diff options
Diffstat (limited to 'Types/NumCopies.hs')
| -rw-r--r-- | Types/NumCopies.hs | 54 |
1 files changed, 40 insertions, 14 deletions
diff --git a/Types/NumCopies.hs b/Types/NumCopies.hs index bbd1b3831..60e0db580 100644 --- a/Types/NumCopies.hs +++ b/Types/NumCopies.hs @@ -19,9 +19,11 @@ module Types.NumCopies ( isSafeDrop, SafeDropProof, mkSafeDropProof, + ContentRemovalLock(..), ) where import Types.UUID +import Types.Key import Utility.Exception (bracketIO) import qualified Data.Map as M @@ -36,6 +38,11 @@ newtype NumCopies = NumCopies Int 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 @@ -48,7 +55,7 @@ data VerifiedCopy {- 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 dropped by any git-annex process. -} + - not to be removed by any git-annex process. -} | LockedCopy V deriving (Show) @@ -116,20 +123,39 @@ withVerifiedCopy mk u = bracketIO setup cleanup {- Check whether enough verification has been done of copies to allow - dropping content safely. - - - 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. + - 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 all special remotes - - to support locking. + - this is the best that can be done without requiring that + - all special remotes support locking. -} -isSafeDrop :: NumCopies -> [VerifiedCopy] -> Bool -isSafeDrop (NumCopies n) l +isSafeDrop (NumCopies n) l Nothing | n == 0 = True - | otherwise = length (deDupVerifiedCopies l) >= n && any fullVerification l + | otherwise = and + [ length (deDupVerifiedCopies l) >= n + , any fullVerification l + ] fullVerification :: VerifiedCopy -> Bool fullVerification (LockedCopy _) = True @@ -137,14 +163,14 @@ fullVerification (TrustedCopy _) = True fullVerification (RecentlyVerifiedCopy _) = False -- A proof that it's currently safe to drop an object. -data SafeDropProof = SafeDropProof NumCopies [VerifiedCopy] +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] -> IO (Either [VerifiedCopy] SafeDropProof) -mkSafeDropProof need have = do +mkSafeDropProof :: NumCopies -> [VerifiedCopy] -> Maybe ContentRemovalLock -> IO (Either [VerifiedCopy] SafeDropProof) +mkSafeDropProof need have removallock = do stillhave <- filterM checkVerifiedCopy have - return $ if isSafeDrop need stillhave - then Right (SafeDropProof need stillhave) + return $ if isSafeDrop need stillhave removallock + then Right (SafeDropProof need stillhave removallock) else Left stillhave |