Propagate sshforward send side connection close

PR #3431 caused connections closed on the remote side of a sshforward
session to not always result in the local side reading an EOF from the
connection. This change restores that behavior by closing the write side
of the forwarded connection after reading an EOF from the stream. Since
only the write side is being closed, it doesn't prevent the remote side
from continuing to read from the connection.

Signed-off-by: Aaron Lehmann <[email protected]>
(cherry picked from commit 8c978cf)

session/sshforward/copy.go

@@ -23,7 +23,12 @@ func Copy(ctx context.Context, conn io.ReadWriteCloser, stream Stream, closeStre
 			if err := stream.RecvMsg(p); err != nil {
 				if err == io.EOF {
 					// indicates client performed CloseSend, but they may still be
-					// reading data, so don't close conn yet
+					// reading data
+					if conn, ok := conn.(interface {
+						CloseWrite() error
+					}); ok {
+						conn.CloseWrite()
+					}
 					return nil
 				}
 				conn.Close()