runtime: stray println in runtime epoll function

[mdwhatcott]

After seeing the following message (that looks like an error) I found a stray println that doesn't seem to be very helpful since there is retry logic in place. The program's exit code was 0 and the results were as expected so there actually wasn't a problem.

https://github.com/golang/go/blob/master/src/runtime/netpoll_epoll.go#L72

I'm proposing that the stray println be removed unless it is otherwise helpful or necessary.

[gopherbot]

CL https://golang.org/cl/12047 mentions this issue.

[ianlancetaylor]

@mdwhatcott You neglected to include the actual error message. Do you still have it?

CC @dvyukov

[mdwhatcott]

@ianlancetaylor:

The exact error text was:

runtime: epollwait on fd 7 failed with 9

I believe it was generated here:

https://github.com/golang/go/blob/master/src/runtime/netpoll_epoll.go#L72

[ianlancetaylor]

Thanks. @dvyukov This is failing with EBADF. I admit that I don't know how that could happen.

I continue to believe that we must either remove this println or add a throw after it. We can not have Go programs print out random messages in the middle of their execution. Which approach do you prefer?

[dvyukov]

@ianlancetaylor That can happen if the program closes random file descriptors (due to a race for example).
I think we need to throw, but it is somewhat risky just before release. I would prefer to do it early in 1.6 cycle. It is possible that windows/darwin/openbsd return some legitimate temp failures other than EINTR. This does not look very urgent as it is most likely a bug in user program.

[mdwhatcott]

It could very well be a bug, but it's only happened for one user running Linux version 2.6.18-400.1.1.el5 (mockbuild@x86-012.build.bos.redhat.com) (gcc version 4.1.2 20080704 (Red Hat 4.1.2-55)). We have many users running this program across various versions of windows, linux (x64 and x86), and mac.

I think either plan that was suggested (print and throw or just remove the print) is better than the current state of affairs.

[ianlancetaylor]

@dvyukov I agree that throwing is risky just before the release. In that case I think we should commit http://golang.org/cl/12047 and revisit after the release. The current code is not what we want. Can you +2 that CL?

(I actually don't see how epoll_wait can return EBADF even if the program closes random file descriptors. epoll_wait doesn't care whether the file descriptors it uses have been closed. I can't see any kernel path that returns EBADF, except of course if the epoll descriptor itself is closed.)

[dvyukov]

@ianlancetaylor Yes, I mean that the epoll fd is closed.
In the case of EBADF the thread that calls netpoll will hang in an active spin, and the program will most likely hang. I agree that proper error handling of any transient errors and a throw on non-transient failures would be better. But if we now choose between a silent non-diagnosable hang and a "runtime: epollwait on fd 7 failed with 9" error message before hang; it is not clear to me what is better. And note that this print should not happen at all in correct programs (I don't remember any mentions of this print in the last two years). For example, this issue would not be failed if we would removed the print, which is bad. So what exactly situation you are trying to solve/improve by removing the print?

[ianlancetaylor]

How could the epoll FD be closed? Nothing ever closes it.

The original issue report says that the program completed normally. That is the situation I am trying to improve: the one reported in this issue. The program ran normally, except that it printed out a meaningless message.

If the choice is a meaningless message and a hanging program, or a hanging program without a meaningless message, that is no choice at all. If those are the real choices--which if this issue is correct is not actually the case--then I choose the throw.

[ianlancetaylor]

In fact, if the epoll FD is closed, then as far as I can tell by looking at the code we aren't going to see a single message. We're going to see a program that loops endlessly printing the same message over and over. So, again, we should do the throw.

[ianlancetaylor]

Oh, I see, netpolllasterr avoids the endless print.

I'm going to change the CL to throw.

[dvyukov]

How could the epoll FD be closed? Nothing ever closes it.

Well, that's trivial, you just do:

syscall.Close(7)

We're going to see a program that loops endlessly printing the same message over and over.

That's what netpolllasterr for.

The original issue report says that the program completed normally.

I think normal completion is pure luck in this case. If it would wait on some network IO, it would hang because epoll is broken.