net: use runtime.Keepalive for *netFD values

The net package sets a finalizer on *netFD. I looked through all the
uses of *netFD in the package, looking for each case where a *netFD
was passed as an argument and the final reference to the argument was
not a function or method call. I added a call to runtime.KeepAlive after
each such final reference (there were only three).

The code is safe today without the KeepAlive calls because the compiler
keeps arguments alive for the duration of the function. However, that is
not a language requirement, so adding the KeepAlive calls ensures that
this code remains safe even if the compiler changes in the future.

Change-Id: I4e2bd7c5a946035dc509ccefb4828f72335a9ee3
Reviewed-on: https://go-review.googlesource.com/27650
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/fd_poll_nacl.go

@@ -5,6 +5,7 @@
 package net
 
 import (
+	"runtime"
 	"syscall"
 	"time"
 )
@@ -22,6 +23,7 @@ func (pd *pollDesc) evict() {
 	pd.closing = true
 	if pd.fd != nil {
 		syscall.StopIO(pd.fd.sysfd)
+		runtime.KeepAlive(pd.fd)
 	}
 }
 

src/net/fd_poll_runtime.go

@@ -7,6 +7,7 @@
 package net
 
 import (
+	"runtime"
 	"sync"
 	"syscall"
 	"time"
@@ -33,6 +34,7 @@ var serverInit sync.Once
 func (pd *pollDesc) init(fd *netFD) error {
 	serverInit.Do(runtime_pollServerInit)
 	ctx, errno := runtime_pollOpen(uintptr(fd.sysfd))
+	runtime.KeepAlive(fd)
 	if errno != 0 {
 		return syscall.Errno(errno)
 	}

src/net/fd_windows.go

@@ -541,7 +541,7 @@ func (fd *netFD) acceptOne(rawsa []syscall.RawSockaddrAny, o *operation) (*netFD
 		netfd.Close()
 		return nil, os.NewSyscallError("setsockopt", err)
 	}
-
+	runtime.KeepAlive(fd)
 	return netfd, nil
 }