    {-# LANGUAGE CPP #-}
    module System.Process.PID1
        ( RunOptions
        , defaultRunOptions
        , getRunEnv
        , getRunExitTimeoutSec
        , getRunGroup
        , getRunUser
        , getRunWorkDir
        , run
        , runWithOptions
        , setRunEnv
        , setRunExitTimeoutSec
        , setRunGroup
        , setRunUser
        , setRunWorkDir
        ) where

    import           Control.Concurrent       (forkIO, newEmptyMVar, takeMVar,
                                               threadDelay, tryPutMVar)
    import           Control.Exception        (assert, catch, throwIO)
    import           Control.Monad            (forever, void)
    import           Data.Foldable            (for_)
    import           System.Directory         (setCurrentDirectory)
    import           System.Exit              (ExitCode (ExitFailure), exitWith)
    import           System.IO.Error          (isDoesNotExistError)
    import           System.Posix.Process     (ProcessStatus (..), executeFile,
                                               exitImmediately, getAnyProcessStatus,
                                               getProcessID)
    import           System.Posix.Signals     (Handler (Catch), Signal,
                                               installHandler, sigINT, sigKILL,
                                               sigTERM, signalProcess)
    import           System.Posix.Types       (CPid)
    import           System.Posix.User        (getGroupEntryForName,
                                               getUserEntryForName,
                                               groupID, setGroupID,
                                               setUserID, userID)
    import           System.Process           (createProcess, proc, env)
    import           System.Process.Internals (ProcessHandle__ (..),
                                               modifyProcessHandle)

    -- | Holder for pid1 run options
    data RunOptions = RunOptions
      { -- optional environment variable override, default is current env
        runEnv :: Maybe [(String, String)]
        -- optional posix user name
      , runUser :: Maybe String
        -- optional posix group name
      , runGroup :: Maybe String
        -- optional working directory
      , runWorkDir :: Maybe FilePath
        -- timeout (in seconds) to wait for all child processes to exit after
        -- receiving SIGTERM or SIGINT signal
      , runExitTimeoutSec :: Int
      } deriving Show

    -- | return default `RunOptions`
    --
    -- @since 0.1.1.0
    defaultRunOptions :: RunOptions
    defaultRunOptions = RunOptions
      { runEnv = Nothing
      , runUser = Nothing
      , runGroup = Nothing
      , runWorkDir = Nothing
      , runExitTimeoutSec = 5 }

    -- | Get environment variable overrides for the given `RunOptions`
    --
    -- @since 0.1.1.0
    getRunEnv :: RunOptions -> Maybe [(String, String)]
    getRunEnv = runEnv

    -- | Set environment variable overrides for the given `RunOptions`
    --
    -- @since 0.1.1.0
    setRunEnv :: [(String, String)] -> RunOptions -> RunOptions
    setRunEnv env' opts = opts { runEnv = Just env' }

    -- | Get the process 'setUserID' user for the given `RunOptions`
    --
    -- @since 0.1.1.0
    getRunUser :: RunOptions -> Maybe String
    getRunUser = runUser

    -- | Set the process 'setUserID' user for the given `RunOptions`
    --
    -- @since 0.1.1.0
    setRunUser :: String -> RunOptions -> RunOptions
    setRunUser user opts = opts { runUser = Just user }

    -- | Get the process 'setGroupID' group for the given `RunOptions`
    --
    -- @since 0.1.1.0
    getRunGroup :: RunOptions -> Maybe String
    getRunGroup = runGroup

    -- | Set the process 'setGroupID' group for the given `RunOptions`
    --
    -- @since 0.1.1.0
    setRunGroup :: String -> RunOptions -> RunOptions
    setRunGroup group opts = opts { runGroup = Just group }

    -- | Get the process current directory for the given `RunOptions`
    --
    -- @since 0.1.1.0
    getRunWorkDir :: RunOptions -> Maybe FilePath
    getRunWorkDir = runWorkDir

    -- | Set the process current directory for the given `RunOptions`
    --
    -- @since 0.1.1.0
    setRunWorkDir :: FilePath -> RunOptions -> RunOptions
    setRunWorkDir dir opts = opts { runWorkDir = Just dir }

    -- | Return the timeout (in seconds) timeout (in seconds) to wait for all child
    -- processes to exit after receiving SIGTERM or SIGINT signal
    --
    -- @since 0.1.2.0
    getRunExitTimeoutSec :: RunOptions -> Int
    getRunExitTimeoutSec = runExitTimeoutSec

    -- | Set the timeout in seconds for the process reaper to wait for all child
    -- processes to exit after receiving SIGTERM or SIGINT signal
    --
    -- @since 0.1.2.0
    setRunExitTimeoutSec :: Int -> RunOptions -> RunOptions
    setRunExitTimeoutSec sec opts = opts { runExitTimeoutSec = sec }

    -- | Run the given command with specified arguments, with optional environment
    -- variable override (default is to use the current process's environment).
    --
    -- This function will check if the current process has a process ID of 1. If it
    -- does, it will install signal handlers for SIGTERM and SIGINT, set up a loop
    -- to reap all orphans, spawn a child process, and when that child dies, kill
    -- all other processes (first with a SIGTERM and then a SIGKILL) and exit with
    -- the child's exit code.
    --
    -- If this process is not PID1, then it will simply @exec@ the given command.
    --
    -- This function will never exit: it will always terminate your process, unless
    -- some exception is thrown.
    --
    -- @since 0.1.0.0
    run :: FilePath -- ^ command to run
        -> [String] -- ^ command line arguments
        -> Maybe [(String, String)]
        -- ^ optional environment variable override, default is current env
        -> IO a
    run cmd args env' = runWithOptions (defaultRunOptions {runEnv = env'}) cmd args

    -- | Variant of 'run' that runs a command, with optional environment posix
    -- user/group and working directory (default is to use the current process's
    -- user, group, environment, and current directory).
    --
    -- @since 0.1.1.0
    runWithOptions :: RunOptions -- ^ run options
                   -> FilePath -- ^ command to run
                   -> [String] -- ^ command line arguments
                   -> IO a
    runWithOptions opts cmd args = do
      for_ (runGroup opts) $ \name -> do
        entry <- getGroupEntryForName name
        setGroupID $ groupID entry
      for_ (runUser opts) $ \name -> do
        entry <- getUserEntryForName name
        setUserID $ userID entry
      for_ (runWorkDir opts) setCurrentDirectory
      let env' = runEnv opts
          timeout = runExitTimeoutSec opts
      -- check if we should act as pid1 or just exec the process
      myID <- getProcessID
      if myID == 1
        then runAsPID1 cmd args env' timeout
        else executeFile cmd True args env'

    -- | Run as a child with signal handling and orphan reaping.
    runAsPID1 :: FilePath -> [String] -> Maybe [(String, String)] -> Int -> IO a
    runAsPID1 cmd args env' timeout = do
        -- Set up an MVar to indicate we're ready to start killing all
        -- children processes. Then start a thread waiting for that
        -- variable to be filled and do the actual killing.
        killChildrenVar <- newEmptyMVar
        _ <- forkIO $ do
            takeMVar killChildrenVar
            killAllChildren timeout

        -- Helper function to start killing, used below
        let startKilling = void $ tryPutMVar killChildrenVar ()

        -- Install signal handlers for TERM and INT, which will start
        -- killing all children
        void $ installHandler sigTERM (Catch startKilling) Nothing
        void $ installHandler sigINT  (Catch startKilling) Nothing

        -- Spawn the child process
        (Nothing, Nothing, Nothing, ph) <- createProcess (proc cmd args)
            { env = env'
            }

        -- Determine the child PID. We want to exit once this child
        -- process is dead.
        p_ <- modifyProcessHandle ph $ \p_ -> return (p_, p_)
        child <-
            case p_ of
                ClosedHandle e -> assert False (exitWith e)
                OpenHandle pid -> return pid

        -- Loop on reaping child processes
        reap startKilling child

    reap :: IO () -> CPid -> IO a
    reap startKilling child = do
        -- Track the ProcessStatus of the child
        childStatus <- newEmptyMVar

        -- Keep reaping one child. Eventually, when all children are dead,
        -- we'll get an exception. We catch that exception and, assuming
        -- it's the DoesNotExistError we're expecting, know that all
        -- children are dead and exit.
        forever (reapOne childStatus) `catch` \e ->
            if isDoesNotExistError e
                -- no more child processes
                then do
                    takeMVar childStatus >>= exitImmediately . toExitCode
                    error "This can never be reached"
                -- some other exception occurred, reraise it
                else throwIO e
      where
        reapOne childStatus = do
            -- Block until a child process exits
            mres <- getAnyProcessStatus True False
            case mres of
                -- This should never happen, if there are no more child
                -- processes we'll get an exception instead
                Nothing -> assert False (return ())
                -- Got a new dead child. If it's the child we created in
                -- main, then start killing all other children. Otherwise,
                -- we're just reaping.
                Just (pid, status)
                    | pid == child -> do
                        -- Take the first status of the child. It's possible -
                        -- however unlikely - that the process ID could end up
                        -- getting reused and there will be another child exiting
                        -- with the same PID. Just ignore that.
                        void $ tryPutMVar childStatus status
                        startKilling
                    | otherwise -> return ()

    killAllChildren :: Int -> IO ()
    killAllChildren timeout = do
        -- Send all children processes the TERM signal
        signalProcess sigTERM (-1) `catch` \e ->
            if isDoesNotExistError e
                then return ()
                else throwIO e

        -- Wait for `timeout` seconds. We don't need to put in any logic about
        -- whether there are still child processes; if all children have
        -- exited, then the reap loop will exit and our process will shut
        -- down.
        threadDelay $ timeout * 1000 * 1000

        -- OK, some children didn't exit. Now time to get serious!
        signalProcess sigKILL (-1) `catch` \e ->
            if isDoesNotExistError e
                then return ()
                else throwIO e

    -- | Convert a ProcessStatus to an ExitCode. In the case of a signal being the
    -- cause of termination, see 'signalToEC'.
    toExitCode :: ProcessStatus -> ExitCode
    toExitCode (Exited ec) = ec
    #if MIN_VERSION_unix(2, 7, 0)
    toExitCode (Terminated sig _) = signalToEC sig
    #else
    toExitCode (Terminated sig) = signalToEC sig
    #endif
    toExitCode (Stopped sig) = signalToEC sig

    -- | Follow the convention of converting a signal into an exit code by adding
    -- 128.
    signalToEC :: Signal -> ExitCode
    signalToEC sig = ExitFailure (fromIntegral sig + 128)