{-# LANGUAGE OverloadedStrings, TupleSections #-}

module Role.User where

import Types
import Pty
import Memory
import Log
import Session
import Crypto
import CmdLine
import WebSockets
import SessionID

import Control.Concurrent.Async
import Control.Concurrent.STM
import System.Process
import System.Exit
import qualified Data.ByteString as B
import Data.List.NonEmpty (NonEmpty(..), toList)
import Data.Monoid
import Data.Maybe
import Data.Time.Clock.POSIX
import System.IO
import System.Environment

run :: UserOpts -> IO ExitCode
run os = do
	(cmd, cmdparams) <- shellCommand os
	go cmd cmdparams startSession
  where
	go cmd cmdparams startmsg = do
		putStr "Connecting to debug-me server..."
		hFlush stdout
		esv <- newEmptyTMVarIO
		runClientApp $ clientApp (InitMode mempty) $ \ichan ochan sid -> do
			let url = sessionIDUrl sid "localhost" 8081
			putStrLn ""
			putStrLn $ "Others can connect to this session by running: debug-me --debug " ++ url
			hFlush stdout
			runWithPty cmd cmdparams $ \(p, ph) -> do
				uthread <- async (user startmsg p ichan ochan)
				exitstatus <- waitForProcess ph
				cancel uthread
				atomically $ putTMVar esv exitstatus
		sessionDone
		fromMaybe (ExitFailure 101) <$> atomically (tryReadTMVar esv)

shellCommand :: UserOpts -> IO (String, [String])
shellCommand os = case cmdToRun os of
	Just v -> return v
	Nothing -> maybe ("bash", ["-l"]) (, []) <$> lookupEnv "SHELL"

-- | Log of recent Activity, with the most recent first.
type Backlog = NonEmpty Log

data UserState = UserState
	{ backLog :: Backlog
	, userSessionKey :: MySessionKey
	, userSigVerifier :: SigVerifier
	}

user :: B.ByteString -> Pty -> TChan (Message Seen) -> TChan (Message Entered) -> IO ()
user starttxt p ochan ichan = withLogger "debug-me.log" $ \logger -> do
	-- Start by establishing our session key, and displaying the starttxt.
	let initialmessage msg = do
		atomically $ writeTChan ochan msg
		logger $ User msg
	sk <- genMySessionKey
	pk <- myPublicKey sk
	let c = mkSigned sk $ Control (SessionKey pk)
	initialmessage $ ControlMessage c
	let starttxt' = starttxt <> "\r\n"
	let act = mkSigned sk $ Activity (Seen (Val starttxt')) Nothing
	let startmsg = ActivityMessage act
	B.hPut stdout starttxt'
	hFlush stdout
	initialmessage startmsg
	l <- mkLog (User startmsg) <$> getPOSIXTime
	us <- newTVarIO $ UserState
		{ backLog = l :| []
		, userSessionKey = sk
		, userSigVerifier = mempty
		}
	_ <- sendPtyOutput p ochan us logger
		`concurrently` sendPtyInput ichan ochan p us logger
			`concurrently` forwardTtyInputToPty p
	return ()

-- | Forward things the user types to the Pty.
forwardTtyInputToPty :: Pty -> IO ()
forwardTtyInputToPty p = do
	b <- B.hGetSome stdin 1024
	if B.null b
		then return ()
		else do
			writePty p b
			forwardTtyInputToPty p

-- | Forward things written to the Pty out the TChan, and also display
-- it on their Tty.
sendPtyOutput :: Pty -> TChan (Message Seen) -> TVar UserState -> Logger -> IO ()
sendPtyOutput p ochan us logger = go
  where
	go = do
		b <- readPty p
		B.hPut stdout b
		hFlush stdout
		now <- getPOSIXTime
		l <- atomically $ do
			let seen = Seen (Val b)
			sendDeveloper ochan us seen now
		logger $ User l
		go

class SendableToDeveloper t where
	sendDeveloper :: TChan (Message Seen) -> TVar UserState -> t -> POSIXTime -> STM (Message Seen)

instance SendableToDeveloper Seen where
	sendDeveloper ochan us seen now = do
		st <- readTVar us
		let bl@(prev :| _) = backLog st
		let msg = ActivityMessage $
			mkSigned (userSessionKey st) $ 
				Activity seen (loggedHash prev)
		let l = mkLog (User msg) now
		writeTChan ochan msg
		writeTVar us $ st { backLog = l :| toList bl }
		return msg

instance SendableToDeveloper ControlAction where
	sendDeveloper ochan us c _now = do
		st <- readTVar us
		let msg = ControlMessage $
			mkSigned (userSessionKey st) (Control c)
		-- Control messages are not kept in the backlog.
		writeTChan ochan msg
		return msg

-- | Read things to be entered from the TChan, verify if they're legal,
-- and send them to the Pty.
sendPtyInput :: TChan (Message Entered) -> TChan (Message Seen) -> Pty -> TVar UserState -> Logger -> IO ()
sendPtyInput ichan ochan p us logger = go
  where
	go = do
		now <- getPOSIXTime
		v <- atomically $ getDeveloperMessage ichan ochan us now
		case v of
			InputMessage msg@(ActivityMessage entered) -> do
				logger $ Developer msg
				writePty p $ val $ enteredData $ activity entered
				go
			InputMessage msg@(ControlMessage (Control c _)) -> do
				logger $ Developer msg
				case c of
					SessionKey pk -> do
						checkDeveloperPublicKey ochan us logger pk
						go
					Rejected r -> error $ "User side received a Rejected: " ++ show r
					SessionKeyAccepted _ -> error "User side received a SessionKeyAccepted"
					SessionKeyRejected _ -> error "User side received a SessionKeyRejected"
			RejectedMessage rej -> do
				logger $ User rej
				go
			BadlySignedMessage _ -> go

data Input
	= InputMessage (Message Entered)
	| RejectedMessage (Message Seen)
	| BadlySignedMessage (Message Entered)

-- Get message from developer, verify its signature is from a developer we
-- have allowed (unless it's a SessionKey control message, then the
-- signature of the message is only verified against the key in it), and
-- make sure it's legal before returning it. If it's not legal, sends a
-- Reject message.
getDeveloperMessage :: TChan (Message Entered) -> TChan (Message Seen) -> TVar UserState -> POSIXTime -> STM Input
getDeveloperMessage ichan ochan us now = do
	msg <- readTChan ichan
	st <- readTVar us
	case msg of
		ControlMessage (Control (SessionKey pk) _) -> do
			let sigverifier = mkSigVerifier pk
			if verifySigned sigverifier msg
				then return (InputMessage msg)
				else return (BadlySignedMessage msg)
		_ -> if verifySigned (userSigVerifier st) msg
			then case msg of
				ActivityMessage entered -> do
					-- Don't need to retain backlog
					-- before the Activity that entered
					-- references.
					let bl' = reduceBacklog $
						truncateBacklog (backLog st) entered
					if isLegalEntered entered (st { backLog = bl' })
						then do
							let l = mkLog (Developer msg) now
							writeTVar us (st { backLog = l :| toList bl' })
							return (InputMessage msg)
						else do
							let reject = Rejected entered
							RejectedMessage <$> sendDeveloper ochan us reject now
				ControlMessage (Control _ _) ->
					return (InputMessage msg)
			else return (BadlySignedMessage msg)

-- | Check if the public key a developer presented is one we want to use,
-- and if so, add it to the userSigVerifier.
checkDeveloperPublicKey :: TChan (Message Seen) -> TVar UserState -> Logger -> PublicKey -> IO ()
checkDeveloperPublicKey ochan us logger pk = do
	now <- getPOSIXTime
	-- TODO check gpg sig..
	msg <- atomically $ do
		st <- readTVar us
		let sv = userSigVerifier st
		let sv' = sv `mappend` mkSigVerifier pk
		let st' = st { userSigVerifier = sv' }
		writeTVar us st'
		sendDeveloper ochan us (SessionKeyAccepted pk) now
	logger $ User msg

-- | Truncate the Backlog to remove entries older than the one
-- that the Activity Entered refers to, but only if the referred
-- to Activity is an Activity Seen. 
--
-- Once the developer has referred to a given Activity Seen in
-- their Activity Entered, they cannot refer backwards to anything
-- that came before it.
--
-- If the Activity refers to an item not in the backlog, no truncation is
-- done.
truncateBacklog :: Backlog -> Activity Entered -> Backlog
truncateBacklog (b :| l) (Activity _ (Just hp) _)
	| truncationpoint b = b :| []
	| otherwise = b :| go [] l
  where
	go c [] = reverse c
	go c (x:xs)
		| truncationpoint x = reverse (x:c)
		| otherwise = go (x:c) xs
	truncationpoint x@(Log { loggedMessage = User {}}) = loggedHash x == Just hp
	truncationpoint _ = False
truncateBacklog bl (Activity _ Nothing _) = bl

-- | To avoid DOS attacks that try to fill up the backlog and so use all
-- memory, don't let the backlog contain more than 1000 items, or
-- more than 16 megabytes of total data. (Excluding the most recent
-- item).
reduceBacklog :: Backlog -> Backlog
reduceBacklog (b :| l) = b :| go 0 (take 1000 l)
  where
	go _ [] = []
	go n (x:xs)
		| n > 16777216 = []
		| otherwise = x : go (n + dataSize x) xs

-- | Entered activity is legal when it points to the last logged activity,
-- because this guarantees that the person who entered it saw
-- the current state of the system before manipulating it.
--
-- To support typeahead on slow links, some echoData may be provided
-- in the Entered activity. If the Entered activity points
-- to an older activity, then the echoData must match the
-- concatenation of all Seen activities after that one, up to the
-- last logged activity.
--
-- Activities that do not enter data point to the first message
-- sent in the debug-me session.
--
-- Does not check the signature.
isLegalEntered :: Activity Entered -> UserState -> Bool
isLegalEntered (Activity _ Nothing _) _ = False
isLegalEntered (Activity a (Just hp) _) us
	| loggedHash lastact == Just hp = True
	| B.null (val (echoData a)) = False -- optimisation
	| any (== Just hp) (map loggedHash bl) =
		let sincehp = reverse (lastact : takeWhile (\l -> loggedHash l /= Just hp) bl)
		in echoData a == mconcat (map (getseen . loggedMessage) sincehp)
	| otherwise = False
  where
	(lastact :| bl) = backLog us
	getseen (User (ActivityMessage as)) = seenData $ activity as
	getseen _ = mempty