support being run by ssh as ssh-askpass replacement

To use, set GIT_ANNEX_SSHASKPASS to point to a fifo or regular file
(FIFO is better, avoids touching disk or multiple readers) that contains
the password. Then set SSH_ASKPASS=git-annex, and when ssh runs it, it will
tell ssh the password.

This is not yet used..

Annex/Ssh.hs

@@ -16,6 +16,8 @@ module Annex.Ssh (
 	sshCachingTo,
 	inRepoWithSshCachingTo,
 	runSshCaching,
+	sshAskPassEnv,
+	runSshAskPass
 ) where
 
 import qualified Data.Map as M
@@ -230,7 +232,7 @@ sshReadPort params = (port, reverse args)
 {- When this env var is set, git-annex runs ssh with parameters
  - to use the socket file that the env var contains.
  -
- - This is a workaround for GiT_SSH not being able to contain
+ - This is a workaround for GIT_SSH not being able to contain
  - additional parameters to pass to ssh. -}
 sshCachingEnv :: String
 sshCachingEnv = "GIT_ANNEX_SSHCACHING"
@@ -268,8 +270,17 @@ sshCachingTo remote g
   where
 	uncached = return g
 
-runSshCaching :: [String] -> String -> IO ()
+runSshCaching :: [String] -> FilePath -> IO ()
 runSshCaching args sockfile = do
 	let args' = toCommand (sshConnectionCachingParams sockfile) ++ args
 	let p = proc "ssh" args'
 	exitWith =<< waitForProcess . processHandle =<< createProcess p
+
+{- When this env var is set, git-annex is being used as a ssh-askpass
+ - program, and should read the password from the specified location,
+ - and output it for ssh to read. -}
+sshAskPassEnv :: String
+sshAskPassEnv = "GIT_ANNEX_SSHASKPASS"
+
+runSshAskPass :: FilePath -> IO ()
+runSshAskPass passfile = putStrLn =<< readFile passfile

CmdLine/GitAnnex.hs

@@ -199,5 +199,11 @@ run args = do
 #ifdef WITH_EKG
 	_ <- forkServer "localhost" 4242
 #endif
-	maybe (dispatch True args cmds gitAnnexOptions [] header Git.CurrentRepo.get)
-		(runSshCaching args) =<< getEnv sshCachingEnv
+	go envmodes
+  where
+	go [] = dispatch True args cmds gitAnnexOptions [] header Git.CurrentRepo.get
+	go ((v, a):rest) = maybe (go rest) a =<< getEnv v
+	envmodes =
+		[ (sshCachingEnv, runSshCaching args)
+		, (sshAskPassEnv, runSshAskPass)
+		]

doc/design/assistant/sshpassword.mdwn

@@ -41,7 +41,7 @@ which gets the password from the webapp, and outputs it to stdout.
 Seems to call for the webapp and program to communicate over a local
 socket (locked down so only user can access) or environment.
 Environment is not as secure (easily snooped by root).
-Local socket probably won't work on Windows.
+Local socket probably won't work on Windows. Could just use a temp file.
 
 Note that the webapp can probe to see if ssh needs a password, and can
 prompt the user for it before running ssh and the ssh-askpass shim.