There is a bug in speculateFileOrUrl.
It puts downloaded file nonatomically.

There is a window when copyFileOrUrl can (and does)
copy partially downloaded file.

Darcs-bug: http://bugs.darcs.net/issue2364
diff --git a/src/Darcs/External.hs b/src/Darcs/External.hs
index 2e0e791..d5a0b9f 100644
--- a/src/Darcs/External.hs
+++ b/src/Darcs/External.hs
@@ -184,7 +184,7 @@ copyFileOrUrl rd   fou out _     | isSshUrl  fou = copySSH rd (splitSshUrl fou)
 copyFileOrUrl _    fou _   _     = fail $ "unknown transport protocol: " ++ fou
 
 speculateFileOrUrl :: String -> FilePath -> IO ()
-speculateFileOrUrl fou out | isHttpUrl fou = speculateRemote fou out
+speculateFileOrUrl fou out | isHttpUrl fou = speculateRemote fou out >> waitUrl fou
                            | otherwise = return ()
 
 copyLocal  :: String -> FilePath -> IO ()