Reduce code duplication and fix logging.

This commit reduced duplicated code that creates a socket to any
of the DNS-SRV-based hosts for an XMPP domain by moving that code
to a new utility class.

Additionally, S2S logging was improved: references to 'remote hosts'
where replaced by 'remote domains' to differentiate between servers
and domains (that can consist of multiple servers).

src/java/org/jivesoftware/openfire/net/SocketUtil.java

+package org.jivesoftware.openfire.net;
+
+import org.jivesoftware.openfire.server.RemoteServerManager;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.io.IOException;
+import java.net.InetSocketAddress;
+import java.net.Socket;
+import java.util.List;
+
+/**
+ * Utility class to generate Socket instances.
+ *
+ * @author Guus der Kinderen, guus.der.kinderen@gmail.com
+ */
+public class SocketUtil
+{
+    private final static Logger Log = LoggerFactory.getLogger( SocketUtil.class );
+
+    /**
+     * Creates a socket connection to an XMPP domain.
+     *
+     * This implementation uses DNS SRV records to find a list of remote hosts for the XMPP domain (as implemented by
+     * {@link DNSUtil#resolveXMPPDomain(String, int)}. It then iteratively tries to create a socket connection to each
+     * of them, until one socket connection succeeds.
+     *
+     * Either the connected Socket instance is returned, or null if no connection could be established.
+     *
+     * Note that this method blocks while performing network IO. The timeout as defined by
+     * {@link RemoteServerManager#getSocketTimeout()} is observed.
+     *
+     * @param xmppDomain The XMPP domain to connect to.
+     * @param port The port to connect to when DNS resolution fails.
+     * @return a Socket instance that is connected, or null.
+     * @see DNSUtil#resolveXMPPDomain(String, int)
+     */
+    public static Socket createSocketToXmppDomain( String xmppDomain, int port )
+    {
+        Log.debug( "Creating a socket connection to XMPP domain '{}' ...", xmppDomain );
+
+        Log.debug( "Use DNS to resolve remote hosts for the provided XMPP domain '{}' (default port: {}) ...", xmppDomain, port );
+        final List<DNSUtil.HostAddress> remoteHosts = DNSUtil.resolveXMPPDomain( xmppDomain, port );
+        Log.debug( "Found {} host(s) for XMPP domain '{}'.", remoteHosts.size(), xmppDomain );
+
+        Socket socket = null;
+        final int socketTimeout = RemoteServerManager.getSocketTimeout();
+        for ( DNSUtil.HostAddress remoteHost : remoteHosts )
+        {
+            final String realHostname = remoteHost.getHost();
+            final int realPort = remoteHost.getPort();
+
+            try
+            {
+                // (re)initialize the socket.
+                socket = new Socket();
+
+                Log.debug( "Trying to create socket connection to XMPP domain '{}' using remote host: {}:{} (blocks up to {} ms) ...", xmppDomain, realHostname, realPort, socketTimeout );
+                socket.connect( new InetSocketAddress( realHostname, realPort ), socketTimeout );
+                Log.debug( "Successfully created socket connection to XMPP domain '{}' using remote host: {}:{}!", xmppDomain, realHostname, realPort );
+                return socket;
+            }
+            catch ( Exception e )
+            {
+                Log.debug( "An exception occurred while trying to create a socket connection to XMPP domain '{}' using remote host {}:{}", xmppDomain, realHostname, realPort, e );
+                Log.warn( "Unable to create a socket connection to XMPP domain '{}' using remote host: {}:{}. Cause: {} (a full stacktrace is logged on debug level)", xmppDomain, realHostname, realPort, e.getMessage() );
+                try
+                {
+                    if ( socket != null )
+                    {
+                        socket.close();
+                        socket = null;
+                    }
+                }
+                catch ( IOException ex )
+                {
+                    Log.debug( "An additional exception occurred while trying to close a socket when creating a connection to {}:{} failed.", realHostname, realPort, ex );
+                }
+            }
+        }
+
+        Log.warn( "Unable to create a socket connection to XMPP domain '{}': Unable to connect to any of its remote hosts.", xmppDomain );
+        return null;
+    }
+}