make fileCopy more robust

[kjk]

It's unlikely to happen, but Close() on a file being written to can fail due to unflushed os buffers. Close might fail even though io.Copy said it wrote all the data to the destination.

Does not apply to files being read from so no need to check error from source.Close()

[yevgenypats]

Interesting. Wasn't aware of that in Go. Can you link to some blog/ data about this?

[kjk]

It's not really a Go thing but an operating system thing. On the lowest level, a close syscall might fail which will propagate to file.Close().

https://www.joeshaw.org/dont-defer-close-on-writable-files/
https://www.reddit.com/r/golang/comments/3735so/do_we_have_to_check_for_errors_when_we_call_close/

https://linux.die.net/man/2/close:

Not checking the return value of close() is a common but nevertheless serious programming error.

[yevgenypats]

cool.