pushed checkPresent exception handling out of Remote implementations

I tend to prefer moving toward explicit exception handling, not away from
it, but in this case, I think there are good reasons to let checkPresent
throw exceptions:

1. They can all be caught in one place (Remote.hasKey), and we know
   every possible exception is caught there now, which we didn't before.
2. It simplified the code of the Remotes. I think it makes sense for
   Remotes to be able to be implemented without needing to worry about
   catching exceptions inside them. (Mostly.)
3. Types.StoreRetrieve.Preparer can only work on things that return a
   Bool, which all the other relevant remote methods already did.
   I do not see a good way to generalize that type; my previous attempts
   failed miserably.

1 files changed, 8 insertions, 0 deletions

diff --git a/Remote.hs b/Remote.hs
index 29097f77d..5ee75823f 100644
--- a/Remote.hs
+++ b/Remote.hs
@@ -56,6 +56,7 @@ import Data.Ord
 import Common.Annex
 import Types.Remote
 import qualified Annex
+import Annex.Exception
 import Annex.UUID
 import Logs.UUID
 import Logs.Trust
@@ -312,3 +313,10 @@ isXMPPRemote :: Remote -> Bool
 isXMPPRemote remote = Git.repoIsUrl r && "xmpp::" `isPrefixOf` Git.repoLocation r
   where
 	r = repo remote
+
+hasKey :: Remote -> Key -> Annex (Either String Bool)
+hasKey r k = either (Left  . show) Right
+	<$> tryNonAsyncAnnex (checkPresent r k)
+
+hasKeyCheap :: Remote -> Bool
+hasKeyCheap = checkPresentCheap