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authorGravatar Joey Hess <joey@kitenet.net>2011-12-11 18:39:53 -0400
committerGravatar Joey Hess <joey@kitenet.net>2011-12-11 20:41:35 -0400
commitc4c965d602687a6277d43ae003c12dc4c132ba28 (patch)
tree2d114981e8779bbb1738a15cc12c1804cefdee54 /Annex/CatFile.hs
parent81f311103d99ec5bfd31ae5a76d6add05ff40121 (diff)
detect and recover from branch push/commit race
Dealing with a race without using locking is exceedingly difficult and tricky. Fully tested, I hope. There are three places left where the branch can be updated, that are not covered by the race recovery code. Let's prove they're all immune to the race: 1. tryFastForwardTo checks to see if a fast-forward can be done, and then does git-update-ref on the branch to fast-forward it. If a push comes in before the check, then either no fast-forward will be done (ok), or the push set the branch to a ref that can still be fast-forwarded (also ok) If a push comes in after the check, the git-update-ref will undo the ref change made by the push. It's as if the push did not come in, and the next git-push will see this, and try to re-do it. (acceptable) 2. When creating the branch for the very first time, an empty index is created, and a commit of it made to the branch. The commit's ref is recorded as the current state of the index. If a push came in during that, it will be noticed the next time a commit is made to the branch, since the branch will have changed. (ok) 3. Creating the branch from an existing remote branch involves making the branch, and then getting its ref, and recording that the index reflects that ref. If a push creates the branch first, git-branch will fail (ok). If the branch is created and a racing push is then able to change it (highly unlikely!) we're still ok, because it first records the ref into the index.lck, and then updating the index. The race can cause the index.lck to have the old branch ref, while the index has the newly pushed branch merged into it, but that only results in an unnecessary update of the index file later on.
Diffstat (limited to 'Annex/CatFile.hs')
-rw-r--r--Annex/CatFile.hs6
1 files changed, 6 insertions, 0 deletions
diff --git a/Annex/CatFile.hs b/Annex/CatFile.hs
index 1d996edfd..bcf44551e 100644
--- a/Annex/CatFile.hs
+++ b/Annex/CatFile.hs
@@ -7,6 +7,7 @@
module Annex.CatFile (
catFile,
+ catObject,
catFileHandle
) where
@@ -22,6 +23,11 @@ catFile branch file = do
h <- catFileHandle
liftIO $ Git.CatFile.catFile h branch file
+catObject :: Git.Ref -> Annex L.ByteString
+catObject ref = do
+ h <- catFileHandle
+ liftIO $ Git.CatFile.catObject h ref
+
catFileHandle :: Annex Git.CatFile.CatFileHandle
catFileHandle = maybe startup return =<< Annex.getState Annex.catfilehandle
where