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|
{- git-annex numcopies types
-
- Copyright 2014-2015 Joey Hess <id@joeyh.name>
-
- Licensed under the GNU GPL version 3 or higher.
-}
module Types.NumCopies (
NumCopies(..),
fromNumCopies,
VerifiedCopy(..),
checkVerifiedCopy,
invalidateVerifiedCopy,
strongestVerifiedCopy,
deDupVerifiedCopies,
mkVerifiedCopy,
invalidatableVerifiedCopy,
withVerifiedCopy,
isSafeDrop,
SafeDropProof,
mkSafeDropProof,
ContentRemovalLock(..),
) where
import Types.UUID
import Types.Key
import Utility.Exception (bracketIO)
import qualified Data.Map as M
import Control.Concurrent.MVar
import Control.Monad.Catch (MonadMask)
import Control.Monad.IO.Class (MonadIO)
import Control.Monad
newtype NumCopies = NumCopies Int
deriving (Ord, Eq, Show)
fromNumCopies :: NumCopies -> Int
fromNumCopies (NumCopies n) = n
-- Indicates that a key's content is exclusively
-- locked locally, pending removal.
newtype ContentRemovalLock = ContentRemovalLock Key
deriving (Show)
-- A verification that a copy of a key exists in a repository.
data VerifiedCopy
{- Represents a recent verification that a copy of an
- object exists in a repository with the given UUID. -}
= RecentlyVerifiedCopy V
{- Use when a repository cannot be accessed, but it's
- a trusted repository, which is on record as containing a key
- and is presumably not going to lose its copy. -}
| TrustedCopy V
{- The strongest proof of the existence of a copy.
- Until its associated action is called to unlock it,
- the copy is locked in the repository and is guaranteed
- not to be removed by any git-annex process. -}
| LockedCopy V
deriving (Show)
data V = V
{ _getUUID :: UUID
, _checkVerifiedCopy :: IO Bool
, _invalidateVerifiedCopy :: IO ()
}
instance Show V where
show v = show (_getUUID v)
instance ToUUID VerifiedCopy where
toUUID = _getUUID . toV
toV :: VerifiedCopy -> V
toV (TrustedCopy v) = v
toV (RecentlyVerifiedCopy v) = v
toV (LockedCopy v) = v
-- Checks that it's still valid.
checkVerifiedCopy :: VerifiedCopy -> IO Bool
checkVerifiedCopy = _checkVerifiedCopy . toV
invalidateVerifiedCopy :: VerifiedCopy -> IO ()
invalidateVerifiedCopy = _invalidateVerifiedCopy . toV
strongestVerifiedCopy :: VerifiedCopy -> VerifiedCopy -> VerifiedCopy
strongestVerifiedCopy a@(LockedCopy _) _ = a
strongestVerifiedCopy _ b@(LockedCopy _) = b
strongestVerifiedCopy a@(TrustedCopy _) _ = a
strongestVerifiedCopy _ b@(TrustedCopy _) = b
strongestVerifiedCopy a@(RecentlyVerifiedCopy _) _ = a
-- Retains stronger verifications over weaker for the same uuid.
deDupVerifiedCopies :: [VerifiedCopy] -> [VerifiedCopy]
deDupVerifiedCopies l = M.elems $
M.fromListWith strongestVerifiedCopy (zip (map toUUID l) l)
mkVerifiedCopy :: ToUUID u => (V -> VerifiedCopy) -> u -> VerifiedCopy
mkVerifiedCopy mk u = mk $ V (toUUID u) (return True) (return ())
invalidatableVerifiedCopy :: ToUUID u => (V -> VerifiedCopy) -> u -> IO VerifiedCopy
invalidatableVerifiedCopy mk u = do
v <- newEmptyMVar
let invalidate = do
_ <- tryPutMVar v ()
return ()
let check = isEmptyMVar v
return $ mk $ V (toUUID u) check invalidate
-- Constructs a VerifiedCopy, and runs the action, ensuring that the
-- verified copy is invalidated when the action returns, or on error.
withVerifiedCopy
:: (Monad m, MonadMask m, MonadIO m, ToUUID u)
=> (V -> VerifiedCopy)
-> u
-> (VerifiedCopy -> m a)
-> m a
withVerifiedCopy mk u = bracketIO setup cleanup
where
setup = invalidatableVerifiedCopy mk u
cleanup = invalidateVerifiedCopy
{- Check whether enough verification has been done of copies to allow
- dropping content safely.
-
- This is carefully balanced to prevent data loss when there are races
- between concurrent drops of the same content in different repos,
- without requiring impractical amounts of locking.
-
- In particular, concurrent drop races may cause the number of copies
- to fall below NumCopies, but it will never fall below 1.
-}
isSafeDrop :: NumCopies -> [VerifiedCopy] -> Maybe ContentRemovalLock -> Bool
{- When a ContentRemovalLock is provided, the content is being
- dropped from the local repo. That lock will prevent other git repos
- that are concurrently dropping from using the local copy as a VerifiedCopy.
- So, no additional locking is needed; all we need is verifications
- of any kind of N other copies of the content. -}
isSafeDrop (NumCopies n) l (Just (ContentRemovalLock _)) =
length (deDupVerifiedCopies l) >= n
{- Dropping from a remote repo.
-
- Unless numcopies is 0, at least one LockedCopy or TrustedCopy is required.
- A LockedCopy prevents races between concurrent drops from
- dropping the last copy, no matter what.
-
- The other N-1 copies can be less strong verifications, like
- RecentlyVerifiedCopy. While those are subject to concurrent drop races,
- and so could be dropped all at once, causing numcopies to be violated,
- this is the best that can be done without requiring that
- all special remotes support locking.
-}
isSafeDrop (NumCopies n) l Nothing
| n == 0 = True
| otherwise = and
[ length (deDupVerifiedCopies l) >= n
, any fullVerification l
]
fullVerification :: VerifiedCopy -> Bool
fullVerification (LockedCopy _) = True
fullVerification (TrustedCopy _) = True
fullVerification (RecentlyVerifiedCopy _) = False
-- A proof that it's currently safe to drop an object.
data SafeDropProof = SafeDropProof NumCopies [VerifiedCopy] (Maybe ContentRemovalLock)
deriving (Show)
-- Make sure that none of the VerifiedCopies have become invalidated
-- before constructing proof.
mkSafeDropProof :: NumCopies -> [VerifiedCopy] -> Maybe ContentRemovalLock -> IO (Either [VerifiedCopy] SafeDropProof)
mkSafeDropProof need have removallock = do
stillhave <- filterM checkVerifiedCopy have
return $ if isSafeDrop need stillhave removallock
then Right (SafeDropProof need stillhave removallock)
else Left stillhave
|
