ensure outgoing sync pushes always succeed when repo is accessible

The fallback branches pushed to contain the uuid of the pusher, which is
ugly. That's why syncing doesn't normally use this method.

The merger deletes fallback branches after merging them, to contain the
ugliness, and so unused doesn't look at data from these branches.
(The fallback git-annex branch is left behind for now.)

1 files changed, 67 insertions, 24 deletions

diff --git a/Assistant/Sync.hs b/Assistant/Sync.hs
index c75767760..b78a2a5a4 100644
--- a/Assistant/Sync.hs
+++ b/Assistant/Sync.hs
@@ -17,10 +17,12 @@ import qualified Command.Sync
 import Utility.Parallel
 import qualified Git
 import qualified Git.Branch
+import qualified Git.Ref
 import qualified Git.Command
 import qualified Remote
 import qualified Types.Remote as Remote
 import qualified Annex.Branch
+import Annex.UUID
 
 import Data.Time.Clock
 import qualified Data.Map as M
@@ -56,48 +58,89 @@ reconnectRemotes threadname st dstatus scanremotes rs = void $
 			return (True, diverged)
 
 {- Updates the local sync branch, then pushes it to all remotes, in
- - parallel.
+ - parallel, along with the git-annex branch. This is the same
+ - as "git annex sync", except in parallel, and will co-exist with use of
+ - "git annex sync".
  -
  - Avoids running possibly long-duration commands in the Annex monad, so
- - as not to block other threads. -}
+ - as not to block other threads.
+ -
+ - This can fail, when the remote's sync branch (or git-annex branch) has
+ - been updated by some other remote pushing into it, or by the remote
+ - itself. To handle failure, a manual pull and merge is done, and the push
+ - is retried.
+ -
+ - When there's a lot of activity, we may fail more than once.
+ - On the other hand, we may fail because the remote is not available.
+ - Rather than retrying indefinitely, after the first retry we enter a
+ - fallback mode, where our push is guarenteed to succeed if the remote is
+ - reachable. If the fallback fails, the push is queued to be retried
+ - later.
+ -
+ - The fallback mode pushes to branches on the remote that have our uuid in
+ - them. While ugly, those branches are reserved for pushing by us, and
+ - so our pushes will succeed.
+ -}
 pushToRemotes :: ThreadName -> UTCTime -> ThreadState -> Maybe FailedPushMap -> [Remote] -> IO Bool
 pushToRemotes threadname now st mpushmap remotes = do
-	(g, branch) <- runThreadState st $
-		(,) <$> fromRepo id <*> inRepo Git.Branch.current
-	go True branch g remotes
+	(g, branch, u) <- runThreadState st $ (,,)
+		<$> fromRepo id
+		<*> inRepo Git.Branch.current
+		<*> getUUID
+	go True branch g u remotes
 	where
-		go _ Nothing _ _ = return True -- no branch, so nothing to do
-		go shouldretry (Just branch) g rs = do
+		go _ Nothing _ _ _ = return True -- no branch, so nothing to do
+		go shouldretry (Just branch) g u rs = do
 			debug threadname
 				[ "pushing to"
 				, show rs
 				]
 			Command.Sync.updateBranch (Command.Sync.syncBranch branch) g
 			(succeeded, failed) <- inParallel (push g branch) rs
+			updatemap succeeded []
 			let ok = null failed
-			case mpushmap of
-				Nothing -> noop
-				Just pushmap -> 
-					changeFailedPushMap pushmap $ \m ->
-						M.union (makemap failed) $
-							M.difference m (makemap succeeded)
-			unless ok $
-				debug threadname
-					[ "failed to push to"
-					, show failed
-					]
-			if ok || not shouldretry
+			if ok
 				then return ok
-				else retry branch g failed
+				else if shouldretry
+					then retry branch g u failed
+					else fallback branch g u failed
 
+		updatemap succeeded failed = case mpushmap of
+			Nothing -> noop
+			Just pushmap -> changeFailedPushMap pushmap $ \m ->
+				M.union (makemap failed) $
+					M.difference m (makemap succeeded)
 		makemap l = M.fromList $ zip l (repeat now)
 
-		push g branch remote = Command.Sync.pushBranch remote branch g
-
-		retry branch g rs = do
+		retry branch g u rs = do
 			debug threadname [ "trying manual pull to resolve failed pushes" ]
 			void $ manualPull st (Just branch) rs
-			go False (Just branch) g rs
+			go False (Just branch) g u rs
+
+		fallback branch g u rs = do
+			debug threadname
+				[ "fallback pushing to"
+				, show rs
+				]
+			(succeeded, failed) <- inParallel (pushfallback g u branch) rs
+			updatemap succeeded failed
+			return $ null failed
+			
+		push g branch remote = Command.Sync.pushBranch remote branch g
+		pushfallback g u branch remote = Git.Command.runBool "push"
+			[ Param $ Remote.name remote
+			, Param $ refspec Annex.Branch.name
+			, Param $ refspec branch
+			] g
+			where
+				refspec b = concat
+					[ s
+					, ":"
+					, show $ Git.Ref.base $
+						Command.Sync.syncBranch $ Git.Ref $
+							"fallback" </> fromUUID u </> s
+					]
+					where s = show $ Git.Ref.base b
 
 {- Manually pull from remotes and merge their branches. -}
 manualPull :: ThreadState -> Maybe Git.Ref -> [Remote] -> IO Bool