server: add env lookups for smp server paths (#817)

* Add env lookups for smp server paths

Allows running smp-server without touching system paths.
Can be helpful for running multiple instances.

* allow empty env var values

* diff

* fix

---------

Co-authored-by: Evgeny Poberezkin <2769109+epoberezkin@users.noreply.github.com>

apps/smp-server/Main.hs

@@ -1,13 +1,17 @@
+{-# LANGUAGE LambdaCase #-}
+
 module Main where
 
 import Control.Logger.Simple
+import Data.Maybe
 import Simplex.Messaging.Server.Main
+import System.Environment
 
-cfgPath :: FilePath
-cfgPath = "/etc/opt/simplex"
+defaultCfgPath :: FilePath
+defaultCfgPath = "/etc/opt/simplex"
 
-logPath :: FilePath
-logPath = "/var/opt/simplex"
+defaultLogPath :: FilePath
+defaultLogPath = "/var/opt/simplex"
 
 logCfg :: LogConfig
 logCfg = LogConfig {lc_file = Nothing, lc_stderr = True}
@@ -15,4 +19,9 @@ logCfg = LogConfig {lc_file = Nothing, lc_stderr = True}
 main :: IO ()
 main = do
   setLogLevel LogDebug
+  cfgPath <- getEnvPath "SMP_SERVER_CFG_PATH" defaultCfgPath
+  logPath <- getEnvPath "SMP_SERVER_LOG_PATH" defaultLogPath
   withGlobalLogging logCfg $ smpServerCLI cfgPath logPath
+    
+getEnvPath :: String -> FilePath -> IO FilePath
+getEnvPath name def = maybe def (\case "" -> def; f -> f) <$> lookupEnv name