Add facility to disable peer discovery

and/or self announcement.

docs/ENVIRONMENT.rst

@@ -215,8 +215,8 @@ raise them.
   functioning Electrum clients by default will send pings roughly
   every 60 seconds.
 
-TOR
----
+PEER DISCOVERY
+--------------
 
 In response to the `server.peers.subscribe` RPC call, ElectrumX will
 only return peer servers that is has recently connected to and
@@ -229,12 +229,36 @@ peers it will fall back to a hard-coded list.
 To give incoming clients a full range of onion servers you will need
 to be running a Tor proxy for ElectrumX to use.
 
+ElectrumX will perform peer-discovery by default and announce itself
+to other peers.  If your server is private you may wish to disable
+some of this.
+
+* **PEER_DISCOVERY**
+
+  If not defined, or non-empty, ElectrumX will occasionally connect to
+  and verify its network of peer servers.  Set to empty to disable
+  peer discovery.
+
+* **PEER_ANNOUNCE**
+
+  Set this environemnt variable to empty to disable announcing itself.
+  If not defined, or non-empty, ElectrumX will announce itself to
+  peers.
+
+  If peer discovery is disabled this environment variable has no
+  effect, because ElectrumX only announces itself to peers when doing
+  peer discovery if it notices it is not present in the peer's
+  returned list.
+
 * **TOR_PROXY_HOST**
 
-  The host where the Tor proxy is running.  Defaults to *127.0.0.1*.
+  The host where your Tor proxy is running.  Defaults to *127.0.0.1*.
   If you use a hostname here rather than an IP address, you must have
   Python version >= 3.5.3, Python 3.5.2 will **not** work.
 
+  If you are not running a Tor proxy just leave this environment
+  variable undefined.
+
 * **TOR_PROXY_PORT**
 
   The port on which the Tor proxy is running.  If not set, ElectrumX

server/env.py

@@ -48,7 +48,9 @@ class Env(LoggedClass):
                                             self.banner_file)
         self.anon_logs = self.default('ANON_LOGS', False)
         self.log_sessions = self.integer('LOG_SESSIONS', 3600)
-        # Tor proxy
+        # Peer discovery
+        self.peer_discovery = bool(self.default('PEER_DISCOVERY', True))
+        self.peer_announce = bool(self.default('PEER_ANNOUNCE', True))
         # Python 3.5.3 - revert back to localhost?
         self.tor_proxy_host = self.default('TOR_PROXY_HOST', '127.0.0.1')
         self.tor_proxy_port = self.integer('TOR_PROXY_PORT', None)

server/peers.py

@@ -112,13 +112,16 @@ class PeerSession(JSONSession):
             return
 
         self.peer_mgr.add_peers(peers)
+
+        if not self.peer_mgr.env.peer_announce:
+            return
+
+        # Announce ourself if not present
         my = self.peer_mgr.myself
         for peer in my.matches(peers):
             if peer.tcp_port == my.tcp_port and peer.ssl_port == my.ssl_port:
                 return
-
-        # Announce ourself if not present
-        self.log_info('registering with server.add_peer')
+        self.log_info('registering ourself with server.add_peer')
         self.send_request(self.on_add_peer, 'server.add_peer', [my.features])
 
     def on_add_peer(self, result, error):
@@ -407,6 +410,11 @@ class PeerManager(util.LoggedClass):
           3) Retrying old peers at regular intervals.
         '''
         self.connect_to_irc()
+        if not self.env.peer_discovery:
+            self.logger.info('peer discovery is disabled')
+            return
+
+        self.logger.info('beginning peer discovery')
         try:
             while True:
                 timeout = self.loop.call_later(WAKEUP_SECS,