fix ssh warmup hang

Fix race condition in ssh warmup that caused git-annex to get stuck and
never process some while when run with high levels of concurrency.

So far, I've isolated the problem to processTranscript, which hangs
reading output from ssh in this situation. I don't yet understand why
processTranscript behaves that way.

Since here we don't care about the ssh output, and only want to /dev/null
it, changed to not use processTranscript, avoiding its problem.

This commit was supported by the NSF-funded DataLad project.

Annex/Ssh.hs

@@ -34,7 +34,6 @@ import Annex.Path
 import Utility.Env
 import Utility.FileSystemEncoding
 import Utility.Hash
-import Utility.Process.Transcript
 import Types.CleanupActions
 import Types.Concurrency
 import Git.Env
@@ -219,13 +218,17 @@ prepSocket socketfile gc sshhost sshparams = do
 	-- return True.
 	-- (Except there's an unlikely false positive where a forced
 	-- ssh command exits 255.)
-	tryssh extraps = liftIO $ do
+	tryssh extraps = liftIO $ withNullHandle $ \nullh -> do
 		let p = proc "ssh" $ concat
 			[ extraps
 			, toCommand sshparams
 			, [fromSshHost sshhost, "true"]
 			]
-		(_, exitcode) <- processTranscript'' p Nothing
+		(Nothing, Nothing, Nothing, pid) <- createProcess $ p
+			{ std_out = UseHandle nullh
+			, std_err = UseHandle nullh
+			}
+		exitcode <- waitForProcess pid
 		return $ case exitcode of
 			ExitFailure 255 -> False
 			_ -> True