run Preparer to get Remover and CheckPresent actions

This will allow special remotes to eg, open a http connection and reuse it,
while checking if chunks are present, or removing chunks.

S3 and WebDAV both need this to support chunks with reasonable speed.

Note that a special remote might want to cache a http connection across
multiple requests. A simple case of this is that CheckPresent is typically
called before Store or Remove. A remote using this interface can certianly
use a Preparer that eg, uses a MVar to cache a http connection.

However, it's up to the remote to then deal with things like stale or
stalled http connections when eg, doing a series of downloads from a remote
and other places. There could be long delays between calls to a remote,
which could lead to eg, http connection stalls; the machine might even
move to a new network, etc.

It might be nice to improve this interface later to allow
the simple case without needing to handle the full complex case.
One way to do it would be to have a `Transaction SpecialRemote cache`,
where SpecialRemote contains methods for Storer, Retriever, Remover, and
CheckPresent, that all expect to be passed a `cache`.

1 files changed, 12 insertions, 2 deletions

diff --git a/Remote/Helper/Messages.hs b/Remote/Helper/Messages.hs
index 3088a9ab2..774716ca1 100644
--- a/Remote/Helper/Messages.hs
+++ b/Remote/Helper/Messages.hs
@@ -9,9 +9,19 @@ module Remote.Helper.Messages where
 
 import Common.Annex
 import qualified Git
+import qualified Types.Remote as Remote
 
 showChecking :: Git.Repo -> Annex ()
 showChecking r = showAction $ "checking " ++ Git.repoDescribe r
 
-cantCheck :: Git.Repo -> a
-cantCheck r = error $ "unable to check " ++ Git.repoDescribe r
+class Checkable a where
+	descCheckable :: a -> String
+
+instance Checkable Git.Repo where
+	descCheckable = Git.repoDescribe
+
+instance Checkable (Remote.RemoteA a) where
+	descCheckable = Remote.name
+
+cantCheck :: Checkable a => a -> e
+cantCheck v = error $ "unable to check " ++ descCheckable v