os: race condition between (*os.File).Stat and os.Chdir on windows

[hirochachacha]

You can reproduce by

package main


import(
    "os"
    "fmt"
)

func main() {
    f, err := os.Open(".")
    if err != nil {
        panic(err)
    }

    fi, err := f.Stat()
    if err != nil {
        panic(err)
    }

    fmt.Println(fi)

    err = os.Chdir("..")
    if err != nil {
        panic(err)
    }

    fi, err = f.Stat()
    if err != nil {
        panic(err)
    }

    fmt.Println(fi)
}

Output is:

&{. {16 {2734709284 30489263} {2684316263 30491099} {2684316263 30491099} 0 4096} {0 0} C:\msys64\home\hiro\work 0 0 0}
&{. {16 {2657275984 30489261} {2713337936 30491099} {2713337936 30491099} 0 8192} {0 0} C:\msys64\home\hiro 0 0 0}

First and second line should be the same.

[gopherbot]

CL https://golang.org/cl/18181 mentions this issue.

[rsc]

FWIW, it is always nice to include the actual output from the program along with what you expected. I cannot easily reproduce, since I don't have a Windows machine handy. (And maybe my Windows machine behaves differently from yours.)

[hirochachacha]

I've edited it.

[alexbrainman]

First and second line should be the same.

Why?
I don't see where the race is. Can you provide more details, please?
Thank you.

Alex

[hirochachacha]

(*os.File).Stat() should return a os.FileInfo of the file we opened,
without dependence on process's current directory.

see https://github.com/golang/go/blob/master/src/os/stat_windows.go#L23

if file.name is given as relative path, the problem is occurred.

see also my CL.

Thank you.

hiro

[alexbrainman]

the problem is occurred.

What is that problem? I don't see how those internal bits that your CL changes affect anything. Can you point me to an example that demonstrates that problem? Thank you.

Alex

[hirochachacha]

Hmm, I don't understand what you mean.

Do you try the code above? What output you get?
Of course, you should not run the code on root directory.

Thank you.

hiro

[alexbrainman]

Do you try the code above? ...

I didn't not. I disagree with your report. You say "First and second line should be the same.", but I don't see why the two lines shouldn't be different. Your output display internal state of os package. Where does it say that this internal state should be one way or other?

Unless you can demonstarte that this internal state affects something visible to os package user, I don't see how this is a bug.

Alex

[hirochachacha]

From my understanding, File.Stat should behave like fstat in the unix world.
At least, I expect such a behavior.

I hope os package provide platform-independent interface.

Although it's a corner case for you, I think this is a bug.

[alexbrainman]

From my understanding, File.Stat should behave like fstat in the unix world.
At least, I expect such a behavior.

Which File.Stat behavior does not meat your expectations? Please provide an example to demonstrate. Just complaining about File.Stat internal state gets us nowhere. Internal state is not behavior.

Alex

[hirochachacha]

I don't care the internal state, too.

I say:

(*os.File).Stat() should return a os.FileInfo of the file we opened,
without dependence on process's current directory. 

Thank you.

hiro

[hirochachacha]

Sorry about my poor English.

[alexbrainman]

I am sorry, but I don't see anything broken here. You don't need to explain me why something is broken, you need to provide a small program to demonstrate broken behaviour. Program that outputs something undocumented or unexpected.

Alex

[hirochachacha]

but I don't see anything broken here.

Is this mean, you tried my code above? or another way?
Can you show me your tests and results?

[bradfitz]

@alexbrainman, I understand his bug report at least.

See the CL's test: https://go-review.googlesource.com/#/c/18181/4/src/os/os_windows_test.go

[hirochachacha]

Maybe, we talked about the difference between equality and identity...

[alexbrainman]

@bradfitz, are you saying that it is important that reflect.DeepEqual(fi, fi2) is true in new test in CL 18181? If yes, then why? If no, then what are we fixing here?

Alex

[mattn]

@alexbrainman

(*os.File).Stat is stat not fstat. And the FileInfo is created from name given Open().

https://github.com/golang/go/blob/master/src/os/stat_windows.go#L23

So if the name is relative path, Chdir make different point to the path.

[bradfitz]

@alexbrainman, maybe not DeepEqual (internals are outside of scope), but os.SameFile(fi, fi2) at least. And if that test passes on Linux, it should pass on Windows. (I haven't tried)

[alexbrainman]

but os.SameFile(fi, fi2) at least

Yes os.SameFile should work, and it does not. So we have a problem. I'll review CL 18181. Thank you, Brad.

Alex