cmd/gofmt: go fmt eats my source code if it gets ENOSPC

[gopherbot]

by tycho@tycho.ws:

What does 'go version' print?

go version go1.2.1 linux/amd64

What steps reproduce the problem?
If possible, include a link to a program on play.golang.org.

1. write some code
2. fill up your disk
3. run `go fmt`

What happened?

go fmt deletes my code, then tells me ENOSPC. go fmt should ideally be smart enough to
realize that (preferably by keeping the old file around until the write of the new file
succeeds) and not delete my source code.

[ianlancetaylor]

Comment 1:

Labels changed: added repo-tools, release-none.

[bradfitz]

Comment 2:

go fmt calls gofmt, which reads the file, reformats it in memory, and then does:
                if *write {
                        err = ioutil.WriteFile(filename, res, 0644)
            if err != nil {
                                return err
                    }
            }
WriteFile is just:
// otherwise WriteFile truncates it before writing.
func WriteFile(filename string, data []byte, perm os.FileMode) error {
        f, err := os.OpenFile(filename, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, perm)
        if err != nil {
                return err
        }
        n, err := f.Write(data)
        if err == nil && n < len(data) {
                err = io.ErrShortWrite
        }
        if err1 := f.Close(); err == nil {
                err = err1
        }
        return err
}
So yes, we're overwriting it in place.  I suppose we could write it to a temp file in
the same dir and then try to automatically rename it into place, at least on Unix (see
Windows issue #8914).  But I think we should make a helper function for that rather than
ask each tool to try to do it.

Status changed to Accepted.

[griesemer]

Comment 3:

Perhaps WriteFile should be smart enough and do it for everybody?