @alexbrainman @mattn

[mattn]

#include <stdio.h>
#include <sys/stat.h>

int
main(int argc, char* argv[]) {
  FILE* fp = fopen("CON", "r");
  if (fp == NULL) {
	perror("fopen");
	return 1;
  }
  fclose(fp);

  struct stat st;
  int r = stat("CON", &st);
  printf("result of stat: %d\n", r);

  printf("st_dev = %d\n", st.st_dev);
  printf("st_ino = %d\n", st.st_ino);
  printf("st_mode = %x\n", st.st_mode);
  printf("st_nlink = %d\n", st.st_nlink);
  printf("st_uid = %d\n", st.st_uid);
  printf("st_gid = %d\n", st.st_gid);
  printf("st_rdev = %d\n", st.st_rdev);
  printf("st_size = %lu\n", st.st_size);
  return 0;
}

result of stat: 0
st_dev = 2
st_ino = 0
st_mode = 81b6
st_nlink = 1
st_uid = 0
st_gid = 0
st_rdev = 2
st_size = 0

In this C program, stat(2) return 0 with successful. Go also should return nil for err.

[mattn]

Hopefully this will work.

diff --git a/src/os/stat_windows.go b/src/os/stat_windows.go
index 3e0e0a59ed..a6830c0f6e 100644
--- a/src/os/stat_windows.go
+++ b/src/os/stat_windows.go
@@ -74,6 +74,18 @@ func stat(funcname, name string, createFileAttrs uint32) (FileInfo, error) {
 		}
 		return fs, nil
 	}
+	if err == syscall.ERROR_FILE_NOT_FOUND {
+		if attr, err := syscall.GetFileAttributes(namep); err == nil && attr == syscall.FILE_ATTRIBUTE_ARCHIVE {
+			return &fileStat{
+				name: name,
+				// hopefully this will work for CON, PRN, AUX, CONOUT$ or etc.
+				vol:   0,
+				idxhi: 0,
+				idxlo: 0,
+			}, nil
+		}
+	}
+
 	// GetFileAttributesEx fails with ERROR_SHARING_VIOLATION error for
 	// files, like c:\pagefile.sys. Use FindFirstFile for such files.
 	if err == windows.ERROR_SHARING_VIOLATION {

[gopherbot]

Change https://golang.org/cl/201157 mentions this issue: os: Fix Stat() with special file on Windows

[lifenjoiner]

Thanks, you guys are really quick~

[lifenjoiner]

@mattn
If your solution works well, maybe we can remove:

go/src/os/stat_windows.go

Lines 24 to 26 in 902d5aa

	if isWindowsNulName(file.name) {
	return &devNullStat, nil
	}

go/src/os/stat_windows.go

Lines 24 to 26 in 902d5aa

	if isWindowsNulName(file.name) {
	return &devNullStat, nil
	}

and maybe isWindowsNulName can be cleaned:

go/src/os/file.go

Lines 565 to 581 in a38a917

	// isWindowsNulName reports whether name is os.DevNull ('NUL') on Windows.
	// True is returned if name is 'NUL' whatever the case.
	func isWindowsNulName(name string) bool {
	if len(name) != 3 {
	return false
	}
	if name[0] != 'n' && name[0] != 'N' {
	return false
	}
	if name[1] != 'u' && name[1] != 'U' {
	return false
	}
	if name[2] != 'l' && name[2] != 'L' {
	return false
	}
	return true
	}

WinAPI CreateDirectory should have covered these reserved names:

go/src/syscall/syscall_windows.go

Lines 443 to 449 in a38a917

	func Mkdir(path string, mode uint32) (err error) {
	pathp, err := UTF16PtrFromString(path)
	if err != nil {
	return err
	}
	return CreateDirectory(pathp, nil)
	}

confiremed by

package main

import (
	"os"
)

func main() {
	err := os.Mkdir("con", os.ModePerm)
	if err != nil {
		panic(err)
	}
}

[lifenjoiner]

append:
on windows, mkdir nul goes silence and returns success, mkdir conin$ and mkdir conin$ have no visible effect and also succeed, while other reserved name trigger errors ...

[gopherbot]

Change https://golang.org/cl/373355 mentions this issue: os: return value for Stat() for files with a reserved name on windows