unused: Reduce memory usage significantly.

Much of the memory bloat turned out to be due to getKeysReferenced
containing a mapM, which is strict and buffered the whole list
rather than streaming it.

The other half of the bloat was due to building a temporary Set
in order to call S.difference. While that is more cpu efficient,
I switched to successive S.delete, since with it, I can run a whole
git annex unused in less than 8 mb of memory.

The whole Set of keys with content available is still stored in memory,
so running unused in a repo with a whole lot of file content will still
use more memory. In a repo containing 6000 files, it needed 40 mb.

Note that the status command still uses the bloatful getKeysReferenced.

3 files changed, 48 insertions, 21 deletions

diff --git a/Command/Unused.hs b/Command/Unused.hs
index bbef835f7..ba14bfc4a 100644
--- a/Command/Unused.hs
+++ b/Command/Unused.hs
@@ -155,9 +155,9 @@ unusedKeys = do
 excludeReferenced :: [Key] -> Annex [Key]
 excludeReferenced [] = return [] -- optimisation
 excludeReferenced l = do
-	c <- inRepo $ Git.Command.pipeRead [Param "show-ref"]
-	removewith (getKeysReferenced : map getKeysReferencedInGit (refs c))
-		(S.fromList l)
+	let s = S.fromList l
+	!s' <- withKeysReferenced s S.delete
+	go s' =<< refs <$> (inRepo $ Git.Command.pipeRead [Param "show-ref"])
 	where
 		-- Skip the git-annex branches, and get all other unique refs.
 		refs = map (Git.Ref .  snd) .
@@ -167,13 +167,12 @@ excludeReferenced l = do
 		uniqref (a, _) (b, _) = a == b
 		ourbranchend = '/' : show Annex.Branch.name
 		ourbranches (_, b) = not $ ourbranchend `isSuffixOf` b
-		removewith [] s = return $ S.toList s
-		removewith (a:as) s
+		go s [] = return $ S.toList s
+		go s (r:rs)
 			| s == S.empty = return [] -- optimisation
 			| otherwise = do
-				referenced <- a
-				let !s' = s `S.difference` S.fromList referenced
-				removewith as s'
+				!s' <- withKeysReferencedInGit r s S.delete
+				go s' rs
 
 {- Finds items in the first, smaller list, that are not
  - present in the second, larger list.
@@ -195,20 +194,37 @@ getKeysReferenced = do
 	keypairs <- mapM Backend.lookupFile files
 	return $ map fst $ catMaybes keypairs
 
-{- List of keys referenced by symlinks in a git ref. -}
-getKeysReferencedInGit :: Git.Ref -> Annex [Key]
-getKeysReferencedInGit ref = do
+{- Given an initial value, mutates it using an action for each
+ - key referenced by symlinks in the git repo. -}
+withKeysReferenced :: v -> (Key -> v -> v) -> Annex v
+withKeysReferenced initial a = do
+	top <- fromRepo Git.workTree
+	go initial =<< inRepo (LsFiles.inRepo [top])
+	where
+		go v [] = return v
+		go v (f:fs) = do
+			x <- Backend.lookupFile f
+			case x of
+				Nothing -> go v fs
+				Just (k, _) -> do
+					let !v' = a k v
+					go v' fs
+
+withKeysReferencedInGit :: Git.Ref -> v -> (Key -> v -> v) -> Annex v
+withKeysReferencedInGit ref initial a = do
 	showAction $ "checking " ++ Git.Ref.describe ref
-	findkeys [] =<< inRepo (LsTree.lsTree ref)
+	go initial =<< inRepo (LsTree.lsTree ref)
 	where
-		findkeys c [] = return c
-		findkeys c (l:ls)
+		go v [] = return v
+		go v (l:ls)
 			| isSymLink (LsTree.mode l) = do
 				content <- L.decodeUtf8 <$> catFile ref (LsTree.file l)
 				case fileKey (takeFileName $ L.unpack content) of
-					Nothing -> findkeys c ls
-					Just k -> findkeys (k:c) ls
-			| otherwise = findkeys c ls
+					Nothing -> go v ls
+					Just k -> do
+						let !v' = a k v
+						go v' ls
+			| otherwise = go v ls
 
 {- Looks in the specified directory for bad/tmp keys, and returns a list
  - of those that might still have value, or might be stale and removable. 
diff --git a/debian/changelog b/debian/changelog
index 10dc1c50c..2cb5a1aea 100644
--- a/debian/changelog
+++ b/debian/changelog
@@ -3,6 +3,9 @@ git-annex (3.20120310) UNRELEASED; urgency=low
   * fsck: Fix up any broken links and misplaced content caused by the
     directory hash calculation bug fixed in the last release.
   * sync: Sync to lower cost remotes first.
+  * unused: Reduce memory usage significantly. Still not constant
+    space, but now only needs to store the set of file contents that
+    are present in the annex in memory.
 
  -- Joey Hess <joeyh@debian.org>  Sat, 10 Mar 2012 14:03:22 -0400
 
diff --git a/doc/todo/git-annex_unused_eats_memory.mdwn b/doc/todo/git-annex_unused_eats_memory.mdwn
index 3e9942e98..91f9a33f7 100644
--- a/doc/todo/git-annex_unused_eats_memory.mdwn
+++ b/doc/todo/git-annex_unused_eats_memory.mdwn
@@ -1,12 +1,20 @@
 `git-annex unused` has to compare large sets of data
 (all keys with content present in the repository,
 with all keys used by files in the repository), and so
-uses more memory than git-annex typically needs; around
-50 mb when run in a repository with 80 thousand files.
+uses more memory than git-annex typically needs.
 
-(Used to be 80 mb, but implementation improved.)
+It used to be a lot worse (hundreds of megabytes).
 
-I would like to reduce this. One idea is to use a bloom filter. 
+Now it only needs enough memory to store a Set of all Keys that currently
+have content in the annex. On a lightly populated repository, it runs in
+quite low memory use (like 8 mb) even if the git repo has 100 thousand
+files. On a repository with lots of file contents, it will use more.
+
+Still, I would like to reduce this to a purely constant memory use,
+as running in constant memory no matter the repo size is a git-annex design
+goal.
+
+One idea is to use a bloom filter. 
 For example, construct a bloom filter of all keys used by files in
 the repository. Then for each key with content present, check if it's
 in the bloom filter. Since there can be false positives, this might