In my Go application instead of writing to a file directly I would like to write to a temporary that is renamed into the final file when everything is done. This is to avoid leaving partially written content in the file if the application crashes.
Currently I use ioutil.TempFile, but the issue is that it creates the file with the 0600 permission, not 0666. Thus with typical umask values one gets the 0600 permission, not expected 0644 or 0660. This is not a problem is the destination file already exist as I can fix the permission on the temporary to much the existing ones, but if the file does not exist, then I need somehow to deduce the current umask.
I suppose I can just duplicate ioutil.TempFile implementation to pass 0666 into os.OpenFile, but that does not sound nice. So the question is there a better way?
I don't quite grok your problem.
Temporary files must be created with as tight permissions as possible because the whole idea of having them is to provide your application with secure means of temporary storing data which is too big to fit in memory (or to hand the generated file over to another process). (Note that on POSIX systems, where an opened file counts as a live reference to it, it's even customary to immediately remove the file while having it open so that there's no way to modify its data other than writing it from the process which created it.)
So in my opinion you're trying to use a wrong solution to your problem.
So what I do in a case like yours is:
If you feel like using a fixed suffix is lame, you can "steal" the implementation of picking a unique non-conflicting file name from ioutil.TempFile(). But IMO this would be overengeneering.