path/filepath: Rel() returns invalid path if volume case is different

[pavel-main]

Go version - go1.4.2 windows/amd64
Operating system - Windows 7 64-bit

The problem:

package main

import (
    "path/filepath"
)

func main() {
    relpath, err := filepath.Rel(`E:\Projects\`, `e:\Projects\src`)
    if err != nil {
        println(err.Error())
        return
    }
    println(relpath)
}

Output:

Rel: can't make \Projects\src relative to \Projects

Expected:

src

I've ocassionally found this when some go program's os.Getcwd() call returned me a path with lowercased volume name - not sure why this happened, can't reproduce anymore. Some users, who set their GOPATH to something like e:\Projects might have problems.

Probably, on Windows, VolumeName() should have separate implementation like this:

func VolumeName(path string) string {
    if !isUNC(path) {
        return strings.ToUpper(path[:volumeNameLen(path)])
    }
    return path[:volumeNameLen(path)]
}

Helpful link - Naming Files, Paths, and Namespaces

[alexbrainman]

We can fix drive letter comparison. But the problem is more general - windows paths are not case sensitive. So, for example, filepath.Rel(E:\Projects\, E:\projects\src) will be wrong too. I am not sure it is worth fixing particular case of drive letter. What do you think?

filepath.Rel is also broken when we walking from one drive to another. It has no meaning for something like filepath.Rel(c:\a\, d:\a\b). I would just avoid using filepath.Rel on windows.

Alex

[pavel-main]

Hi, Alex!

I've encountered this call in some command-line app, and I guess, a lot of filesystem-related command line apps are already using or will use this necessary call. Is it really ok to abandon this, when so simple solution is in the air?

filepath.Rel(E:\Projects, E:\projects\src) will be wrong too.

But you can compare their both lower or uppercase versions. The only problem - before returning the result you'll have to return the substring in original case - it will might be a clunky solution, but still possible.

It has no meaning for something like filepath.Rel(c:\a, d:\a\b)

I think it would be trivial to compare both drive letters / isUNC before actual comparsion. So, in your specific example, it would be absolutely ok to return "Rel can't make ..." error.

[alexbrainman]

Fair enough. We can do as much as we can. @kpashka what you like to try and send this change?

Alex

[rsc]

It may be worth fixing this to help the go command (#11409). What is the definition of case-insensitive on Windows? Is it ASCII only, or are c:\δ and c:\Δ the same file name too?

[alexbrainman]

What is the definition of case-insensitive on Windows?

I really don't know. I use English Windows version. For me everything is ASCII.

Is it ASCII only, or are c:\δ and c:\Δ the same file name too?

U:\>type main.go
package main

import (
        "io/ioutil"
        "log"
        "os"
        "path/filepath"
        "strings"
)

func main() {
        tmpdir, err := ioutil.TempDir("", "alex")
        if err != nil {
                log.Fatal(err)
        }
        defer os.RemoveAll(tmpdir)

        const filename = "\u0441\u0430\u0448\u0430_\u03b4.txt"

        err = ioutil.WriteFile(filepath.Join(tmpdir, filename), nil, 0644)
        if err != nil {
                log.Fatal(err)
        }

        f, err := os.Open(filepath.Join(tmpdir, strings.ToUpper(filename)))
        if err != nil {
                log.Fatal(err)
        }
        defer f.Close()
}

U:\>go run main.go

U:\>

It may be worth fixing this to help the go command (#11409).

This issue is easy, because drive letters are always 'a' to 'z' - there is no unicode here. You were always against treating windows paths as case-incensitive. So I stopped arguing. Maybe we should start ignoring case in drive letter and see where it gets us. I was fighting similar battle (see issue #11459) just the other day.

Mind you this issue is about filepath.Rel. filepath.Rel coubld be overkill for #11409?

Alex

[AngangGuo]

From filepath.Rel:

That is, Join(basepath, Rel(basepath, targpath)) is equivalent to targpath itself

But in Windows I got an error as shown here:
https://play.golang.org/p/uoltpVJeng

Andrew

[gopherbot]

CL https://golang.org/cl/16795 mentions this issue.

[alexbrainman]

@AngangGuo I don't see your problem is the same as stated in this issue. If you think you have a problem, you should create new issue.

Alex

[amkgo]

Thanks. I've create a new issue here:
#13208

Andrew