Add daemon_url RPC call and document it.

Closes #111

docs/RPC-INTERFACE.rst

@@ -155,6 +155,19 @@ The following commands are available:
 
   Currently this is data gleaned from an IRC session.
 
+* **daemon_url**
+
+  This command takes an option argument that is interpreted
+  identically to the **DAEMON_URL** environment variable.  If default
+  value of the argument is the **DAEMON_URL** environment variable.
+
+  The command replaces the daemon's URL at run-time, and rotates to the
+  first in the list.
+
+  For example, in case ElectrumX has rotated to a secondary daemon and
+  you want to revert to the first after fixing the issue, call this
+  command without an argument.
+
 * **reorg**
 
   Force a block chain reorg.  This command takes an optional

server/controller.py

@@ -78,8 +78,8 @@ class Controller(util.LoggedClass):
         env.max_send = max(350000, env.max_send)
         self.setup_bands()
         # Set up the RPC request handlers
-        cmds = ('disconnect getinfo groups log peers reorg sessions stop'
-                .split())
+        cmds = ('daemon_url disconnect getinfo groups log peers reorg '
+                'sessions stop'.split())
         self.rpc_handlers = {cmd: getattr(self, 'rpc_' + cmd) for cmd in cmds}
         # Set up the ElectrumX request handlers
         rpcs = [
@@ -592,6 +592,15 @@ class Controller(util.LoggedClass):
         '''
         return self.for_each_session(session_ids, self.toggle_logging)
 
+    def rpc_daemon_url(self, daemon_url=None):
+        '''Replace the daemon URL.'''
+        daemon_url = daemon_url or self.env.daemon_url
+        try:
+            self.daemon.set_urls(self.env.coin.daemon_urls(daemon_url))
+        except Exception as e:
+            raise RPCError('an error occured: {}'.format(e))
+        return 'set daemon URL to {}'.format(daemon_url)
+
     def rpc_stop(self):
         '''Shut down the server cleanly.'''
         self.initiate_shutdown()

server/daemon.py

@@ -30,12 +30,7 @@ class Daemon(util.LoggedClass):
 
     def __init__(self, urls):
         super().__init__()
-        if not urls:
-            raise DaemonError('no daemon URLs provided')
-        for url in urls:
-            self.logger.info('daemon at {}'.format(self.logged_url(url)))
-        self.urls = urls
-        self.url_index = 0
+        self.set_urls(urls)
         self._height = None
         self._mempool_hashes = set()
         self.mempool_refresh_event = asyncio.Event()
@@ -43,6 +38,15 @@ class Daemon(util.LoggedClass):
         # See DEFAULT_HTTP_WORKQUEUE in bitcoind, which is typically 16
         self.workqueue_semaphore = asyncio.Semaphore(value=10)
 
+    def set_urls(self, urls):
+        '''Set the URLS to the given list, and switch to the first one.'''
+        if not urls:
+            raise DaemonError('no daemon URLs provided')
+        for url in urls:
+            self.logger.info('daemon at {}'.format(self.logged_url(url)))
+        self.urls = urls
+        self.url_index = 0
+
     async def _send(self, payload, processor):
         '''Send a payload to be converted to JSON.