os: Windows: MkdirAll fails with valid path that starts with \\? (e.g. \\?\D:\test)

[sandreas]

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

1.9

Does this issue reproduce with the latest release?

yes

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

set GOARCH=amd64
set GOBIN=
set GOEXE=.exe
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOOS=windows
set GOPATH=C:\Users\xxx\go
set GORACE=
set GOROOT=C:\Go
set GOTOOLDIR=C:\Go\pkg\tool\windows_amd64
set GCCGO=gccgo
set CC=gcc
set GOGCCFLAGS=-m64 -mthreads -fmessage-length=0
set CXX=g++
set CGO_ENABLED=1
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

What did you do?

precondition: Directory D:\test does not exist

dir := "\\\\?\\D:\\test"
err = os.MkdirAll(dir, 0777) // results in error
err = os.Mkdir(dir, 0777) // works

https://play.golang.org/p/Z2ONpbvDyV

What did you expect to see?

os.Mkdir and os.MkdirAll will create the directory

What did you see instead?

os.MkdirAll fails with an error

[ianlancetaylor]

CC @alexbrainman

[alexbrainman]

Mkdir just calls correspondent Windows API, so it succeeds with whatever path it is given. But MkdirAll recursively calls itself for its parameters parent directory. It finds parent directory by looking for path delimiter in the file path and taking whatever is on the left. When file path is \\?\D:\test, it obviously finds empty path at the end. We could fix MkdirAll by removing \\?\ prefix on Windows. But I suspect MkdirAll is also broken for network shares, like \\server\share\a\b\c. Do we want to fix \\?\ (it will be easy), or do we want to fix both?

I also noticed that path/filepath does not handle \\?\ at all. Do we want to try and fix that? It is bit confusing some code works with \\?\ and some don't. Should we say we support long paths everywhere and will try and fix whatever breaks?

Alex

[ncw]

An rclone user has noticed this in rclone/rclone#1754

However it seemed to work for me in Windows 7 so maybe this is a specific Windows 10 problem?

[alexbrainman]

However it seemed to work for me in Windows 7 so maybe this is a specific Windows 10 problem?

I replied on rclone/rclone#1754 . Perhaps that answers your question.

Alex

[gopherbot]

Change https://golang.org/cl/86295 mentions this issue: os: Windows: MkdirAll should work with extended-length form path