    {-# LANGUAGE DataKinds #-}
    {-# LANGUAGE TypeFamilies #-}

    module Propellor.Property.Atomic (
            atomicDirUpdate,
            atomicDirSync,
            atomicUpdate,
            AtomicResourcePair(..),
            flipAtomicResourcePair,
            SwapAtomicResourcePair,
            CheckAtomicResourcePair,
    ) where

    import Propellor.Base
    import Propellor.Types.Core
    import Propellor.Types.MetaTypes
    import Propellor.EnsureProperty
    import Propellor.Property.File
    import Propellor.Property.Rsync (syncDir)

    import System.Posix.Files

    -- | A pair of resources, one active and one inactive, which can swap
    -- positions atomically.
    data AtomicResourcePair a = AtomicResourcePair
            { activeAtomicResource :: a
            , inactiveAtomicResource :: a
            }

    flipAtomicResourcePair :: AtomicResourcePair a -> AtomicResourcePair a
    flipAtomicResourcePair a = AtomicResourcePair
            { activeAtomicResource = inactiveAtomicResource a
            , inactiveAtomicResource = activeAtomicResource a
            }

    -- | Action that activates the inactiveAtomicResource, and deactivates
    -- the activeAtomicResource. This action must be fully atomic.
    type SwapAtomicResourcePair a = AtomicResourcePair a -> Propellor Bool

    -- | Checks which of the pair of resources is currently active and
    -- which is inactive, and puts them in the correct poisition in
    -- the AtomicResourcePair.
    type CheckAtomicResourcePair a = AtomicResourcePair a -> Propellor (AtomicResourcePair a)

    -- | Makes a non-atomic Property be atomic, by applying it to the
    -- inactiveAtomicResource, and if it was successful,
    -- atomically activating that resource.
    atomicUpdate
            -- Constriaint inherited from ensureProperty.
            :: EnsurePropertyAllowed t t
            => SingI t
            => AtomicResourcePair a
            -> CheckAtomicResourcePair a
            -> SwapAtomicResourcePair a
            -> (a -> Property (MetaTypes t))
            -> Property (MetaTypes t)
    atomicUpdate rbase rcheck rswap mkp = property' d $ \w -> do
            r <- rcheck rbase
            res <- ensureProperty w $ mkp $ inactiveAtomicResource r
            case res of
                    FailedChange -> return FailedChange
                    NoChange -> return NoChange
                    MadeChange -> do
                            ok <- rswap r
                            if ok
                                    then return res
                                    else return FailedChange
      where
            d = getDesc $ mkp $ activeAtomicResource rbase

    -- | Applies a Property to a directory such that the directory is updated
    -- fully atomically; there is no point in time in which the directory will
    -- be in an inconsistent state.
    --
    -- For example, git repositories are not usually updated atomically,
    -- and so while the repository is being updated, the files in it can be a
    -- mixture of two different versions, which could cause unexpected
    -- behavior to consumers. To avoid such problems:
    --
    -- >	& atomicDirUpdate "/srv/web/example.com"
    -- >		(\d -> Git.pulled "joey" "http://.." d Nothing)
    --
    -- This operates by making a second copy of the directory, and passing it
    -- to the Property, which can make whatever changes it needs to that copy,
    -- non-atomically. After the Property successfully makes a change, the
    -- copy is swapped into place, fully atomically.
    --
    -- This necessarily uses double the disk space, since there are two copies
    -- of the directory. The parent directory will actually contain three
    -- children: a symlink with the name of the directory itself, and two copies
    -- of the directory, with names suffixed with ".1" and ".2"
    atomicDirUpdate
            -- Constriaint inherited from ensureProperty.
            :: EnsurePropertyAllowed t t
            => SingI t
            => FilePath
            -> (FilePath -> Property (MetaTypes t))
            -> Property (MetaTypes t)
    atomicDirUpdate d = atomicUpdate (mkDirLink d) (checkDirLink d) (swapDirLink d)

    mkDirLink :: FilePath -> AtomicResourcePair FilePath
    mkDirLink d = AtomicResourcePair
            { activeAtomicResource = addext ".1"
            , inactiveAtomicResource = addext ".2"
            }
      where
            addext = addExtension (dropTrailingPathSeparator d)

    inactiveLinkTarget :: AtomicResourcePair FilePath -> FilePath
    inactiveLinkTarget = takeFileName . inactiveAtomicResource

    swapDirLink :: FilePath -> SwapAtomicResourcePair FilePath
    swapDirLink d rp = liftIO $ do
            v <- tryIO $ createSymbolicLink (inactiveLinkTarget rp)
                    `viaStableTmp` d
            case v of
                    Right () -> return True
                    Left e -> do
                            warningMessage $ "Unable to update symlink at " ++ d ++ " (" ++ show e ++ ")"
                            return False

    checkDirLink :: FilePath -> CheckAtomicResourcePair FilePath
    checkDirLink d rp = liftIO $ do
            v <- tryIO $ readSymbolicLink d
            return $ case v of
                    Right t | t == inactiveLinkTarget rp ->
                            flipAtomicResourcePair rp
                    _ -> rp

    -- | This can optionally be used after atomicDirUpdate to rsync the changes
    -- that were made over to the other copy of the directory. It's not
    -- necessary to use this, but it can improve efficiency.
    --
    -- For example:
    --
    -- >	& atomicDirUpdate "/srv/web/example.com"
    -- >		(\d -> Git.pulled "joey" "http://.." d Nothing)
    -- >		`onChange` atomicDirSync "/srv/web/example.com"
    --
    -- Using atomicDirSync in the above example lets git only download
    -- the changes once, rather than the same changes being downloaded a second
    -- time to update the other copy of the directory the next time propellor
    -- runs.
    --
    -- Suppose that a web server program is run from the git repository,
    -- and needs to be restarted after the pull. That restart should be done
    -- after the atomicDirUpdate, but before the atomicDirSync. That way,
    -- the old web server process will not have its files changed out from
    -- under it.
    --
    -- >	& atomicDirUpdate "/srv/web/example.com"
    -- >		(\d -> Git.pulled "joey" "http://.." d Nothing)
    -- >		`onChange` (webServerRestart `before` atomicDirSync "/srv/web/example.com")
    atomicDirSync :: FilePath -> Property (DebianLike + ArchLinux)
    atomicDirSync d = syncDir (activeAtomicResource rp) (inactiveAtomicResource rp)
      where
            rp = mkDirLink d