net: handle hangup in read on Plan 9

On Plan 9, when closing a TCP connection, we
write the "hangup" string to the TCP ctl file.

The next read on the TCP data file will return
an error like "/net/tcp/18/data: Hangup", while
in Go, we expect to return io.EOF.

This change makes Read to return io.EOF when
an error string containing "Hangup" is returned.

Change-Id: I3f71ed543704190b441cac4787488a77f46d88a1
Reviewed-on: https://go-review.googlesource.com/22149Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/fd_plan9.go

@@ -77,6 +77,9 @@ func (fd *netFD) Read(b []byte) (n int, err error) {
 	}
 	defer fd.readUnlock()
 	n, err = fd.data.Read(b)
+	if isHangup(err) {
+		err = io.EOF
+	}
 	if fd.net == "udp" && err == io.EOF {
 		n = 0
 		err = nil
@@ -179,3 +182,7 @@ func setReadBuffer(fd *netFD, bytes int) error {
 func setWriteBuffer(fd *netFD, bytes int) error {
 	return syscall.EPLAN9
 }
+
+func isHangup(err error) bool {
+	return err != nil && stringsHasSuffix(err.Error(), "Hangup")
+}