implement pair request broadcasts

Pair requests are sent on all network interfaces, and contain the best
available hostname to use to contact the host on that interface.

Added a pairing in progress page.

Revert "reduce some boilerplate using ghc extensions", because it caused
overlapping instances for Text.

Assistant/Pairing.hs

@@ -7,17 +7,24 @@
 
 module Assistant.Pairing where
 
-import Assistant.Common
+import Common
 import Utility.Verifiable
+import Utility.ThreadScheduler
+import Utility.Network
 
-import Network.Socket (HostName)
+import Network.Multicast
+import Network.Info
+import Network.Socket
+import Control.Concurrent
+import qualified Data.Map as M
 
-{- "I'd like to pair with somebody who knows a secret." -}
+{- "I'll pair with anybody who shares the secret that can be used to verify
+ - this request." -}
 data PairReq = PairReq (Verifiable PairData)
 	deriving (Eq, Read, Show)
 
-{- "I've checked your PairReq, and like it.
- - I set up your ssh key already. Here's mine for you to set up." -}
+{- "I've verified your request, and you can verify mine to see that I know
+ - the secret. I set up your ssh key already. Here's mine for you to set up." -}
 data PairAck = PairAck (Verifiable PairData)
 	deriving (Eq, Read, Show)
 
@@ -35,9 +42,102 @@ data PairMsg
 data PairData = PairData
 	{ remoteHostName :: HostName
 	, remoteUserName :: UserName
-	, sshPubKey :: Maybe SshPubKey
+	, sshPubKey :: SshPubKey
 	}
 	deriving (Eq, Read, Show)
 
 type SshPubKey = String
 type UserName = String
+
+{- A pairing that is in progress has a secret, and a thread that is
+ - broadcasting pairing requests. -}
+data PairingInProgress = PairingInProgress Secret ThreadId
+
+{- This is an arbitrary port in the dynamic port range, that could
+ - conceivably be used for some other broadcast messages.
+ - If so, hope they ignore the garbage from us; we'll certianly
+ - ignore garbage from them. Wild wild west. -}
+pairingPort :: PortNumber
+pairingPort = 55556
+
+{- This is the All Hosts multicast group, which should reach all hosts
+ - on the same network segment. -}
+multicastAddress :: HostName
+multicastAddress = "224.0.0.1"
+
+type MkPairMsg = HostName -> PairMsg
+
+{- Multicasts a message repeatedly on all interfaces until its thread
+ - is killed, with a 2 second delay between each transmission.
+ -
+ - The remoteHostName is set to the best host name that can be found for
+ - each interface's IP address. When possible, that's a .local name.
+ - If not, it's whatever is found in the DNS for the address, or failing
+ - that, the IP address.
+ -
+ - Note that new sockets are opened each time. This is hardly efficient,
+ - but it allows new network interfaces to be used as they come up.
+ - On the other hand, the expensive DNS lookups are cached. -}
+multicastPairMsg :: MkPairMsg -> IO ThreadId
+multicastPairMsg mkmsg = forkIO $ go =<< initMsgCache mkmsg
+	where
+		go cache = do
+			addrs <- activeNetworkAddresses
+			cache' <- updateMsgCache mkmsg cache addrs
+			mapM_ (sendinterface cache') addrs
+			threadDelaySeconds (Seconds 2)
+			go cache'
+		sendinterface cache i = void $ catchMaybeIO $ withSocketsDo $ do
+			(sock, addr) <- multicastSender multicastAddress pairingPort
+			setInterface sock (show i)
+			maybe noop (\s -> void $ sendTo sock s addr)
+				(M.lookup i cache)
+
+{- A cache of serialized messages. -}
+type MsgCache = M.Map SomeAddr String
+
+{- Ensures that the cache has messages for each address. -}
+updateMsgCache :: MkPairMsg -> MsgCache -> [SomeAddr] -> IO MsgCache
+updateMsgCache _ m [] = return m
+updateMsgCache mkmsg m (v:vs)
+	| M.member v m = updateMsgCache mkmsg m vs
+	| otherwise = do
+		let sockaddr = case v of
+			IPv4Addr (IPv4 a) -> SockAddrInet (PortNum 0) a
+			IPv6Addr (IPv6 o1 o2 o3 o4) -> SockAddrInet6 (PortNum 0) 0 (o1, o2, o3, o4) 0
+		mhostname <- catchDefaultIO (fst <$> getNameInfo [] True False sockaddr) Nothing
+		let cache' = M.insert v (show $ mkmsg $ fromMaybe (show v) mhostname) m
+		updateMsgCache mkmsg cache' vs
+
+{- An initial message cache. Look up hostname.local, and if found, 
+ - put it in the cache. -}
+initMsgCache :: MkPairMsg -> IO MsgCache
+initMsgCache mkmsg = go =<< getHostname
+	where
+		go Nothing = return M.empty
+		go (Just n) = do
+			let localname = n ++ ".local"
+			addrs <- catchDefaultIO (getAddrInfo Nothing (Just localname) Nothing) []
+			case headMaybe addrs of
+				Nothing -> return M.empty
+				Just addr -> case addrAddress addr of
+					SockAddrInet _ a ->
+						use localname $
+							IPv4Addr $ IPv4 a
+					SockAddrInet6 _ _ (o1, o2, o3, o4) _ ->
+						use localname $	
+							IPv6Addr $ IPv6 o1 o2 o3 o4
+					_ -> return M.empty
+		use hostname addr = return $ M.fromList [(addr, show $ mkmsg hostname)]
+
+data SomeAddr = IPv4Addr IPv4 | IPv6Addr IPv6
+	deriving (Ord, Eq)
+
+instance Show SomeAddr where
+	show (IPv4Addr x) = show x
+	show (IPv6Addr x) = show x
+
+activeNetworkAddresses :: IO [SomeAddr]
+activeNetworkAddresses = filter (not . all (`elem` "0.:") . show)
+	. concat . map (\ni -> [IPv4Addr $ ipv4 ni, IPv6Addr $ ipv6 ni])
+	<$> getNetworkInterfaces