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|
{- git-annex numcopies types
-
- Copyright 2014-2022 Joey Hess <id@joeyh.name>
-
- Licensed under the GNU AGPL version 3 or higher.
-}
module Types.NumCopies (
NumCopies,
configuredNumCopies,
fromNumCopies,
MinCopies,
configuredMinCopies,
fromMinCopies,
VerifiedCopy(..),
checkVerifiedCopy,
invalidateVerifiedCopy,
strongestVerifiedCopy,
deDupVerifiedCopies,
mkVerifiedCopy,
invalidatableVerifiedCopy,
withVerifiedCopy,
isSafeDrop,
SafeDropProof,
mkSafeDropProof,
ContentRemovalLock(..),
) where
import Types.UUID
import Types.Key
import Utility.Exception (bracketIO)
import Utility.Monad
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)
-- Smart constructor; prevent configuring numcopies to 0 which would
-- cause data loss.
configuredNumCopies :: Int -> NumCopies
configuredNumCopies n
| n > 0 = NumCopies n
| otherwise = NumCopies 1
fromNumCopies :: NumCopies -> Int
fromNumCopies (NumCopies n) = n
newtype MinCopies = MinCopies Int
deriving (Ord, Eq, Show)
configuredMinCopies :: Int -> MinCopies
configuredMinCopies n
| n > 0 = MinCopies n
| otherwise = MinCopies 1
fromMinCopies :: MinCopies -> Int
fromMinCopies (MinCopies 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 Bool -> IO VerifiedCopy
invalidatableVerifiedCopy mk u check = do
v <- newEmptyMVar
let invalidate = do
_ <- tryPutMVar v ()
return ()
let check' = isEmptyMVar v <&&> check
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
:: (MonadMask m, MonadIO m, ToUUID u)
=> (V -> VerifiedCopy)
-> u
-> IO Bool
-> (VerifiedCopy -> m a)
-> m a
withVerifiedCopy mk u check = bracketIO setup cleanup
where
setup = invalidatableVerifiedCopy mk u check
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 MinCopies.
-}
isSafeDrop :: NumCopies -> MinCopies -> [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 enough other copies of the content. -}
isSafeDrop (NumCopies n) (MinCopies m) l (Just (ContentRemovalLock _)) =
length (deDupVerifiedCopies l) >= max n m
{- Dropping from a remote repo.
-
- To guarantee MinCopies is never violated, at least that many LockedCopy
- or TrustedCopy are required. A LockedCopy prevents races between
- concurrent drops from dropping the last copy, no matter what.
-
- The other copies required by NumCopies 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) (MinCopies m) l Nothing
| n == 0 && m == 0 = True
| otherwise = and
[ length (deDupVerifiedCopies l) >= n
, length (filter fullVerification l) >= m
]
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 MinCopies [VerifiedCopy] (Maybe ContentRemovalLock)
deriving (Show)
-- Makes sure that none of the VerifiedCopies have become invalidated
-- before constructing proof.
mkSafeDropProof :: NumCopies -> MinCopies -> [VerifiedCopy] -> Maybe ContentRemovalLock -> IO (Either [VerifiedCopy] SafeDropProof)
mkSafeDropProof need mincopies have removallock = do
stillhave <- filterM checkVerifiedCopy have
return $ if isSafeDrop need mincopies stillhave removallock
then Right (SafeDropProof need mincopies stillhave removallock)
else Left stillhave
|
