runtime: on unexpected netpoll error, throw instead of looping

The current code prints an error message and then tries to carry on.
This is not helpful for Go users: they see a message that means
nothing and that they can do nothing about.  In the only known case of
this message, in issue 11498, the best guess is that the netpoll code
went into an infinite loop.  Instead of doing that, crash the program.

Fixes #11498.

Change-Id: Idda3456c5b708f0df6a6b56c5bb4e796bbc39d7c
Reviewed-on: https://go-review.googlesource.com/12047
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

src/runtime/netpoll_epoll.go

@@ -19,8 +19,7 @@ func epollwait(epfd int32, ev *epollevent, nev, timeout int32) int32
 func closeonexec(fd int32)
 
 var (
-	epfd           int32 = -1 // epoll descriptor
-	netpolllasterr int32
+	epfd int32 = -1 // epoll descriptor
 )
 
 func netpollinit() {
@@ -67,9 +66,9 @@ func netpoll(block bool) *g {
 retry:
 	n := epollwait(epfd, &events[0], int32(len(events)), waitms)
 	if n < 0 {
-		if n != -_EINTR && n != netpolllasterr {
-			netpolllasterr = n
+		if n != -_EINTR {
 			println("runtime: epollwait on fd", epfd, "failed with", -n)
+			throw("epollwait failed")
 		}
 		goto retry
 	}

src/runtime/netpoll_kqueue.go

@@ -17,8 +17,7 @@ func kevent(kq int32, ch *keventt, nch int32, ev *keventt, nev int32, ts *timesp
 func closeonexec(fd int32)
 
 var (
-	kq             int32 = -1
-	netpolllasterr int32
+	kq int32 = -1
 )
 
 func netpollinit() {
@@ -75,9 +74,9 @@ func netpoll(block bool) *g {
 retry:
 	n := kevent(kq, nil, 0, &events[0], int32(len(events)), tp)
 	if n < 0 {
-		if n != -_EINTR && n != netpolllasterr {
-			netpolllasterr = n
+		if n != -_EINTR {
 			println("runtime: kevent on fd", kq, "failed with", -n)
+			throw("kevent failed")
 		}
 		goto retry
 	}

src/runtime/netpoll_solaris.go

@@ -174,9 +174,6 @@ func netpollarm(pd *pollDesc, mode int) {
 	unlock(&pd.lock)
 }
 
-// netpolllasterr holds the last error code returned by port_getn to prevent log spamming
-var netpolllasterr int32
-
 // polls for ready network connections
 // returns list of goroutines that become runnable
 func netpoll(block bool) *g {
@@ -194,9 +191,9 @@ func netpoll(block bool) *g {
 retry:
 	var n uint32 = 1
 	if port_getn(portfd, &events[0], uint32(len(events)), &n, wait) < 0 {
-		if e := errno(); e != _EINTR && e != netpolllasterr {
-			netpolllasterr = e
+		if e := errno(); e != _EINTR {
 			print("runtime: port_getn on fd ", portfd, " failed with ", e, "\n")
+			throw("port_getn failed")
 		}
 		goto retry
 	}