WebHDFSClient calls "conn.disconnect()", which disconnects from the NameNode. When we use webhdfs as the source in distcp, this used up all ephemeral ports on the client side since all closed connections continue to occupy the port with TIME_WAIT status for some time.
According to http://tinyurl.com/java7-http-keepalive, we should call conn.getInputStream().close() instead to make sure the connection is kept alive. This will get rid of the ephemeral port problem.
Manual steps used to verify the bug fix:
1. Build original hadoop jar.
2. Try out distcp from webhdfs as source, and "netstat -n | grep TIME_WAIT | grep -c 50070" on the local machine shows a big number (100s).
3. Build hadoop jar with this diff.
4. Try out distcp from webhdfs as source, and "netstat -n | grep TIME_WAIT | grep -c 50070" on the local machine shows 0.
5. The explanation: distcp's client side does a lot of directory scanning, which would create and close a lot of connections to the namenode HTTP port.