mincopies

This is conceptually very simple, just making a 1 that was hard coded be
exposed as a config option. The hard part was plumbing all that, and
dealing with complexities like reading it from git attributes at the
same time that numcopies is read.

Behavior change: When numcopies is set to 0, git-annex used to drop
content without requiring any copies. Now to get that (highly unsafe)
behavior, mincopies also needs to be set to 0. It seemed better to
remove that edge case, than complicate mincopies by ignoring it when
numcopies is 0.

This commit was sponsored by Denis Dzyubenko on Patreon.

Types/NumCopies.hs

@@ -1,6 +1,6 @@
 {- git-annex numcopies types
  -
- - Copyright 2014-2015 Joey Hess <id@joeyh.name>
+ - Copyright 2014-2021 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -8,6 +8,8 @@
 module Types.NumCopies (
 	NumCopies(..),
 	fromNumCopies,
+	MinCopies(..),
+	fromMinCopies,
 	VerifiedCopy(..),
 	checkVerifiedCopy,
 	invalidateVerifiedCopy,
@@ -39,6 +41,12 @@ newtype NumCopies = NumCopies Int
 fromNumCopies :: NumCopies -> Int
 fromNumCopies (NumCopies n) = n
 
+newtype MinCopies = MinCopies Int
+	deriving (Ord, Eq, Show)
+
+fromMinCopies :: MinCopies -> Int
+fromMinCopies (MinCopies n) = n
+
 -- Indicates that a key's content is exclusively
 -- locked locally, pending removal.
 newtype ContentRemovalLock = ContentRemovalLock Key
@@ -130,33 +138,33 @@ withVerifiedCopy mk u check = bracketIO setup cleanup
  - 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.
+ - to fall below NumCopies, but it will never fall below MinCopies.
  -}
-isSafeDrop :: NumCopies -> [VerifiedCopy] -> Maybe ContentRemovalLock -> Bool
+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 N other copies of the content. -}
-isSafeDrop (NumCopies n) l (Just (ContentRemovalLock _)) = 
+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.
+ - 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 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 
+ - 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) l Nothing
-	| n == 0 = True
+isSafeDrop (NumCopies n) (MinCopies m) l Nothing
+	| n == 0 && m == 0 = True
 	| otherwise = and
 		[ length (deDupVerifiedCopies l) >= n
-		, any fullVerification l
+		, length (filter fullVerification l) >= m
 		]
 
 fullVerification :: VerifiedCopy -> Bool
@@ -165,14 +173,14 @@ fullVerification (TrustedCopy _) = True
 fullVerification (RecentlyVerifiedCopy _) = False
 
 -- A proof that it's currently safe to drop an object.
-data SafeDropProof = SafeDropProof NumCopies [VerifiedCopy] (Maybe ContentRemovalLock)
+data SafeDropProof = SafeDropProof NumCopies MinCopies [VerifiedCopy] (Maybe ContentRemovalLock)
 	deriving (Show)
 
--- Make sure that none of the VerifiedCopies have become invalidated
+-- Makes 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
+mkSafeDropProof :: NumCopies -> MinCopies -> [VerifiedCopy] -> Maybe ContentRemovalLock -> IO (Either [VerifiedCopy] SafeDropProof)
+mkSafeDropProof need mincopies have removallock = do
 	stillhave <- filterM checkVerifiedCopy have
-	return $ if isSafeDrop need stillhave removallock
-		then Right (SafeDropProof need stillhave removallock)
+	return $ if isSafeDrop need mincopies stillhave removallock
+		then Right (SafeDropProof need mincopies stillhave removallock)
 		else Left stillhave