Fix File to Close

[hangscer8]

Need close file in time.

[lifubang]

LGTM
Furthermore, do you know whether there is a way to detect such errors automatically or not?

[kolyshkin]

@hangscer8 thanks, LGTM.

Furthermore, do you know whether there is a way to detect such errors automatically or not?

One way is to do a runtime check. I have added a test case doing that in #2802 (further improved in #3735). Problem is, it does not test every code path, and only checks for those leaks that are not a one-time thing. Thus, it does not detect issues fixed in this PR, because:

1. tryDefaultCgroupRoot is only called once.
2. The "leak" in update.go is not a leak, since it's in a short-lived binary (runc update) which frees all resources (incl. open file descriptors) upon exiting.

One other thing to note is, Go uses runtime.SetFinalizer to call close on every os.File which is being garbage-collected, meaning that close will be called implicitly and eventually -- some time after the file variable goes out of scope and is garbage-collected. Surely we should still close all fds explicitly nevertheless.

[kolyshkin]

LGTM