Enhance PEER_DISCOVERY environment variable

It is now tri-state - ON, OFF or SELF.

Fixes #287

docs/ENVIRONMENT.rst

@@ -270,9 +270,15 @@ 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.
+  This environment variable is case-insensitive and defaults to `on`.
+
+  If `on`, ElectrumX will occasionally connect to and verify its
+  network of peer servers.
+
+  If `off`, peer discovery is disabled and a hard-coded default list
+  of servers will be read in and served.  If set to `self` then peer
+  discovery is disabled and the server will only return itself in the
+  peers list.
 
 * **PEER_ANNOUNCE**
 

server/env.py

@@ -23,6 +23,9 @@ NetIdentity = namedtuple('NetIdentity', 'host tcp_port ssl_port nick_suffix')
 class Env(EnvBase):
     '''Wraps environment configuration.'''
 
+    # Peer discovery
+    PD_OFF, PD_SELF, PD_ON = range(3)
+
     def __init__(self):
         super().__init__()
         self.obsolete(['UTXO_MB', 'HIST_MB', 'NETWORK'])
@@ -49,7 +52,7 @@ class Env(EnvBase):
         self.anon_logs = self.boolean('ANON_LOGS', False)
         self.log_sessions = self.integer('LOG_SESSIONS', 3600)
         # Peer discovery
-        self.peer_discovery = self.boolean('PEER_DISCOVERY', True)
+        self.peer_discovery = self.peer_discovery_enum()
         self.peer_announce = self.boolean('PEER_ANNOUNCE', True)
         self.force_proxy = self.boolean('FORCE_PROXY', False)
         self.tor_proxy_host = self.default('TOR_PROXY_HOST', 'localhost')
@@ -143,3 +146,12 @@ class Env(EnvBase):
             ssl_port,
             '_tor',
         )
+
+    def peer_discovery_enum(self):
+        pd = self.default('PEER_DISCOVERY', 'on').strip().lower()
+        if pd in ('off', ''):
+            return self.PD_OFF
+        elif pd == 'self':
+            return self.PD_SELF
+        else:
+            return self.PD_ON

server/peers.py

@@ -424,9 +424,10 @@ class PeerManager(util.LoggedClass):
     def import_peers(self):
         '''Import hard-coded peers from a file or the coin defaults.'''
         self.add_peers(self.myselves)
-        coin_peers = self.env.coin.PEERS
 
-        # Add the hard-coded ones
+        # Add the hard-coded ones unless only returning self
+        if self.env.peer_discovery != self.env.PD_SELF:
+            coin_peers = self.env.coin.PEERS
             peers = [Peer.from_real_name(real_name, 'coins.py')
                      for real_name in coin_peers]
             self.add_peers(peers, limit=None)
@@ -459,7 +460,7 @@ class PeerManager(util.LoggedClass):
           3) Retrying old peers at regular intervals.
         '''
         self.connect_to_irc()
-        if not self.env.peer_discovery:
+        if self.env.peer_discovery != self.env.PD_ON:
             self.logger.info('peer discovery is disabled')
             return
 

tests/server/test_env.py

@@ -199,7 +199,17 @@ def test_ANON_LOGS():
     assert_boolean('ANON_LOGS', 'anon_logs', False)
 
 def test_PEER_DISCOVERY():
-    assert_boolean('PEER_DISCOVERY', 'peer_discovery', True)
+    e = Env()
+    assert e.peer_discovery == Env.PD_ON
+    os.environ['PEER_DISCOVERY'] = ' '
+    e = Env()
+    assert e.peer_discovery == Env.PD_OFF
+    os.environ['PEER_DISCOVERY'] = 'ON'
+    e = Env()
+    assert e.peer_discovery == Env.PD_ON
+    os.environ['PEER_DISCOVERY'] = 'self'
+    e = Env()
+    assert e.peer_discovery == Env.PD_SELF
 
 def test_PEER_ANNOUNCE():
     assert_boolean('PEER_ANNOUNCE', 'peer_announce', True)