Someone on the gophers slack confirmed having the same result under windows 7 with my code. So either my code is wrong, or there really is an issue.

Thank you for the report @FabienTregan!

I'll page some some wizards /cc @alexbrainman @ianlancetaylor

trying to investigate the problem with people on gophers' slack,
I was hinted this :

Kale Blankenship [22 h 37]
@Fabien I think this is due to how the Windows API works.
> If CREATE_ALWAYS and FILE_ATTRIBUTE_NORMAL are specified, CreateFile
> fails and sets the last error to ERROR_ACCESS_DENIED if the file exists
> and has the FILE_ATTRIBUTE_HIDDEN or FILE_ATTRIBUTE_SYSTEM attribute. To
> avoid the error, specify the same attributes as the existing file.
https://msdn.microsoft.com/en-us/library/windows/desktop/aa363858(v=vs.85).aspx
The typical path sets `FILE_ATTRIBUTE_NORMAL`
(https://github.com/golang/go/blob/master/src/syscall/syscall_windows.go#L289),
which is probably a reasonable thing to do. From my brief reading of the
docs, the way around it would be to first read the existing attributes
before opening. Conceivably, you could work around it using
`syscall.CreateFile` and `os.NewFile` yourself .

But the flags to File.Open do not seem to contain a bit for visibility.
I could not find how to workaround my problem with this information.

What happens with the C fopen call on WIndows? Does it support opening hidden files?

@ianlancetaylor I've not been writting any C for at least 15 years, but made this test :

$ cat write.c
#include <stdio.h>

int main (int argc, char *argv[]) {

  FILE *f = fopen("file.txt", "w");
  if (f == NULL)
  {
    printf("Error opening file!\n");
    return -1;
  }

  fprintf(f, "written");

}

It writes Error opening file! if file.txt is hidden.
Opening the file in "a" mode works, though.

It seems that fopen on windows calls CreateFile, which documentation says :

If CREATE_ALWAYS and FILE_ATTRIBUTE_NORMAL are specified, CreateFile fails and sets the last error to ERROR_ACCESS_DENIED if the file exists and has the FILE_ATTRIBUTE_HIDDEN or FILE_ATTRIBUTE_SYSTEM attribute. To avoid the error, specify the same attributes as the existing file.

@FabienTregan Thanks, we also call CreateFile. What I was wondering was whether the C fopen function passed FILE_ATTRIBUTE_NORMAL, but it seems that it doesn't. That makes me wonder whether we should change Go's syscall.Open to not pass FILE_ATTRIBUTE_NORMAL.

It looks like the use of FILE_ATTRIBUTE_NORMAL dates back to when @alexbrainman added the code in https://golang.org/cl/770041.

But the flags to File.Open do not seem to contain a bit for visibility.
I could not find how to workaround my problem with this information.

I think others suggest you use syscall.CreateFile directly in your program as you see fit.

That makes me wonder whether we should change Go's syscall.Open to not pass FILE_ATTRIBUTE_NORMAL.

Sure, we, probably, could do that. I changed:

diff --git a/src/syscall/syscall_windows.go b/src/syscall/syscall_windows.goindex 5cfdb76e2b..d46ce1a0c4 100644
--- a/src/syscall/syscall_windows.go
+++ b/src/syscall/syscall_windows.go
@@ -286,7 +286,7 @@ func Open(path string, mode int, perm uint32) (fd Handle, err error) {
        default:
                createmode = OPEN_EXISTING
        }
-       h, e := CreateFile(pathp, access, sharemode, sa, createmode, FILE_ATTRIBUTE_NORMAL, 0)
+       h, e := CreateFile(pathp, access, sharemode, sa, createmode, 0, 0)
        return h, e
 }

and that still passes on my computer. But, according to the documentation:

CreateFile fails and sets the last error to ERROR_ACCESS_DENIED if the file exists and has the FILE_ATTRIBUTE_HIDDEN or FILE_ATTRIBUTE_SYSTEM attribute. To avoid the error, specify the same attributes as the existing file.

and I don't see how my change above will help here - the documentation requires us to pass "the same attributes as the existing file".

I also not sure, if we want to change the way we treat hidden files.

Alex