runtime: add netpollBreak

The new netpollBreak function can be used to interrupt a blocking netpoll.
This function is not currently used; it will be used by later CLs.

Updates #27707

Change-Id: I5cb936609ba13c3c127ea1368a49194fc58c9f4d
Reviewed-on: https://go-review.googlesource.com/c/go/+/171824
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/runtime/defs1_netbsd_386.go

@@ -6,6 +6,7 @@ package runtime
 const (
 	_EINTR  = 0x4
 	_EFAULT = 0xe
+	_EAGAIN = 0x23
 	_ENOSYS = 0x4e
 
 	_O_NONBLOCK = 0x4

src/runtime/defs1_netbsd_amd64.go

@@ -6,6 +6,7 @@ package runtime
 const (
 	_EINTR  = 0x4
 	_EFAULT = 0xe
+	_EAGAIN = 0x23
 	_ENOSYS = 0x4e
 
 	_O_NONBLOCK = 0x4

src/runtime/defs1_netbsd_arm.go

@@ -6,6 +6,7 @@ package runtime
 const (
 	_EINTR  = 0x4
 	_EFAULT = 0xe
+	_EAGAIN = 0x23
 	_ENOSYS = 0x4e
 
 	_O_NONBLOCK = 0x4

src/runtime/defs1_netbsd_arm64.go

@@ -6,6 +6,7 @@ package runtime
 const (
 	_EINTR  = 0x4
 	_EFAULT = 0xe
+	_EAGAIN = 0x23
 	_ENOSYS = 0x4e
 
 	_O_NONBLOCK = 0x4

src/runtime/defs1_solaris_amd64.go

@@ -8,6 +8,7 @@ const (
 	_EBADF       = 0x9
 	_EFAULT      = 0xe
 	_EAGAIN      = 0xb
+	_EBUSY       = 0x10
 	_ETIME       = 0x3e
 	_ETIMEDOUT   = 0x91
 	_EWOULDBLOCK = 0xb
@@ -101,6 +102,8 @@ const (
 	_POLLERR = 0x8
 
 	_PORT_SOURCE_FD    = 0x4
+	_PORT_SOURCE_ALERT = 0x5
+	_PORT_ALERT_UPDATE = 0x2
 )
 
 type semt struct {

src/runtime/defs_darwin.go

@@ -30,6 +30,7 @@ import "C"
 const (
 	EINTR     = C.EINTR
 	EFAULT    = C.EFAULT
+	EAGAIN    = C.EAGAIN
 	ETIMEDOUT = C.ETIMEDOUT
 
 	PROT_NONE  = C.PROT_NONE

src/runtime/defs_darwin_386.go

@@ -8,6 +8,7 @@ import "unsafe"
 const (
 	_EINTR     = 0x4
 	_EFAULT    = 0xe
+	_EAGAIN    = 0x23
 	_ETIMEDOUT = 0x3c
 
 	_PROT_NONE  = 0x0

src/runtime/defs_darwin_amd64.go

@@ -8,6 +8,7 @@ import "unsafe"
 const (
 	_EINTR     = 0x4
 	_EFAULT    = 0xe
+	_EAGAIN    = 0x23
 	_ETIMEDOUT = 0x3c
 
 	_PROT_NONE  = 0x0

src/runtime/defs_darwin_arm.go

@@ -10,6 +10,7 @@ import "unsafe"
 const (
 	_EINTR     = 0x4
 	_EFAULT    = 0xe
+	_EAGAIN    = 0x23
 	_ETIMEDOUT = 0x3c
 
 	_PROT_NONE  = 0x0

src/runtime/defs_darwin_arm64.go

@@ -8,6 +8,7 @@ import "unsafe"
 const (
 	_EINTR     = 0x4
 	_EFAULT    = 0xe
+	_EAGAIN    = 0x23
 	_ETIMEDOUT = 0x3c
 
 	_PROT_NONE  = 0x0

src/runtime/defs_freebsd.go

@@ -47,6 +47,7 @@ const (
 const (
 	EINTR  = C.EINTR
 	EFAULT = C.EFAULT
+	EAGAIN = C.EAGAIN
 	ENOSYS = C.ENOSYS
 
 	O_NONBLOCK = C.O_NONBLOCK

src/runtime/defs_freebsd_386.go

@@ -15,6 +15,7 @@ const (
 const (
 	_EINTR  = 0x4
 	_EFAULT = 0xe
+	_EAGAIN = 0x23
 	_ENOSYS = 0x4e
 
 	_O_NONBLOCK = 0x4

src/runtime/defs_freebsd_amd64.go

@@ -15,6 +15,7 @@ const (
 const (
 	_EINTR  = 0x4
 	_EFAULT = 0xe
+	_EAGAIN = 0x23
 	_ENOSYS = 0x4e
 
 	_O_NONBLOCK = 0x4

src/runtime/defs_freebsd_arm.go

@@ -15,6 +15,7 @@ const (
 const (
 	_EINTR  = 0x4
 	_EFAULT = 0xe
+	_EAGAIN = 0x23
 	_ENOSYS = 0x4e
 
 	_O_NONBLOCK = 0x4

src/runtime/defs_netbsd.go

@@ -32,6 +32,7 @@ import "C"
 const (
 	EINTR  = C.EINTR
 	EFAULT = C.EFAULT
+	EAGAIN = C.EAGAIN
 	ENOSYS = C.ENOSYS
 
 	O_NONBLOCK = C.O_NONBLOCK

src/runtime/defs_openbsd.go

@@ -28,6 +28,7 @@ import "C"
 const (
 	EINTR  = C.EINTR
 	EFAULT = C.EFAULT
+	EAGAIN = C.EAGAIN
 	ENOSYS = C.ENOSYS
 
 	O_NONBLOCK = C.O_NONBLOCK

src/runtime/defs_openbsd_386.go

@@ -8,6 +8,7 @@ import "unsafe"
 const (
 	_EINTR  = 0x4
 	_EFAULT = 0xe
+	_EAGAIN = 0x23
 	_ENOSYS = 0x4e
 
 	_O_NONBLOCK = 0x4

src/runtime/defs_openbsd_amd64.go

@@ -8,6 +8,7 @@ import "unsafe"
 const (
 	_EINTR  = 0x4
 	_EFAULT = 0xe
+	_EAGAIN = 0x23
 	_ENOSYS = 0x4e
 
 	_O_NONBLOCK = 0x4

src/runtime/defs_openbsd_arm.go

@@ -8,6 +8,7 @@ import "unsafe"
 const (
 	_EINTR  = 0x4
 	_EFAULT = 0xe
+	_EAGAIN = 0x23
 	_ENOSYS = 0x4e
 
 	_O_NONBLOCK = 0x4

src/runtime/defs_openbsd_arm64.go

@@ -5,6 +5,7 @@ import "unsafe"
 const (
 	_EINTR  = 0x4
 	_EFAULT = 0xe
+	_EAGAIN = 0x23
 	_ENOSYS = 0x4e
 
 	_O_NONBLOCK = 0x4

src/runtime/defs_solaris.go

@@ -38,6 +38,7 @@ const (
 	EBADF       = C.EBADF
 	EFAULT      = C.EFAULT
 	EAGAIN      = C.EAGAIN
+	EBUSY       = C.EBUSY
 	ETIME       = C.ETIME
 	ETIMEDOUT   = C.ETIMEDOUT
 	EWOULDBLOCK = C.EWOULDBLOCK
@@ -130,6 +131,8 @@ const (
 	POLLERR = C.POLLERR
 
 	PORT_SOURCE_FD    = C.PORT_SOURCE_FD
+	PORT_SOURCE_ALERT = C.PORT_SOURCE_ALERT
+	PORT_ALERT_UPDATE = C.PORT_ALERT_UPDATE
 )
 
 type SemT C.sem_t

src/runtime/export_test.go

@@ -35,6 +35,9 @@ var Atoi = atoi
 var Atoi32 = atoi32
 
 var Nanotime = nanotime
+var Netpoll = netpoll
+var NetpollBreak = netpollBreak
+var Usleep = usleep
 
 var PhysHugePageSize = physHugePageSize
 

src/runtime/netpoll.go

@@ -12,12 +12,26 @@ import (
 )
 
 // Integrated network poller (platform-independent part).
-// A particular implementation (epoll/kqueue) must define the following functions:
-// func netpollinit()			// to initialize the poller
-// func netpollopen(fd uintptr, pd *pollDesc) int32	// to arm edge-triggered notifications
-// and associate fd with pd.
-// An implementation must call the following function to denote that the pd is ready.
-// func netpollready(gpp **g, pd *pollDesc, mode int32)
+// A particular implementation (epoll/kqueue/port/AIX/Windows)
+// must define the following functions:
+//
+// func netpollinit()
+//     Initialize the poller. Only called once.
+//
+// func netpollopen(fd uintptr, pd *pollDesc) int32
+//     Arm edge-triggered notifications for fd. The pd argument is to pass
+//     back to netpollready when fd is ready. Return an errno value.
+//
+// func netpoll(delta int64) gList
+//     Poll the network. If delta < 0, block indefinitely. If delta == 0,
+//     poll without blocking. If delta > 0, block for up to delta nanoseconds.
+//     Return a list of goroutines built by calling netpollready.
+//
+// func netpollBreak()
+//     Wake up the network poller, assumed to be blocked in netpoll.
+//
+// func netpollIsPollDescriptor(fd uintptr) bool
+//     Reports whether fd is a file descriptor used by the poller.
 
 // pollDesc contains 2 binary semaphores, rg and wg, to park reader and writer
 // goroutines respectively. The semaphore can be in the following states:
@@ -99,14 +113,7 @@ func netpollinited() bool {
 // poll_runtime_isPollServerDescriptor reports whether fd is a
 // descriptor being used by netpoll.
 func poll_runtime_isPollServerDescriptor(fd uintptr) bool {
-	fds := netpolldescriptor()
-	if GOOS != "aix" {
-		return fd == fds
-	} else {
-		// AIX have a pipe in its netpoll implementation.
-		// Therefore, two fd are returned by netpolldescriptor using a mask.
-		return fd == fds&0xFFFF || fd == (fds>>16)&0xFFFF
-	}
+	return netpollIsPollDescriptor(fd)
 }
 
 //go:linkname poll_runtime_pollOpen internal/poll.runtime_pollOpen
@@ -316,8 +323,13 @@ func poll_runtime_pollUnblock(pd *pollDesc) {
 	}
 }
 
-// make pd ready, newly runnable goroutines (if any) are added to toRun.
-// May run during STW, so write barriers are not allowed.
+// netpollready is called by the platform-specific netpoll function.
+// It declares that the fd associated with pd is ready for I/O.
+// The toRun argument is used to build a list of goroutines to return
+// from netpoll. The mode argument is 'r', 'w', or 'r'+'w' to indicate
+// whether the fd is ready for reading or writing or both.
+//
+// This may run while the world is stopped, so write barriers are not allowed.
 //go:nowritebarrier
 func netpollready(toRun *gList, pd *pollDesc, mode int32) {
 	var rg, wg *g

src/runtime/netpoll_aix.go

@@ -63,12 +63,8 @@ func netpollinit() {
 	pds[0] = nil
 }
 
-func netpolldescriptor() uintptr {
-	// Both fd must be returned
-	if rdwake > 0xFFFF || wrwake > 0xFFFF {
-		throw("netpolldescriptor: invalid fd number")
-	}
-	return uintptr(rdwake<<16 | wrwake)
+func netpollIsPollDescriptor(fd uintptr) bool {
+	return fd == uintptr(rdwake) || fd == uintptr(wrwake)
 }
 
 // netpollwakeup writes on wrwake to wakeup poll before any changes.
@@ -132,6 +128,11 @@ func netpollarm(pd *pollDesc, mode int) {
 	unlock(&mtxset)
 }
 
+// netpollBreak interrupts an epollwait.
+func netpollBreak() {
+	netpollwakeup()
+}
+
 // netpoll checks for ready network connections.
 // Returns list of goroutines that become runnable.
 // delay < 0: blocks indefinitely
@@ -176,9 +177,14 @@ retry:
 	}
 	// Check if some descriptors need to be changed
 	if n != 0 && pfds[0].revents&(_POLLIN|_POLLHUP|_POLLERR) != 0 {
+		if delay != 0 {
+			// A netpollwakeup could be picked up by a
+			// non-blocking poll. Only clear the wakeup
+			// if blocking.
 			var b [1]byte
 			for read(rdwake, unsafe.Pointer(&b[0]), 1) == 1 {
 			}
+		}
 		// Do not look at the other fds in this case as the mode may have changed
 		// XXX only additions of flags are made, so maybe it is ok
 		unlock(&mtxset)

src/runtime/netpoll_epoll.go

@@ -20,24 +20,40 @@ func closeonexec(fd int32)
 
 var (
 	epfd int32 = -1 // epoll descriptor
+
+	netpollBreakRd, netpollBreakWr uintptr // for netpollBreak
 )
 
 func netpollinit() {
 	epfd = epollcreate1(_EPOLL_CLOEXEC)
-	if epfd >= 0 {
-		return
-	}
+	if epfd < 0 {
 		epfd = epollcreate(1024)
-	if epfd >= 0 {
-		closeonexec(epfd)
-		return
-	}
+		if epfd < 0 {
 			println("runtime: epollcreate failed with", -epfd)
 			throw("runtime: netpollinit failed")
+		}
+		closeonexec(epfd)
+	}
+	r, w, errno := nonblockingPipe()
+	if errno != 0 {
+		println("runtime: pipe failed with", -errno)
+		throw("runtime: pipe failed")
+	}
+	ev := epollevent{
+		events: _EPOLLIN,
+	}
+	*(**uintptr)(unsafe.Pointer(&ev.data)) = &netpollBreakRd
+	errno = epollctl(epfd, _EPOLL_CTL_ADD, r, &ev)
+	if errno != 0 {
+		println("runtime: epollctl failed with", -errno)
+		throw("runtime: epollctl failed")
+	}
+	netpollBreakRd = uintptr(r)
+	netpollBreakWr = uintptr(w)
 }
 
-func netpolldescriptor() uintptr {
-	return uintptr(epfd)
+func netpollIsPollDescriptor(fd uintptr) bool {
+	return fd == uintptr(epfd) || fd == netpollBreakRd || fd == netpollBreakWr
 }
 
 func netpollopen(fd uintptr, pd *pollDesc) int32 {
@@ -56,6 +72,25 @@ func netpollarm(pd *pollDesc, mode int) {
 	throw("runtime: unused")
 }
 
+// netpollBreak interrupts an epollwait.
+func netpollBreak() {
+	for {
+		var b byte
+		n := write(netpollBreakWr, unsafe.Pointer(&b), 1)
+		if n == 1 {
+			break
+		}
+		if n == -_EINTR {
+			continue
+		}
+		if n == -_EAGAIN {
+			return
+		}
+		println("runtime: netpollBreak write failed with", -n)
+		throw("runtime: netpollBreak write failed")
+	}
+}
+
 // netpoll checks for ready network connections.
 // Returns list of goroutines that become runnable.
 // delay < 0: blocks indefinitely
@@ -100,6 +135,22 @@ retry:
 		if ev.events == 0 {
 			continue
 		}
+
+		if *(**uintptr)(unsafe.Pointer(&ev.data)) == &netpollBreakRd {
+			if ev.events != _EPOLLIN {
+				println("runtime: netpoll: break fd ready for", ev.events)
+				throw("runtime: netpoll: break fd ready for something unexpected")
+			}
+			if delay != 0 {
+				// netpollBreak could be picked up by a
+				// nonblocking poll. Only read the byte
+				// if blocking.
+				var tmp [16]byte
+				read(int32(netpollBreakRd), noescape(unsafe.Pointer(&tmp[0])), int32(len(tmp)))
+			}
+			continue
+		}
+
 		var mode int32
 		if ev.events&(_EPOLLIN|_EPOLLRDHUP|_EPOLLHUP|_EPOLLERR) != 0 {
 			mode += 'r'

src/runtime/netpoll_fake.go

@@ -12,8 +12,8 @@ package runtime
 func netpollinit() {
 }
 
-func netpolldescriptor() uintptr {
-	return ^uintptr(0)
+func netpollIsPollDescriptor(fd uintptr) bool {
+	return false
 }
 
 func netpollopen(fd uintptr, pd *pollDesc) int32 {
@@ -27,6 +27,9 @@ func netpollclose(fd uintptr) int32 {
 func netpollarm(pd *pollDesc, mode int) {
 }
 
+func netpollBreak() {
+}
+
 func netpoll(delay int64) gList {
 	return gList{}
 }

src/runtime/netpoll_kqueue.go

@@ -12,6 +12,8 @@ import "unsafe"
 
 var (
 	kq int32 = -1
+
+	netpollBreakRd, netpollBreakWr uintptr // for netpollBreak
 )
 
 func netpollinit() {
@@ -21,10 +23,27 @@ func netpollinit() {
 		throw("runtime: netpollinit failed")
 	}
 	closeonexec(kq)
+	r, w, errno := nonblockingPipe()
+	if errno != 0 {
+		println("runtime: pipe failed with", -errno)
+		throw("runtime: pipe failed")
+	}
+	ev := keventt{
+		filter: _EVFILT_READ,
+		flags:  _EV_ADD,
+	}
+	*(*uintptr)(unsafe.Pointer(&ev.ident)) = uintptr(r)
+	n := kevent(kq, &ev, 1, nil, 0, nil)
+	if n < 0 {
+		println("runtime: kevent failed with", -errno)
+		throw("runtime: kevent failed")
+	}
+	netpollBreakRd = uintptr(r)
+	netpollBreakWr = uintptr(w)
 }
 
-func netpolldescriptor() uintptr {
-	return uintptr(kq)
+func netpollIsPollDescriptor(fd uintptr) bool {
+	return fd == uintptr(kq) || fd == netpollBreakRd || fd == netpollBreakWr
 }
 
 func netpollopen(fd uintptr, pd *pollDesc) int32 {
@@ -57,6 +76,22 @@ func netpollarm(pd *pollDesc, mode int) {
 	throw("runtime: unused")
 }
 
+// netpollBreak interrupts an epollwait.
+func netpollBreak() {
+	for {
+		var b byte
+		n := write(netpollBreakWr, unsafe.Pointer(&b), 1)
+		if n == 1 || n == -_EAGAIN {
+			break
+		}
+		if n == -_EINTR {
+			continue
+		}
+		println("runtime: netpollBreak write failed with", -n)
+		throw("runtime: netpollBreak write failed")
+	}
+}
+
 // netpoll checks for ready network connections.
 // Returns list of goroutines that become runnable.
 // delay < 0: blocks indefinitely
@@ -98,6 +133,22 @@ retry:
 	var toRun gList
 	for i := 0; i < int(n); i++ {
 		ev := &events[i]
+
+		if uintptr(ev.ident) == netpollBreakRd {
+			if ev.filter != _EVFILT_READ {
+				println("runtime: netpoll: break fd ready for", ev.filter)
+				throw("runtime: netpoll: break fd ready for something unexpected")
+			}
+			if delay != 0 {
+				// netpollBreak could be picked up by a
+				// nonblocking poll. Only read the byte
+				// if blocking.
+				var tmp [16]byte
+				read(int32(netpollBreakRd), noescape(unsafe.Pointer(&tmp[0])), int32(len(tmp)))
+			}
+			continue
+		}
+
 		var mode int32
 		switch ev.filter {
 		case _EVFILT_READ:

src/runtime/netpoll_solaris.go

@@ -71,17 +71,20 @@ import "unsafe"
 //go:cgo_import_dynamic libc_port_associate port_associate "libc.so"
 //go:cgo_import_dynamic libc_port_dissociate port_dissociate "libc.so"
 //go:cgo_import_dynamic libc_port_getn port_getn "libc.so"
+//go:cgo_import_dynamic libc_port_alert port_alert "libc.so"
 
 //go:linkname libc_port_create libc_port_create
 //go:linkname libc_port_associate libc_port_associate
 //go:linkname libc_port_dissociate libc_port_dissociate
 //go:linkname libc_port_getn libc_port_getn
+//go:linkname libc_port_alert libc_port_alert
 
 var (
 	libc_port_create,
 	libc_port_associate,
 	libc_port_dissociate,
-	libc_port_getn libcFunc
+	libc_port_getn,
+	libc_port_alert libcFunc
 )
 
 func errno() int32 {
@@ -108,6 +111,10 @@ func port_getn(port int32, evs *portevent, max uint32, nget *uint32, timeout *ti
 	return int32(sysvicall5(&libc_port_getn, uintptr(port), uintptr(unsafe.Pointer(evs)), uintptr(max), uintptr(unsafe.Pointer(nget)), uintptr(unsafe.Pointer(timeout))))
 }
 
+func port_alert(port int32, flags, events uint32, user uintptr) int32 {
+	return int32(sysvicall4(&libc_port_alert, uintptr(port), uintptr(flags), uintptr(events), user))
+}
+
 var portfd int32 = -1
 
 func netpollinit() {
@@ -121,8 +128,8 @@ func netpollinit() {
 	throw("runtime: netpollinit failed")
 }
 
-func netpolldescriptor() uintptr {
-	return uintptr(portfd)
+func netpollIsPollDescriptor(fd uintptr) bool {
+	return fd == uintptr(portfd)
 }
 
 func netpollopen(fd uintptr, pd *pollDesc) int32 {
@@ -178,6 +185,21 @@ func netpollarm(pd *pollDesc, mode int) {
 	unlock(&pd.lock)
 }
 
+// netpollBreak interrupts a port_getn wait.
+func netpollBreak() {
+	// Use port_alert to put portfd into alert mode.
+	// This will wake up all threads sleeping in port_getn on portfd,
+	// and cause their calls to port_getn to return immediately.
+	// Further, until portfd is taken out of alert mode,
+	// all calls to port_getn will return immediately.
+	if port_alert(portfd, _PORT_ALERT_UPDATE, _POLLHUP, uintptr(unsafe.Pointer(&portfd))) < 0 {
+		if e := errno(); e != _EBUSY {
+			println("runtime: port_alert failed with", e)
+			throw("runtime: netpoll: port_alert failed")
+		}
+	}
+}
+
 // netpoll checks for ready network connections.
 // Returns list of goroutines that become runnable.
 // delay < 0: blocks indefinitely
@@ -224,6 +246,24 @@ retry:
 	for i := 0; i < int(n); i++ {
 		ev := &events[i]
 
+		if ev.portev_source == _PORT_SOURCE_ALERT {
+			if ev.portev_events != _POLLHUP || unsafe.Pointer(ev.portev_user) != unsafe.Pointer(&portfd) {
+				throw("runtime: netpoll: bad port_alert wakeup")
+			}
+			if delay != 0 {
+				// Now that a blocking call to netpoll
+				// has seen the alert, take portfd
+				// back out of alert mode.
+				// See the comment in netpollBreak.
+				if port_alert(portfd, 0, 0, 0) < 0 {
+					e := errno()
+					println("runtime: port_alert failed with", e)
+					throw("runtime: netpoll: port_alert failed")
+				}
+			}
+			continue
+		}
+
 		if ev.portev_events == 0 {
 			continue
 		}

src/runtime/netpoll_stub.go

@@ -6,13 +6,30 @@
 
 package runtime
 
+import "runtime/internal/atomic"
+
 var netpollWaiters uint32
 
+var netpollStubLock mutex
+var netpollNote note
+var netpollBroken uint32
+
+func netpollBreak() {
+	if atomic.Cas(&netpollBroken, 0, 1) {
+		notewakeup(&netpollNote)
+	}
+}
+
 // Polls for ready network connections.
 // Returns list of goroutines that become runnable.
 func netpoll(delay int64) gList {
 	// Implementation for platforms that do not support
 	// integrated network poller.
+	if delay != 0 {
+		noteclear(&netpollNote)
+		atomic.Store(&netpollBroken, 0)
+		notetsleep(&netpollNote, delay)
+	}
 	return gList{}
 }
 

src/runtime/netpoll_windows.go

@@ -41,8 +41,8 @@ func netpollinit() {
 	}
 }
 
-func netpolldescriptor() uintptr {
-	return iocphandle
+func netpollIsPollDescriptor(fd uintptr) bool {
+	return fd == iocphandle
 }
 
 func netpollopen(fd uintptr, pd *pollDesc) int32 {
@@ -61,6 +61,13 @@ func netpollarm(pd *pollDesc, mode int) {
 	throw("runtime: unused")
 }
 
+func netpollBreak() {
+	if stdcall4(_PostQueuedCompletionStatus, iocphandle, 0, 0, 0) == 0 {
+		println("runtime: netpoll: PostQueuedCompletionStatus failed (errno=", getlasterror(), ")")
+		throw("runtime: netpoll: PostQueuedCompletionStatus failed")
+	}
+}
+
 // netpoll checks for ready network connections.
 // Returns list of goroutines that become runnable.
 // delay < 0: blocks indefinitely
@@ -112,12 +119,20 @@ func netpoll(delay int64) gList {
 		mp.blocked = false
 		for i = 0; i < n; i++ {
 			op = entries[i].op
+			if op != nil {
 				errno = 0
 				qty = 0
 				if stdcall5(_WSAGetOverlappedResult, op.pd.fd, uintptr(unsafe.Pointer(op)), uintptr(unsafe.Pointer(&qty)), 0, uintptr(unsafe.Pointer(&flags))) == 0 {
 					errno = int32(getlasterror())
 				}
 				handlecompletion(&toRun, op, errno, qty)
+			} else {
+				if delay == 0 {
+					// Forward the notification to the
+					// blocked poller.
+					netpollBreak()
+				}
+			}
 		}
 	} else {
 		op = nil
@@ -139,6 +154,14 @@ func netpoll(delay int64) gList {
 			// dequeued failed IO packet, so report that
 		}
 		mp.blocked = false
+		if op == nil {
+			if delay == 0 {
+				// Forward the notification to the
+				// blocked poller.
+				netpollBreak()
+			}
+			return gList{}
+		}
 		handlecompletion(&toRun, op, errno, qty)
 	}
 	return toRun

src/runtime/os_netbsd.go

@@ -24,8 +24,6 @@ const (
 
 	// From <sys/lwp.h>
 	_LWP_DETACHED = 0x00000040
-
-	_EAGAIN = 35
 )
 
 type mOS struct {

src/runtime/os_openbsd.go

@@ -65,7 +65,6 @@ func setNonblock(fd int32)
 
 const (
 	_ESRCH       = 3
-	_EAGAIN      = 35
 	_EWOULDBLOCK = _EAGAIN
 	_ENOTSUP     = 91
 

src/runtime/os_windows.go

@@ -34,6 +34,7 @@ const (
 //go:cgo_import_dynamic runtime._GetThreadContext GetThreadContext%2 "kernel32.dll"
 //go:cgo_import_dynamic runtime._LoadLibraryW LoadLibraryW%1 "kernel32.dll"
 //go:cgo_import_dynamic runtime._LoadLibraryA LoadLibraryA%1 "kernel32.dll"
+//go:cgo_import_dynamic runtime._PostQueuedCompletionStatus PostQueuedCompletionStatus%4 "kernel32.dll"
 //go:cgo_import_dynamic runtime._ResumeThread ResumeThread%1 "kernel32.dll"
 //go:cgo_import_dynamic runtime._SetConsoleCtrlHandler SetConsoleCtrlHandler%2 "kernel32.dll"
 //go:cgo_import_dynamic runtime._SetErrorMode SetErrorMode%1 "kernel32.dll"
@@ -80,6 +81,7 @@ var (
 	_GetThreadContext,
 	_LoadLibraryW,
 	_LoadLibraryA,
+	_PostQueuedCompletionStatus,
 	_QueryPerformanceCounter,
 	_QueryPerformanceFrequency,
 	_ResumeThread,

src/runtime/proc_test.go

@@ -981,3 +981,42 @@ func TestPreemptionAfterSyscall(t *testing.T) {
 func TestGetgThreadSwitch(t *testing.T) {
 	runtime.RunGetgThreadSwitchTest()
 }
+
+// TestNetpollBreak tests that netpollBreak can break a netpoll.
+// This test is not particularly safe since the call to netpoll
+// will pick up any stray files that are ready, but it should work
+// OK as long it is not run in parallel.
+func TestNetpollBreak(t *testing.T) {
+	if runtime.GOMAXPROCS(0) == 1 {
+		t.Skip("skipping: GOMAXPROCS=1")
+	}
+
+	// Make sure that netpoll is initialized.
+	time.Sleep(1)
+
+	start := time.Now()
+	c := make(chan bool, 2)
+	go func() {
+		c <- true
+		runtime.Netpoll(10 * time.Second.Nanoseconds())
+		c <- true
+	}()
+	<-c
+	// Loop because the break might get eaten by the scheduler.
+	// Break twice to break both the netpoll we started and the
+	// scheduler netpoll.
+loop:
+	for {
+		runtime.Usleep(100)
+		runtime.NetpollBreak()
+		runtime.NetpollBreak()
+		select {
+		case <-c:
+			break loop
+		default:
+		}
+	}
+	if dur := time.Since(start); dur > 5*time.Second {
+		t.Errorf("netpollBreak did not interrupt netpoll: slept for: %v", dur)
+	}
+}