Add non-blocking notes

[WeidiDeng]

I have done some local testing, it turns out if the file is in non-blocking mode, Getsize will return zero sizes without any error. Setting the file in block mode will allow the result to be correct.

Instead, we could document that we can manually set a file to be non-blocking mode using syscall.SetNonblock, this will allow deadlines to work. I have set a larger timeout so the tests will succeed every time. Possibly this is due to how golang internal poller works, but that's up to golang to fix it.

[WeidiDeng]

All tests passed in my workflow.

[WeidiDeng]

I think -timeout parameter actually means the total time of running the test, not individual test. During my test, I found each test iteration took about 0.2 seconds, and 100 tests took 20 seconds. Definitely the polling is working as intended.

I did experience random get/set size errors when using non-blocking mode (manually set instead of using syscall.RawConn). And that's the reason I think we should document this case, let users configure by themselves.

[WeidiDeng]

If I revert your changes in 3194d69. The tests do block forever. syscall.SetNonblock has no such problem if added to ioctl function.

[creack]

Thanks!

The last attempt to get that working tried to remove the uses of .Fd() (which sets blocking mode), but that resulted in various races. I'll keep testing in some of my projects with this version to be sure, but seem like it works as expected indeed.