os: ReadFile on directories does not return EISDIR in windows

[dop251]

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

$ go version
go version 1.15.6 windows/amd64

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 GO111MODULE=
set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\vagrant\AppData\Local\go-build
set GOENV=C:\Users\vagrant\AppData\Roaming\go\env
set GOEXE=.exe
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GOMODCACHE=C:\Users\vagrant\go\pkg\mod
set GONOPROXY=
set GONOSUMDB=
set GOOS=windows
set GOPATH=C:\Users\vagrant\go
set GOPRIVATE=
set GOPROXY=https://proxy.golang.org,direct
set GOROOT=c:\go
set GOSUMDB=sum.golang.org
set GOTMPDIR=
set GOTOOLDIR=c:\go\pkg\tool\windows_amd64
set GCCGO=gccgo
set AR=ar
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set GOMOD=C:\Users\vagrant\go\wintest\go.mod
set CGO_CFLAGS=-g -O2
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-g -O2
set CGO_FFLAGS=-g -O2
set CGO_LDFLAGS=-g -O2
set PKG_CONFIG=pkg-config
set GOGCCFLAGS=-m64 -mthreads -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=C:\Users\vagrant\AppData\Local\Temp\go-build505615858=/tmp/go-build -gno-record-gcc-switches

What did you do?

https://play.golang.org/p/GwvDhx3PqK-

What did you expect to see?

All good

What did you see instead?

Unexpected error: read /: The handle is invalid

[egonelbre]

Internally it ends up doing code that is equivalent to:

#include <windows.h>
#include <stdio.h>

void main() {
	WIN32_FIND_DATA data;
	HANDLE h = FindFirstFileA("\\", &data);

	char buf[100];
	BOOL r = ReadFile(h, &buf[0], 100, 0, 0);
	DWORD err = GetLastError();
	printf("%d\n", err);
}

Which does return the invalid handle error.

[dop251]

I believe it should be fixed. Opening a directory in read/write mode does produce the correct error because there is a special handling for it:

go/src/os/file_windows.go

Line 177 in 4d27c4c

r, errd := openDir(name)

What's missing is returning of the same error when a Read() is attempted on a directory file.

[gopherbot]

Change https://golang.org/cl/279852 mentions this issue: os: fix openFileNolog when open dir in windows

[gopherbot]

Change https://golang.org/cl/374394 mentions this issue: os: return EISDIR when ReadFile is attempted on a directory.

[alexbrainman]

I believe it should be fixed. Opening a directory in read/write mode does produce the correct error ...

@dop251 I am not convinced. Where is it documented that ioutil.ReadFile("/") should return syscall.EISDIR?

Thank you.

Alex

[dop251]

To clarify, it applies not just to ReadFile(), but to any Read(). I don't think it's explicitly documented (at least not in Go), however:

1. It makes it consistent with POSIX OSes thus improving the portability of Go source code.
2. syscall.EISDIR is already returned when you try to open a directory in read/write mode in Windows.
3. It's better than the current The handle is invalid error.