go: os: File implicitly closed when garbage-collected due to runtime finalizer but this behavior is not documented

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

$ go version
go 1.15.2

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (go env)?

Linux and macOS.

What did you do?

Code: https://play.golang.org/p/YmHIIc7sha4

Note: This does not reproduce in go playground, but can reproduce on my mac/linux.

What did you expect to see?

No panic.

What did you see instead?

panic: bad file descriptor

Reason

I found that this is because there’s an implicitly close call when the file object is garbage-collected.

In os/file_unix.go:

func newFile(fd uintptr, name string, kind newFileKind) *File {
	...

	runtime.SetFinalizer(f.file, (*file).close)
	return f
}

What do I expect

I know there’s thousands of method to avoid being garbage-collected such as runtime.KeepAlive, but I think this is unreasonable, either this behaviour should be removed or the file object should not be garbage-collected.

If a user forgets to call file.Close, the memory should be leaking.

Also, this behaviour isn’t documented anywhere.