error if tcp=tor is requested but tor is unimportable

This only catches txtorcon not being installed (which should be fixed by
doing `pip install tahoe-lafs[tor]`). It doesn't notice that the Tor
daemon is not running (which we can't detect during startup, only
afterwards, when it's harder to notify the user), in which case Tor
connections (and all connections when "tcp = tor" is enabled) will just
fail silently.

src/allmydata/node.py

@@ -14,6 +14,21 @@ from allmydata.util.fileutil import abspath_expanduser_unicode
 from allmydata.util.encodingutil import get_filesystem_encoding, quote_output
 from allmydata.util import configutil
 
+def _import_tor():
+    # this exists to be overridden by unit tests
+    try:
+        from foolscap.connections import tor
+        return tor
+    except ImportError: # pragma: no cover
+        return None
+
+def _import_i2p():
+    try:
+        from foolscap.connections import i2p
+        return i2p
+    except ImportError: # pragma: no cover
+        return None
+
 # Add our application versions to the data that Foolscap's LogPublisher
 # reports.
 for thing, things_version in get_package_versions().iteritems():
@@ -175,9 +190,8 @@ class Node(service.MultiService):
         enabled = self.get_config("tor", "enable", True, boolean=True)
         if not enabled:
             return None
-        try:
-            from foolscap.connections import tor
-        except ImportError:
+        tor = _import_tor()
+        if not tor:
             return None
 
         if self.get_config("tor", "launch", False, boolean=True):
@@ -215,9 +229,8 @@ class Node(service.MultiService):
         enabled = self.get_config("i2p", "enable", True, boolean=True)
         if not enabled:
             return None
-        try:
-            from foolscap.connections import i2p
-        except ImportError:
+        i2p = _import_i2p()
+        if not i2p:
             return None
 
         samport = self.get_config("i2p", "sam.port", None)
@@ -259,6 +272,11 @@ class Node(service.MultiService):
             raise ValueError("'tahoe.cfg [connections] tcp='"
                              " uses unknown handler type '%s'"
                              % tcp_handler_name)
+        if not handlers[tcp_handler_name]:
+            raise ValueError("'tahoe.cfg [connections] tcp=' uses "
+                             "unavailable/unimportable handler type '%s'. "
+                             "Please pip install tahoe-lafs[%s] to fix."
+                             % (tcp_handler_name, tcp_handler_name))
         self._default_connection_handlers["tcp"] = tcp_handler_name
 
     def set_tub_options(self):

src/allmydata/test/test_connections.py

@@ -29,6 +29,12 @@ class Tor(unittest.TestCase):
         h = n._make_tor_handler()
         self.assertEqual(h, None)
 
+    def test_unimportable(self):
+        n = FakeNode(BASECONFIG)
+        with mock.patch("allmydata.node._import_tor", return_value=None):
+            h = n._make_tor_handler()
+        self.assertEqual(h, None)
+
     def test_default(self):
         n = FakeNode(BASECONFIG)
         h1 = mock.Mock()
@@ -109,6 +115,12 @@ class I2P(unittest.TestCase):
         h = n._make_i2p_handler()
         self.assertEqual(h, None)
 
+    def test_unimportable(self):
+        n = FakeNode(BASECONFIG)
+        with mock.patch("allmydata.node._import_i2p", return_value=None):
+            h = n._make_i2p_handler()
+        self.assertEqual(h, None)
+
     def test_default(self):
         n = FakeNode(BASECONFIG)
         h1 = mock.Mock()
@@ -204,6 +216,15 @@ class Connections(unittest.TestCase):
         self.assertEqual(n._default_connection_handlers["tor"], "tor")
         self.assertEqual(n._default_connection_handlers["i2p"], "i2p")
 
+    def test_tor_unimportable(self):
+        n = FakeNode(BASECONFIG+"[connections]\ntcp = tor\n")
+        with mock.patch("allmydata.node._import_tor", return_value=None):
+            e = self.assertRaises(ValueError, n.init_connections)
+        self.assertEqual(str(e),
+                         "'tahoe.cfg [connections] tcp='"
+                         " uses unavailable/unimportable handler type 'tor'."
+                         " Please pip install tahoe-lafs[tor] to fix.")
+
     def test_unknown(self):
         n = FakeNode(BASECONFIG+"[connections]\ntcp = unknown\n")
         e = self.assertRaises(ValueError, n.init_connections)