GHC/Event/Manager.hs

    {-# LANGUAGE Trustworthy #-}
    {-# LANGUAGE BangPatterns
               , CPP
               , ExistentialQuantification
               , NoImplicitPrelude
               , RecordWildCards
               , TypeSynonymInstances
               , FlexibleInstances
      #-}

    -- |
    -- The event manager supports event notification on fds. Each fd may
    -- have multiple callbacks registered, each listening for a different
    -- set of events. Registrations may be automatically deactivated after
    -- the occurrence of an event ("one-shot mode") or active until
    -- explicitly unregistered.
    --
    -- If an fd has only one-shot registrations then we use one-shot
    -- polling if available. Otherwise we use multi-shot polling.

    module GHC.Event.Manager
        ( -- * Types
          EventManager

          -- * Creation
        , new
        , newWith
        , newDefaultBackend

          -- * Running
        , finished
        , loop
        , step
        , shutdown
        , release
        , cleanup
        , wakeManager

          -- * State
        , callbackTableVar
        , emControl

          -- * Registering interest in I/O events
        , Lifetime (..)
        , Event
        , evtRead
        , evtWrite
        , IOCallback
        , FdKey(keyFd)
        , FdData
        , registerFd
        , unregisterFd_
        , unregisterFd
        , closeFd
        , closeFd_
        ) where

    #include "EventConfig.h"

    ------------------------------------------------------------------------
    -- Imports

    import Control.Concurrent.MVar (MVar, newMVar, putMVar,
                                    tryPutMVar, takeMVar, withMVar)
    import Control.Exception (onException)
    import Data.Bits ((.&.))
    import Data.Foldable (forM_)
    import Data.Functor (void)
    import Data.IORef (IORef, atomicModifyIORef', mkWeakIORef, newIORef, readIORef,
                       writeIORef)
    import Data.Maybe (maybe)
    import Data.OldList (partition)
    import GHC.Arr (Array, (!), listArray)
    import GHC.Base
    import GHC.Conc.Sync (yield)
    import GHC.List (filter, replicate)
    import GHC.Num (Num(..))
    import GHC.Real (fromIntegral)
    import GHC.Show (Show(..))
    import GHC.Event.Control
    import GHC.Event.IntTable (IntTable)
    import GHC.Event.Internal (Backend, Event, evtClose, evtRead, evtWrite,
                               Lifetime(..), EventLifetime, Timeout(..))
    import GHC.Event.Unique (Unique, UniqueSource, newSource, newUnique)
    import System.Posix.Types (Fd)

    import qualified GHC.Event.IntTable as IT
    import qualified GHC.Event.Internal as I

    #if defined(HAVE_KQUEUE)
    import qualified GHC.Event.KQueue as KQueue
    #elif defined(HAVE_EPOLL)
    import qualified GHC.Event.EPoll  as EPoll
    #elif defined(HAVE_POLL)
    import qualified GHC.Event.Poll   as Poll
    #else
    # error not implemented for this operating system
    #endif

    ------------------------------------------------------------------------
    -- Types

    data FdData = FdData {
          fdKey       :: {-# UNPACK #-} !FdKey
        , fdEvents    :: {-# UNPACK #-} !EventLifetime
        , _fdCallback :: !IOCallback
        }

    -- | A file descriptor registration cookie.
    data FdKey = FdKey {
          keyFd     :: {-# UNPACK #-} !Fd
        , keyUnique :: {-# UNPACK #-} !Unique
        } deriving ( Eq   -- ^ @since 4.4.0.0
                   , Show -- ^ @since 4.4.0.0
                   )

    -- | Callback invoked on I/O events.
    type IOCallback = FdKey -> Event -> IO ()

    data State = Created
               | Running
               | Dying
               | Releasing
               | Finished
                 deriving ( Eq   -- ^ @since 4.4.0.0
                          , Show -- ^ @since 4.4.0.0
                          )

    -- | The event manager state.
    data EventManager = EventManager
        { emBackend      :: !Backend
        , emFds          :: {-# UNPACK #-} !(Array Int (MVar (IntTable [FdData])))
        , emState        :: {-# UNPACK #-} !(IORef State)
        , emUniqueSource :: {-# UNPACK #-} !UniqueSource
        , emControl      :: {-# UNPACK #-} !Control
        , emLock         :: {-# UNPACK #-} !(MVar ())
        }

    -- must be power of 2
    callbackArraySize :: Int
    callbackArraySize = 32

    hashFd :: Fd -> Int
    hashFd fd = fromIntegral fd .&. (callbackArraySize - 1)
    {-# INLINE hashFd #-}

    callbackTableVar :: EventManager -> Fd -> MVar (IntTable [FdData])
    callbackTableVar mgr fd = emFds mgr ! hashFd fd
    {-# INLINE callbackTableVar #-}

    haveOneShot :: Bool
    {-# INLINE haveOneShot #-}
    #if defined(darwin_HOST_OS) || defined(ios_HOST_OS)
    haveOneShot = False
    #elif defined(HAVE_EPOLL) || defined(HAVE_KQUEUE)
    haveOneShot = True
    #else
    haveOneShot = False
    #endif
    ------------------------------------------------------------------------
    -- Creation

    handleControlEvent :: EventManager -> Fd -> Event -> IO ()
    handleControlEvent mgr fd _evt = do
      msg <- readControlMessage (emControl mgr) fd
      case msg of
        CMsgWakeup      -> return ()
        CMsgDie         -> writeIORef (emState mgr) Finished
        _               -> return ()

    newDefaultBackend :: IO Backend
    #if defined(HAVE_KQUEUE)
    newDefaultBackend = KQueue.new
    #elif defined(HAVE_EPOLL)
    newDefaultBackend = EPoll.new
    #elif defined(HAVE_POLL)
    newDefaultBackend = Poll.new
    #else
    newDefaultBackend = errorWithoutStackTrace "no back end for this platform"
    #endif

    -- | Create a new event manager.
    new :: IO EventManager
    new = newWith =<< newDefaultBackend

    -- | Create a new 'EventManager' with the given polling backend.
    newWith :: Backend -> IO EventManager
    newWith be = do
      iofds <- fmap (listArray (0, callbackArraySize-1)) $
               replicateM callbackArraySize (newMVar =<< IT.new 8)
      ctrl <- newControl False
      state <- newIORef Created
      us <- newSource
      _ <- mkWeakIORef state $ do
                   st <- atomicModifyIORef' state $ \s -> (Finished, s)
                   when (st /= Finished) $ do
                     I.delete be
                     closeControl ctrl
      lockVar <- newMVar ()
      let mgr = EventManager { emBackend = be
                             , emFds = iofds
                             , emState = state
                             , emUniqueSource = us
                             , emControl = ctrl
                             , emLock = lockVar
                             }
      registerControlFd mgr (controlReadFd ctrl) evtRead
      registerControlFd mgr (wakeupReadFd ctrl) evtRead
      return mgr
      where
        replicateM n x = sequence (replicate n x)

    failOnInvalidFile :: String -> Fd -> IO Bool -> IO ()
    failOnInvalidFile loc fd m = do
      ok <- m
      when (not ok) $
        let msg = "Failed while attempting to modify registration of file " ++
                  show fd ++ " at location " ++ loc
        in errorWithoutStackTrace msg

    registerControlFd :: EventManager -> Fd -> Event -> IO ()
    registerControlFd mgr fd evs =
      failOnInvalidFile "registerControlFd" fd $
      I.modifyFd (emBackend mgr) fd mempty evs

    -- | Asynchronously shuts down the event manager, if running.
    shutdown :: EventManager -> IO ()
    shutdown mgr = do
      state <- atomicModifyIORef' (emState mgr) $ \s -> (Dying, s)
      when (state == Running) $ sendDie (emControl mgr)

    -- | Asynchronously tell the thread executing the event
    -- manager loop to exit.
    release :: EventManager -> IO ()
    release EventManager{..} = do
      state <- atomicModifyIORef' emState $ \s -> (Releasing, s)
      when (state == Running) $ sendWakeup emControl

    finished :: EventManager -> IO Bool
    finished mgr = (== Finished) `liftM` readIORef (emState mgr)

    cleanup :: EventManager -> IO ()
    cleanup EventManager{..} = do
      writeIORef emState Finished
      void $ tryPutMVar emLock ()
      I.delete emBackend
      closeControl emControl

    ------------------------------------------------------------------------
    -- Event loop

    -- | Start handling events.  This function loops until told to stop,
    -- using 'shutdown'.
    --
    -- /Note/: This loop can only be run once per 'EventManager', as it
    -- closes all of its control resources when it finishes.
    loop :: EventManager -> IO ()
    loop mgr@EventManager{..} = do
      void $ takeMVar emLock
      state <- atomicModifyIORef' emState $ \s -> case s of
        Created -> (Running, s)
        Releasing -> (Running, s)
        _       -> (s, s)
      case state of
        Created   -> go `onException` cleanup mgr
        Releasing -> go `onException` cleanup mgr
        Dying     -> cleanup mgr
        -- While a poll loop is never forked when the event manager is in the
        -- 'Finished' state, its state could read 'Finished' once the new thread
        -- actually runs.  This is not an error, just an unfortunate race condition
        -- in Thread.restartPollLoop.  See #8235
        Finished  -> return ()
        _         -> do cleanup mgr
                        errorWithoutStackTrace $ "GHC.Event.Manager.loop: state is already " ++
                                show state
     where
      go = do state <- step mgr
              case state of
                Running   -> yield >> go
                Releasing -> putMVar emLock ()
                _         -> cleanup mgr

    -- | To make a step, we first do a non-blocking poll, in case
    -- there are already events ready to handle. This improves performance
    -- because we can make an unsafe foreign C call, thereby avoiding
    -- forcing the current Task to release the Capability and forcing a context switch.
    -- If the poll fails to find events, we yield, putting the poll loop thread at
    -- end of the Haskell run queue. When it comes back around, we do one more
    -- non-blocking poll, in case we get lucky and have ready events.
    -- If that also returns no events, then we do a blocking poll.
    step :: EventManager -> IO State
    step mgr@EventManager{..} = do
      waitForIO
      state <- readIORef emState
      state `seq` return state
      where
        waitForIO = do
          n1 <- I.poll emBackend Nothing (onFdEvent mgr)
          when (n1 <= 0) $ do
            yield
            n2 <- I.poll emBackend Nothing (onFdEvent mgr)
            when (n2 <= 0) $ do
              _ <- I.poll emBackend (Just Forever) (onFdEvent mgr)
              return ()

    ------------------------------------------------------------------------
    -- Registering interest in I/O events

    -- | Register interest in the given events, without waking the event
    -- manager thread.  The 'Bool' return value indicates whether the
    -- event manager ought to be woken.
    --
    -- Note that the event manager is generally implemented in terms of the
    -- platform's @select@ or @epoll@ system call, which tend to vary in
    -- what sort of fds are permitted. For instance, waiting on regular files
    -- is not allowed on many platforms.
    registerFd_ :: EventManager -> IOCallback -> Fd -> Event -> Lifetime
                -> IO (FdKey, Bool)
    registerFd_ mgr@(EventManager{..}) cb fd evs lt = do
      u <- newUnique emUniqueSource
      let fd'  = fromIntegral fd
          reg  = FdKey fd u
          el = I.eventLifetime evs lt
          !fdd = FdData reg el cb
      (modify,ok) <- withMVar (callbackTableVar mgr fd) $ \tbl -> do
        oldFdd <- IT.insertWith (++) fd' [fdd] tbl
        let prevEvs :: EventLifetime
            prevEvs = maybe mempty eventsOf oldFdd

            el' :: EventLifetime
            el' = prevEvs `mappend` el
        case I.elLifetime el' of
          -- All registrations want one-shot semantics and this is supported
          OneShot | haveOneShot -> do
            ok <- I.modifyFdOnce emBackend fd (I.elEvent el')
            if ok
              then return (False, True)
              else IT.reset fd' oldFdd tbl >> return (False, False)

          -- We don't want or don't support one-shot semantics
          _ -> do
            let modify = prevEvs /= el'
            ok <- if modify
                  then let newEvs = I.elEvent el'
                           oldEvs = I.elEvent prevEvs
                       in I.modifyFd emBackend fd oldEvs newEvs
                  else return True
            if ok
              then return (modify, True)
              else IT.reset fd' oldFdd tbl >> return (False, False)
      -- this simulates behavior of old IO manager:
      -- i.e. just call the callback if the registration fails.
      when (not ok) (cb reg evs)
      return (reg,modify)
    {-# INLINE registerFd_ #-}

    -- | @registerFd mgr cb fd evs lt@ registers interest in the events @evs@
    -- on the file descriptor @fd@ for lifetime @lt@. @cb@ is called for
    -- each event that occurs.  Returns a cookie that can be handed to
    -- 'unregisterFd'.
    registerFd :: EventManager -> IOCallback -> Fd -> Event -> Lifetime -> IO FdKey
    registerFd mgr cb fd evs lt = do
      (r, wake) <- registerFd_ mgr cb fd evs lt
      when wake $ wakeManager mgr
      return r
    {-# INLINE registerFd #-}

    {-
        Building GHC with parallel IO manager on Mac freezes when
        compiling the dph libraries in the phase 2. As workaround, we
        don't use oneshot and we wake up an IO manager on Mac every time
        when we register an event.

        For more information, please read:
            https://gitlab.haskell.org/ghc/ghc/issues/7651
    -}
    -- | Wake up the event manager.
    wakeManager :: EventManager -> IO ()
    #if defined(darwin_HOST_OS) || defined(ios_HOST_OS)
    wakeManager mgr = sendWakeup (emControl mgr)
    #elif defined(HAVE_EPOLL) || defined(HAVE_KQUEUE)
    wakeManager _ = return ()
    #else
    wakeManager mgr = sendWakeup (emControl mgr)
    #endif

    eventsOf :: [FdData] -> EventLifetime
    eventsOf [fdd] = fdEvents fdd
    eventsOf fdds  = mconcat $ map fdEvents fdds

    -- | Drop a previous file descriptor registration, without waking the
    -- event manager thread.  The return value indicates whether the event
    -- manager ought to be woken.
    unregisterFd_ :: EventManager -> FdKey -> IO Bool
    unregisterFd_ mgr@(EventManager{..}) (FdKey fd u) =
      withMVar (callbackTableVar mgr fd) $ \tbl -> do
        let dropReg = nullToNothing . filter ((/= u) . keyUnique . fdKey)
            fd' = fromIntegral fd
            pairEvents :: [FdData] -> IO (EventLifetime, EventLifetime)
            pairEvents prev = do
              r <- maybe mempty eventsOf `fmap` IT.lookup fd' tbl
              return (eventsOf prev, r)
        (oldEls, newEls) <- IT.updateWith dropReg fd' tbl >>=
                            maybe (return (mempty, mempty)) pairEvents
        let modify = oldEls /= newEls
        when modify $ failOnInvalidFile "unregisterFd_" fd $
          case I.elLifetime newEls of
            OneShot | I.elEvent newEls /= mempty, haveOneShot ->
              I.modifyFdOnce emBackend fd (I.elEvent newEls)
            _ ->
              I.modifyFd emBackend fd (I.elEvent oldEls) (I.elEvent newEls)
        return modify

    -- | Drop a previous file descriptor registration.
    unregisterFd :: EventManager -> FdKey -> IO ()
    unregisterFd mgr reg = do
      wake <- unregisterFd_ mgr reg
      when wake $ wakeManager mgr

    -- | Close a file descriptor in a race-safe way.
    closeFd :: EventManager -> (Fd -> IO ()) -> Fd -> IO ()
    closeFd mgr close fd = do
      fds <- withMVar (callbackTableVar mgr fd) $ \tbl -> do
        prev <- IT.delete (fromIntegral fd) tbl
        case prev of
          Nothing  -> close fd >> return []
          Just fds -> do
            let oldEls = eventsOf fds
            when (I.elEvent oldEls /= mempty) $ do
              _ <- I.modifyFd (emBackend mgr) fd (I.elEvent oldEls) mempty
              wakeManager mgr
            close fd
            return fds
      forM_ fds $ \(FdData reg el cb) -> cb reg (I.elEvent el `mappend` evtClose)

    -- | Close a file descriptor in a race-safe way.
    -- It assumes the caller will update the callback tables and that the caller
    -- holds the callback table lock for the fd. It must hold this lock because
    -- this command executes a backend command on the fd.
    closeFd_ :: EventManager
                -> IntTable [FdData]
                -> Fd
                -> IO (IO ())
    closeFd_ mgr tbl fd = do
      prev <- IT.delete (fromIntegral fd) tbl
      case prev of
        Nothing  -> return (return ())
        Just fds -> do
          let oldEls = eventsOf fds
          when (oldEls /= mempty) $ do
            _ <- I.modifyFd (emBackend mgr) fd (I.elEvent oldEls) mempty
            wakeManager mgr
          return $
            forM_ fds $ \(FdData reg el cb) ->
              cb reg (I.elEvent el `mappend` evtClose)

    ------------------------------------------------------------------------
    -- Utilities

    -- | Call the callbacks corresponding to the given file descriptor.
    onFdEvent :: EventManager -> Fd -> Event -> IO ()
    onFdEvent mgr fd evs
      | fd == controlReadFd (emControl mgr) || fd == wakeupReadFd (emControl mgr) =
        handleControlEvent mgr fd evs

      | otherwise = do
        fdds <- withMVar (callbackTableVar mgr fd) $ \tbl ->
            IT.delete (fromIntegral fd) tbl >>= maybe (return []) (selectCallbacks tbl)
        forM_ fdds $ \(FdData reg _ cb) -> cb reg evs
      where
        -- | Here we look through the list of registrations for the fd of interest
        -- and sort out which match the events that were triggered. We,
        --
        --   1. re-arm the fd as appropriate
        --   2. reinsert registrations that weren't triggered and multishot
        --      registrations
        --   3. return a list containing the callbacks that should be invoked.
        selectCallbacks :: IntTable [FdData] -> [FdData] -> IO [FdData]
        selectCallbacks tbl fdds = do
            let -- figure out which registrations have been triggered
                matches :: FdData -> Bool
                matches fd' = evs `I.eventIs` I.elEvent (fdEvents fd')
                (triggered, notTriggered) = partition matches fdds

                -- sort out which registrations we need to retain
                isMultishot :: FdData -> Bool
                isMultishot fd' = I.elLifetime (fdEvents fd') == MultiShot
                saved = notTriggered ++ filter isMultishot triggered

                savedEls = eventsOf saved
                allEls = eventsOf fdds

            -- Reinsert multishot registrations.
            -- We deleted the table entry for this fd above so we there isn't a preexisting entry
            _ <- IT.insertWith (\_ _ -> saved) (fromIntegral fd) saved tbl

            case I.elLifetime allEls of
              -- we previously armed the fd for multiple shots, no need to rearm
              MultiShot | allEls == savedEls ->
                return ()

              -- either we previously registered for one shot or the
              -- events of interest have changed, we must re-arm
              _ ->
                case I.elLifetime savedEls of
                  OneShot | haveOneShot ->
                    -- if there are no saved events and we registered with one-shot
                    -- semantics then there is no need to re-arm
                    unless (OneShot == I.elLifetime allEls
                            && mempty == I.elEvent savedEls) $ do
                      void $ I.modifyFdOnce (emBackend mgr) fd (I.elEvent savedEls)
                  _ ->
                    -- we need to re-arm with multi-shot semantics
                    void $ I.modifyFd (emBackend mgr) fd
                                      (I.elEvent allEls) (I.elEvent savedEls)

            return triggered

    nullToNothing :: [a] -> Maybe [a]
    nullToNothing []       = Nothing
    nullToNothing xs@(_:_) = Just xs

    unless :: Monad m => Bool -> m () -> m ()
    unless p = when (not p)