Add keyring option to vault program to set the CEPH_KEYRING envvar

So we don't have runtime behavior dependent on a mix of CLI options and
environment variables.

Fixes GitHub issue #79.

src/Vault.hs

@@ -20,6 +20,7 @@ import Data.Word (Word64)
 import Options.Applicative hiding (Parser, option)
 import qualified Options.Applicative as O
 import System.Directory
+import System.Environment
 import System.Log.Logger
 import Text.Read
 import Text.Trifecta
@@ -39,6 +40,7 @@ data Options = Options
   , period    :: Int
   , bound     :: Int
   , name      :: String
+  , keyring   :: String
   , component :: Component }
 
 data Component = Broker
@@ -61,6 +63,7 @@ optionsParser Options{..} = Options <$> parsePool
                                     <*> parsePeriod
                                     <*> parseBound
                                     <*> parseName
+                                    <*> parseKeyring
                                     <*> parseComponents
   where
     parsePool = strOption $
@@ -123,6 +126,13 @@ optionsParser Options{..} = Options <$> parsePool
         <> showDefault
         <> help "Identifiable name for the daemon. Useful for telemetrics."
 
+    parseKeyring = strOption $
+           long "keyring"
+        <> short 'k'
+        <> metavar "CEPH-KEYRING"
+        <> value ""
+        <> help "Path to Ceph keyring file. If set, this will override the CEPH_KEYRING environment variable."
+
     parseComponents = subparser
        (  parseBrokerComponent
        <> parseReaderComponent
@@ -196,6 +206,11 @@ possibleKeys =
 parseArgsWithConfig :: FilePath -> IO Options
 parseArgsWithConfig = parseConfig >=> execParser . helpfulParser
 
+-- If set, override CEPH_KEYRING envvar (used by librados).
+updateCephKeyring :: String -> IO ()
+updateCephKeyring "" = return ()
+updateCephKeyring k  = setEnv "CEPH_KEYRING" k
+
 --
 -- Main program entry point
 --
@@ -208,6 +223,8 @@ main = do
               | quiet     = Quiet
               | otherwise = Normal
 
+    updateCephKeyring keyring
+
     quit <- initializeProgram (package ++ "-" ++ version) level
 
     -- Run daemon(s, at present just one). These are all expected to fork