os: File.Truncate reports unhelpful error in Linux.

[Zilog8]

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

go version go1.4 linux/386
go version go1.4 windows/amd64

What operating system and processor architecture are you using?

Debian Wheezy, x86
Windows 7, x64

What did you do?

    filepath := "/play/p2/test2.txt"
    file, err := os.Open(filepath)
    if err != nil {
        fmt.Println("Error 1", err)
    }
    err = file.Truncate(0)
    if err != nil {
        fmt.Println("Error 2", err)
    }
    err = file.Close()
    if err != nil {
        fmt.Println("Error 3", err)
    }

What did you expect to see?

Error 2 truncate /play/p2/test2.txt: Access is denied

What did you see instead?

Error 2 truncate /play/p2/test2.txt: invalid argument

Executing Truncate(int) rightfully results in an error because os.Open() yields a read-only file handle.
However, in windows this error is helpful (the "expect to see" above), while in linux it is not helpful (the "see instead" above)

[bradfitz]

Assigning to @ianlancetaylor to decide if this is something we should care about. Where does this end? There are surely tons of different error messages between operating systems.

[Zilog8]

I agree it's not exactly a critical issue :-). However, precise and accurate error messages are very useful. I just spent a few hours trying to figure out why my code wasn't working (my project involves joining various filesystems through an abstraction layer, which is then shared over NFS, so it could've been a couple of different things going wrong), until I decided to try various snippets of it on a windows machine. Once I had the windows error message it was just a few moments before I knew exactly what was wrong and why.

My issue isn't that the messages are different. It's that the message in Linux is so un-useful.

[minux]

That's how Linux reports error when you've provided a read only file
descriptor to an operation that requires writing permission. (See man 2
ftruncate)

Actually, if you look at the man page, EBADF / EINVAL is only used for the
"fd is not open for writing error", so the error is not ambiguous.

It's not feasible for the os package to try to provide better error message
than what the underlying syscall provides in every case.

PS: when debugging this kind of issue, I suggest using strace.

[Zilog8]

Hmm.. the documentation that I read showed:

 EINVAL
    The argument length is negative or larger than the maximum file size.

However, further down on the page it does say, which I hadn't noticed earlier:

    EBADF or EINVAL
    fd is not open for writing.
    EINVAL
    fd does not reference a regular file.

Well, if Linux itself is what provides the ambiguous error, I guess there's not much to be done. Adding extra logic just for this is probably not worth it.

[minux]

The first part of the man page is for truncate(2), and the 2nd part is
for ftruncate(2).

Close as nothing to do.