os: fix Windows (*os.File).Seek on directory handle

[bradfitz]

Half of our Windows builders (windows-386-2008, windows-amd64-2016, windows-amd64-longtest) fail to seek an *os.File opened on a directory.

But windows-amd64-2008 and windows-amd64-2012 work.

Figure this out and re-enable the test on Windows. (The test was added late in the Go 1.14 cycle, just before beta)

[gopherbot]

Change https://golang.org/cl/210283 mentions this issue: os: skip a new failing test on Windows

[networkimprov]

cc @alexbrainman @mattn @zx2c4

[alexbrainman]

Go use FindFirstFile, FindNextFile and FindClose Windows API to implement directory reading. The API does not provide functionality to seek to random position of the directory. I don't know what the alternatives are, if any.

Alex

[gopherbot]

Change https://golang.org/cl/213439 mentions this issue: os: document that File.Seek works on directories, but not portably

[networkimprov]

Could File.Seek(0, os.SeekStart) be mapped to FindFirstFile() ?

[mattn]

man 3 fdopendir say:

fd is used internally by the implementation, and should not otherwise be used by the application.

https://linux.die.net/man/3/opendir

As far as I can see the doc, Go should not support Seek for directory.

[zx2c4]

NtQueryDirectoryFile may be of use.

[alexbrainman]

Could File.Seek(0, os.SeekStart) be mapped to FindFirstFile() ?

Yes. But people will still complain that File.Seel(1, os.SeekStart) fails. So, lets not pretend.

NtQueryDirectoryFile

Yes, it might work. But is all extra new code worth ability to File.Seek?

Alex

[networkimprov]

Could File.Seek(0, os.SeekStart) be mapped to FindFirstFile() ?

Yes. But people will still complain that File.Seel(1, os.SeekStart) fails. So, lets not pretend.

Well you could FindNextFile() in a loop for i < seek_pos?

[alexbrainman]

Well you could FindNextFile() in a loop for i < seek_pos?

I don't understand what you are proposing. Please, send a CL with the change, if you know what to do.

Thank you.

Alex

[odeke-em]

Punting to Go1.17 as we didn’t get much action for the Go1.16 development cycle.