7 files changed, 65 insertions, 14 deletions
diff --git a/Annex/Branch.hs b/Annex/Branch.hs
index 5436132d8..ad96a2073 100644
--- a/Annex/Branch.hs
+++ b/Annex/Branch.hs
@@ -18,6 +18,7 @@ module Annex.Branch (
 	get,
 	getHistorical,
 	change,
+	maybeChange,
 	commit,
 	forceCommit,
 	files,
@@ -224,7 +225,15 @@ getRef ref file = withIndex $ decodeBS <$> catFile ref file
  - modifes the current content of the file on the branch.
  -}
 change :: FilePath -> (String -> String) -> Annex ()
-change file a = lockJournal $ \jl -> a <$> getLocal file >>= set jl file
+change file f = lockJournal $ \jl -> f <$> getLocal file >>= set jl file
+
+{- Applies a function which can modify the content of a file, or not. -}
+maybeChange :: FilePath -> (String -> Maybe String) -> Annex ()
+maybeChange file f = lockJournal $ \jl -> do
+	v <- getLocal file
+	case f v of
+		Just v' | v' /= v -> set jl file v'
+		_ -> noop
 
 {- Records new content of a file into the journal -}
 set :: JournalLocked -> FilePath -> String -> Annex ()
diff --git a/Logs/Location.hs b/Logs/Location.hs
index ba9c31abf..89100805b 100644
--- a/Logs/Location.hs
+++ b/Logs/Location.hs
@@ -48,7 +48,7 @@ logChange = logChange' logNow
 logChange' :: (LogStatus -> String -> Annex LogLine) -> Key -> UUID -> LogStatus -> Annex ()
 logChange' mklog key (UUID u) s = do
 	config <- Annex.getGitConfig
-	addLog (locationLogFile config key) =<< mklog s u
+	maybeAddLog (locationLogFile config key) =<< mklog s u
 logChange' _ _ NoUUID _ = noop
 
 {- Returns a list of repository UUIDs that, according to the log, have
diff --git a/Logs/Presence.hs b/Logs/Presence.hs
index 60e0c542a..f90253421 100644
--- a/Logs/Presence.hs
+++ b/Logs/Presence.hs
@@ -4,7 +4,7 @@
  - a way that can be union merged.
  -
  - A line of the log will look like: "date N INFO"
- - Where N=1 when the INFO is present, and 0 otherwise.
+ - Where N=1 when the INFO is present, 0 otherwise.
  - 
  - Copyright 2010-2014 Joey Hess <id@joeyh.name>
  -
@@ -14,6 +14,7 @@
 module Logs.Presence (
 	module X,
 	addLog,
+	maybeAddLog,
 	readLog,
 	logNow,
 	currentLog,
@@ -28,10 +29,21 @@ import Common.Annex
 import qualified Annex.Branch
 import Git.Types (RefDate)
 
+{- Adds a LogLine to the log, removing any LogLines that are obsoleted by
+ - adding it. -}
 addLog :: FilePath -> LogLine -> Annex ()
-addLog file line = Annex.Branch.change file $ \s -> 
+addLog file line = Annex.Branch.change file $ \s ->
 	showLog $ compactLog (line : parseLog s)
 
+{- When a LogLine already exists with the same status and info, but an
+ - older timestamp, that LogLine is preserved, rather than updating the log
+ - with a newer timestamp.
+ -}
+maybeAddLog :: FilePath -> LogLine -> Annex ()
+maybeAddLog file line = Annex.Branch.maybeChange file $ \s -> do
+	m <- insertNewStatus line $ logMap $ parseLog s
+	return $ showLog $ mapLog m
+
 {- Reads a log file.
  - Note that the LogLines returned may be in any order. -}
 readLog :: FilePath -> Annex [LogLine]
diff --git a/Logs/Presence/Pure.hs b/Logs/Presence/Pure.hs
index b1fc212fd..4e5ff68c0 100644
--- a/Logs/Presence/Pure.hs
+++ b/Logs/Presence/Pure.hs
@@ -61,21 +61,39 @@ filterPresent = filter (\l -> InfoPresent == status l) . compactLog
 {- Compacts a set of logs, returning a subset that contains the current
  - status. -}
 compactLog :: [LogLine] -> [LogLine]
-compactLog = M.elems . foldr mapLog M.empty
+compactLog = mapLog . logMap
 
 type LogMap = M.Map String LogLine
 
-{- Inserts a log into a map of logs, if the log has better (ie, newer)
- - information than the other logs in the map -}
-mapLog :: LogLine -> LogMap -> LogMap
-mapLog l m
-	| better = M.insert i l m
-	| otherwise = m
+mapLog :: LogMap -> [LogLine]
+mapLog = M.elems
+
+logMap :: [LogLine] -> LogMap
+logMap = foldr insertNewerLogLine M.empty
+
+insertBetter :: (LogLine -> Bool) -> LogLine -> LogMap -> Maybe LogMap
+insertBetter betterthan l m
+	| better = Just (M.insert i l m)
+	| otherwise = Nothing
   where
-	better = maybe True newer $ M.lookup i m
-	newer l' = date l' <= date l
+	better = maybe True betterthan (M.lookup i m)
 	i = info l
 
+{- Inserts a log into a map of logs, if the log has newer
+ - information than the other logs in the map for the same info. -}
+insertNewerLogLine :: LogLine -> LogMap -> LogMap
+insertNewerLogLine l m = fromMaybe m $ insertBetter newer l m
+  where
+	newer l' = date l' <= date l
+
+{- Inserts the log unless there's already one in the map with
+ - the same status for its info, in which case there's no need to
+ - change anything, to avoid log churn. -}
+insertNewStatus :: LogLine -> LogMap -> Maybe LogMap
+insertNewStatus l m  = insertBetter diffstatus l m
+  where
+	diffstatus l' = status l' /= status l
+
 instance Arbitrary LogLine where
 	arbitrary = LogLine
 		<$> arbitrary
diff --git a/debian/changelog b/debian/changelog
index 250e183a6..1f336b9ec 100644
--- a/debian/changelog
+++ b/debian/changelog
@@ -30,11 +30,13 @@ git-annex (5.20150931) UNRELEASED; urgency=medium
   * copy --auto was checking the wrong repo's preferred content.
     (--from was checking what --to should, and vice-versa.)
     Fixed this bug, which was introduced in version 5.20150727.
+  * Avoid unncessary write to the location log when a file is unlocked
+    and then added back with unchanged content.
   * Debian: Add torrent library to build-depends as it's packaged now,
     and stop recommending bittornado | bittorrent.
   * Debian: Remove dependency on transformers library, as it is now
     included in ghc.
-    
+
  -- Joey Hess <id@joeyh.name>  Thu, 01 Oct 2015 12:42:56 -0400
 
 git-annex (5.20150930) unstable; urgency=medium
diff --git a/doc/todo/wishlist:_allow_re-adding_without_generating_log_entry.mdwn b/doc/todo/wishlist:_allow_re-adding_without_generating_log_entry.mdwn
index d9ea181a8..258b82f6a 100644
--- a/doc/todo/wishlist:_allow_re-adding_without_generating_log_entry.mdwn
+++ b/doc/todo/wishlist:_allow_re-adding_without_generating_log_entry.mdwn
@@ -7,3 +7,5 @@ I keep a database file in git-annex so it stays synced across several machines.
     git annex sync --content
 
 This works fine except it creates an entry in the log for the database file even if that stays unchanged. I would like an option that just returns to the state before `git annex unlock` if the file has not been changed. Maybe `git annex add --restore-unchanged`?
+
+> [[done]] --[[Joey]]
diff --git a/doc/todo/wishlist:_allow_re-adding_without_generating_log_entry/comment_2_3a3d793b32b8440a8213b38bc83ea94a._comment b/doc/todo/wishlist:_allow_re-adding_without_generating_log_entry/comment_2_3a3d793b32b8440a8213b38bc83ea94a._comment
new file mode 100644
index 000000000..aeb98f06e
--- /dev/null
+++ b/doc/todo/wishlist:_allow_re-adding_without_generating_log_entry/comment_2_3a3d793b32b8440a8213b38bc83ea94a._comment
@@ -0,0 +1,8 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 2"""
+ date="2015-10-12T17:37:59Z"
+ content="""
+Ok, I found a way to implement it with no added overhead in the common
+case!
+"""]]