fixup merges now done when needed

Assistant/Threads/Merger.hs

@@ -70,3 +70,11 @@ onAdd g file _
 
 mergeBranch :: Git.Ref -> Git.Repo -> IO Bool
 mergeBranch = Git.Merge.mergeNonInteractive . Command.Sync.syncBranch
+
+{- Manually pull from remotes and merge their branches. Called by the pusher
+ - when a push fails, which can happen due to a remote not having pushed
+ - changes to us. That could be because it doesn't have us as a remote, or
+ - because the assistant is not running there, or other reasons. -}
+manualPull :: Git.Ref -> [Remote] -> Annex ()
+manualPull currentbranch remotes = forM_ remotes $ \r ->
+	Command.Sync.mergeRemote r currentbranch

Assistant/Threads/Pusher.hs

@@ -11,6 +11,7 @@ import Common.Annex
 import Assistant.Commits
 import Assistant.Pushes
 import Assistant.ThreadedMonad
+import Assistant.Threads.Merger
 import qualified Command.Sync
 import Utility.ThreadScheduler
 import Utility.Parallel
@@ -68,11 +69,19 @@ pushToRemotes :: UTCTime -> ThreadState -> FailedPushChan -> [Remote] -> IO ()
 pushToRemotes now st pushchan remotes = do
 	(g, branch) <- runThreadState st $
 		(,) <$> fromRepo id <*> Command.Sync.currentBranch
-	Command.Sync.updateBranch (Command.Sync.syncBranch branch) g
-	failed <- map (`FailedPush` now) <$> inParallel (push g branch) remotes
-	unless (null failed) $
-		refillFailedPushes pushchan failed
+	go True branch g remotes
 	where
+		go shouldretry branch g rs = do
+			Command.Sync.updateBranch (Command.Sync.syncBranch branch) g
+			failed <- inParallel (push g branch) rs
+			unless (null failed) $
+				if shouldretry
+					then retry branch g rs
+					else refillFailedPushes pushchan $
+						map (`FailedPush` now) failed
 		push g branch remote =
 			ifM (Command.Sync.pushBranch remote branch g)
 				( exitSuccess, exitFailure)
+		retry branch g rs = do
+			runThreadState st $ manualPull branch rs
+			go False branch g rs

doc/design/assistant/syncing.mdwn

@@ -15,7 +15,7 @@ all the other git clones, at both the git level and the key/value level.
    **done**
 1. Periodically retry pushes that failed.  **done** (every half an hour)
 1. Also, detect if a push failed due to not being up-to-date, pull,
-   and repush.
+   and repush. **done**
 2. Use a git merge driver that adds both conflicting files,
    so conflicts never break a sync.
 3. Investigate the XMPP approach like dvcs-autosync does, or other ways of