go: os: Symlink on Windows with a to-be-created directory path silently creates a link of the wrong type

(Discovered via debugging on #38772.)

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

gopher@SERVER-2016-V7- C:\workdir\go\src>..\bin\go version
go version devel gomote.XXXXX windows/amd64

Does this issue reproduce with the latest release?

Yes.

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

gopher@SERVER-2016-V7- C:\workdir\go\src>..\bin\go version
go version devel gomote.XXXXX windows/amd64
gopher@SERVER-2016-V7- C:\workdir\go\src>…\bin\go env
set GO111MODULE=
set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\gopher\AppData\Local\go-build
set GOENV=C:\Users\gopher\AppData\Roaming\go\env
set GOEXE=.exe
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GOMODCACHE=C:\Users\gopher\go\pkg\mod
set GONOPROXY=
set GONOSUMDB=
set GOOS=windows
set GOPATH=C:\Users\gopher\go
set GOPRIVATE=
set GOPROXY=https://proxy.golang.org,direct
set GOROOT=C:\workdir\go
set GOSUMDB=sum.golang.org
set GOTMPDIR=
set GOTOOLDIR=C:\workdir\go\pkg\tool\windows_amd64
set GCCGO=gccgo
set GOAMD64=alignedjumps
set AR=ar
set CC=gcc
set CXX=g++
set CGO_ENABLED=0
set GOMOD=C:\workdir\go\src\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 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=
C:\Users\gopher\AppData\Local\Temp\go-build141789646=/tmp/go-build -gno-record-gcc-switches

What did you do?

	err := os.Symlink(to, from)
	if err != nil {
		t.Fatalf(…)
	}

	err = os.Mkdir(to, 0755)
	if err != nil {
		t.Fatalf(…)
	}

	…

	file, err := os.Open(from)
	if err != nil {
		t.Fatalf(…)
	}
	file.Close()

CL 234737 contains a complete example.

What did you expect to see?

Either a non-nil error from the call to os.Symlink, or a successful call to os.Open.

What did you see instead?

gopher@SERVER-2016-V7- C:\workdir\go\src>..\bin\go test os -run=TestSymlink.*
--- FAIL: TestSymlinkBeforeTargetFileExists (0.00s)
    os_test.go:908: Open("earlysymlinktestfrom") failed: open earlysymlinktestfrom: Access is denied
.
FAIL
FAIL    os      0.052s
FAIL

Diagnosis

The Windows CreateSymbolicLinkA system call requires a flag to indicate whether the destination is a file or a directory. (If SYMBOLIC_LINK_FLAG_DIRECTORY is set, the symlink is a directory symlink; otherwise, it is a file symlink.)

The current implementation of os.Symlink on Windows uses a call to os.Stat to determine which kind of link to create. Unfortunately, if that Stat call fails, Symlink assumes that it is a file rather than reporting the error to the caller: https://github.com/golang/go/blob/567556d78657326c99b8fa84ec2a5ee511a0941b/src/os/file_windows.go#L337-L338

That assumption seems like a mistake. If a Windows user needs to create a symlink to a not-yet-existing file or directory on Windows, they should be made aware that the call is missing essential information (the destination type), and can then make an explicit choice to use golang.org/x/sys/windows.CreateSymbolicLink instead of os.Syscall if they are able to supply the missing information.

I think we should change os.Symlink at the beginning of the Go 1.16 cycle so that it propagates the Stat error instead of implicitly assuming that the destination is a file.

CC @fraenkel @alexbrainman @ianlancetaylor