avoid trying to create a content file in order to lock it

The nice refactoring in ec7dd0446a
highlighted a bug in lockContent -- when the content is not present,
this incorrectly created an empty lock file, using the same filename
as the content file.

This seems like it could result in empty objects, which fsck would detect
and complain about. Both drop and move --to call lockContent, as does
Remote.Git.dropKey -- I think we got lucky and this bug didn't show up
because both all of those only operate on files that are present. So
this bug could only manifest if there was a race, and a file's content
was dropped at just the wrong time, just as another process was about to
drop it. (And then only if the other process's dropping failed, otherwise
it'd delete the empty object file.)

Hmm, move --from also called lockContent. Unnecessarily, since the content
is not being removed from the local annex. In this case, the combination of
the 2 bugs could result in an empty lock file being written, and then if
the download of the content failed, left in the object directory as the
content.

This commit also optimises lockContent, avoiding an unncessary
doesFileExist test and instead just catching the exception that's thrown
when the file doesn't exist.

This commit was sponsored by Justine Lam.

Annex/Content.hs

@@ -175,12 +175,10 @@ lockContent key a = do
 	lock _ (Just lockfile) = createLockFile Nothing lockfile >>= dolock . Just
 	{- Since content files are stored with the write bit disabled, have
 	 - to fiddle with permissions to open for an exclusive lock. -}
-	opencontentforlock f = catchMaybeIO $ ifM (doesFileExist f)
-		( withModifiedFileMode f
+	opencontentforlock f = catchDefaultIO Nothing $ 
+		withModifiedFileMode f
 			(`unionFileModes` ownerWriteMode)
-			(createLockFile Nothing f)
-		, createLockFile Nothing f
-		)
+			(openExistingLockFile f)
 	dolock Nothing = return Nothing
 	dolock (Just fd) = do
 		v <- tryIO $ setLock fd (WriteLock, AbsoluteSeek, 0, 0)

Command/Move.hs

@@ -150,11 +150,10 @@ fromOk src key = go =<< Annex.getState Annex.force
 		return $ u /= Remote.uuid src && elem src remotes
 
 fromPerform :: Remote -> Bool -> Key -> AssociatedFile -> CommandPerform
-fromPerform src move key afile = moveLock move key $
-	ifM (inAnnex key)
-		( dispatch move True
-		, dispatch move =<< go
-		)
+fromPerform src move key afile = ifM (inAnnex key)
+	( dispatch move True
+	, dispatch move =<< go
+	)
   where
 	go = notifyTransfer Download afile $ 
 		download (Remote.uuid src) key afile noRetry $ \p -> do
@@ -166,8 +165,9 @@ fromPerform src move key afile = moveLock move key $
 		ok <- Remote.removeKey src key
 		next $ Command.Drop.cleanupRemote key src ok
 
-{- Locks a key in order for it to be moved.
- - No lock is needed when a key is being copied. -}
+{- Locks a key in order for it to be moved away from the current repository.
+ - No lock is needed when a key is being copied, or moved to the current
+ - repository. -}
 moveLock :: Bool -> Key -> Annex a -> Annex a
 moveLock True key a = lockContent key a
 moveLock False _ a = a