sync: Pushes the git-annex branch to remote/synced/git-annex, rather than directly to remote/git-annex.

This fixes a problem I was seeing in the assistant where two remotes would
attempt to sync with one another at the same time, and both failed pushing
the diverged git-annex branch. Then when both tried to resolve the failed
push, they each modified their git-annex branch, which again each blocked
the other from pushing into it. The result was that the git-annex
branches were perpetually diverged (despite having the same content!) and
once the assistant fell into this trap, it couldn't get out and always
had to do the slow push/fail/pull/merge/push/fail cycle.

Command/Sync.hs

@@ -154,15 +154,15 @@ pushRemote remote branch = go =<< needpush
 pushBranch :: Remote -> Git.Ref -> Git.Repo -> IO Bool
 pushBranch remote branch g =
 	Git.Command.runBool "push"
-		[ Param (Remote.name remote)
+		[ Param $ Remote.name remote
-		, Param (show Annex.Branch.name)
+		, Param $ refspec Annex.Branch.name
-		, Param refspec
+		, Param $ refspec branch
 		] g
 	where
-		refspec = concat 
+		refspec b = concat 
-			[ show $ Git.Ref.base branch
+			[ show $ Git.Ref.base b
 			,  ":"
-			, show $ Git.Ref.base $ syncBranch branch
+			, show $ Git.Ref.base $ syncBranch b
 			]
 
 mergeAnnex :: CommandStart

debian/changelog

@@ -22,6 +22,8 @@ git-annex (3.20120826) UNRELEASED; urgency=low
     rather than moving to .git/annex/bad/
   * Avoid crashing on encoding errors in filenames when writing transfer info
     files and reading from checksum commands.
+  * sync: Pushes the git-annex branch to remote/synced/git-annex, rather
+    than directly to remote/git-annex.
 
  -- Joey Hess <joeyh@debian.org>  Mon, 27 Aug 2012 13:27:39 -0400
 

doc/design/assistant/syncing.mdwn

@@ -9,8 +9,37 @@ all the other git clones, at both the git level and the key/value level.
   (in a fresh clone each time) several times in a row, but then stops happening,
   which has prevented me from debugging it.
   This could possibly have been caused by the bug fixed in 750c4ac6c282d14d19f79e0711f858367da145e4.
-* The git repository syncing sometimes fails due to the remote having updated.
+
-  While syncing retries, this sometimes doesn't work.
+* The transfer code doesn't always manage to transfer file contents.
+
+  Besides reconnection events, there are two places where transfers get queued:
+
+  1. When the committer commits a file, it queues uploads.
+  2. When the watcher sees a broken symlink be created, it queues downloads.
+
+  Consider a doubly-linked chain of three repositories, A B and C.
+  (C and A do not directly communicate.)
+  
+  * File is added to A.
+  * A uploads its content to B.
+  * At the same time, A git syncs to B.
+  * Once B gets the git sync, it git syncs to C.
+  * When C's watcher sees the file appear, it tries to download it. But if
+    B had not finished receiving the file from A, C doesn't know B has it,
+    and cannot download it from anywhere.
+
+  Possible solution: After B receives content, it could queue uploads of it 
+  to all remotes that it doesn't know have it yet, which would include C.
+  
+  In practice, this has the problem that when C receives the content,
+  it will queue uploads of it, which can send back to B (or to some other repo
+  that already has the content) and loop, until the git-annex branches catch
+  up and break the cycle.
+
+  Possible solution: C could record a download intent. (Similar to a failed
+  download, but with an unknown source.) When C next receives a git-annex
+  branch push, it could try to requeue downloads that it has such intents
+  registered for.
 
 ## TODO
 

doc/todo/assistant_git_sync_laddering.mdwn

@@ -5,4 +5,6 @@ that then has to be auto-resolved.
 This seems similar to the laddering problem described in this old bug:
 [[bugs/making_annex-merge_try_a_fast-forward]]
 
---[[Joey]] 
+--[[Joey]]
+
+Think I've fixed this. [[done]] --[[Joey]]