Let SessionManager.get require Request object; Check IP addr.

So far there is no use case in which get needs to be called with
something other than a Request, and I don't think there will be.
So let's make that part of the design and we can also take the
opportunity to check IP.

frontends/etiquette_flask/etiquette_flask/sessions.py

@@ -7,6 +7,7 @@ import werkzeug.wrappers
 import etiquette
 
 SESSION_MAX_AGE = 86400
+REQUEST_TYPES = (flask.Request, werkzeug.wrappers.Request, werkzeug.local.LocalProxy)
 
 def _generate_token(length=32):
     randbytes = os.urandom(math.ceil(length / 2))
@@ -15,10 +16,13 @@ def _generate_token(length=32):
     return token
 
 def _normalize_token(token):
-    if isinstance(token, (flask.Request, werkzeug.wrappers.Request, werkzeug.local.LocalProxy)):
+    if isinstance(token, REQUEST_TYPES):
         request = token
         token = request.cookies.get('etiquette_session', None)
         if token is None:
+            # During normal usage, this does not occur because give_token is
+            # applied *before* the request handler even sees the request.
+            # Just a precaution.
             message = 'Cannot normalize token for request with no etiquette_session header.'
             raise TypeError(message, request)
     elif isinstance(token, str):
@@ -35,10 +39,15 @@ class SessionManager:
     def add(self, session):
         self.sessions[session.token] = session
 
-    def get(self, token):
-        token = _normalize_token(token)
+    def get(self, request):
+        token = _normalize_token(request)
         session = self.sessions[token]
-        if session.expired():
+        invalid = (
+            request.remote_addr != session.ip_address or
+            session.expired()
+        )
+        if invalid:
+            self.remove(token)
             raise KeyError(token)
         return session
 
@@ -59,7 +68,7 @@ class SessionManager:
                 request.cookies['etiquette_session'] = token
 
             try:
-                session = self.get(token)
+                session = self.get(request)
             except KeyError:
                 session = Session(request, user=None)
                 self.add(session)
@@ -85,8 +94,11 @@ class SessionManager:
 
     def remove(self, token):
         token = _normalize_token(token)
-        if token in self.sessions:
+        try:
             self.sessions.pop(token)
+        except KeyError:
+            pass
+
 
 class Session:
     def __init__(self, request, user):