net: work around Solaris connect issue when server closes socket

On Solaris, if you do a in-progress connect, and then the
server accepts and closes the socket, the client's later
attempt to complete the connect will fail with EINVAL.  Handle
this case by assuming that the connect succeeded.  This code
is weird enough that it is implemented as Solaris-only so that
it doesn't hide a real error on a different OS.

Update #6828

R=golang-codereviews, bradfitz, dave
CC=golang-codereviews
https://golang.org/cl/46160043

src/pkg/net/fd_unix.go

@@ -80,6 +80,16 @@ func (fd *netFD) connect(la, ra syscall.Sockaddr) error {
 		if err == nil || err == syscall.EISCONN {
 			break
 		}
+
+		// On Solaris we can see EINVAL if the socket has
+		// already been accepted and closed by the server.
+		// Treat this as a successful connection--writes to
+		// the socket will see EOF.  For details and a test
+		// case in C see http://golang.org/issue/6828.
+		if runtime.GOOS == "solaris" && err == syscall.EINVAL {
+			break
+		}
+
 		if err != syscall.EINPROGRESS && err != syscall.EALREADY && err != syscall.EINTR {
 			return err
 		}