Launch service thread with a function that re-throws if they fail

In Haskell by default if a thread has an exception, the main thread isn't
notified at all. This patch changes service thread launching to re-throw their
exceptions in the main thread, so that their failure is noticed.

src/Control/Concurrent/Local.hs

@@ -0,0 +1,31 @@
+{- This file is part of Vervis.
+ -
+ - Written in 2019 by fr33domlover <fr33domlover@riseup.net>.
+ -
+ - ♡ Copying is an act of love. Please copy, reuse and share.
+ -
+ - The author(s) have dedicated all copyright and related and neighboring
+ - rights to this software to the public domain worldwide. This software is
+ - distributed without any warranty.
+ -
+ - You should have received a copy of the CC0 Public Domain Dedication along
+ - with this software. If not, see
+ - <http://creativecommons.org/publicdomain/zero/1.0/>.
+ -}
+
+module Control.Concurrent.Local
+    ( forkCheck
+    )
+where
+
+import Prelude
+
+import Control.Concurrent
+import Data.Functor (void)
+
+-- | Like 'forkIO', but if the new thread terminates with an exception,
+-- re-throw it in the current thread.
+forkCheck :: IO () -> IO ()
+forkCheck run = do
+    tid <- myThreadId
+    void $ forkFinally run $ either (throwTo tid) (const $ return ())

src/Vervis/Application.hs

@@ -1,6 +1,6 @@
 {- This file is part of Vervis.
  -
- - Written in 2016, 2018 by fr33domlover <fr33domlover@riseup.net>.
+ - Written in 2016, 2018, 2019 by fr33domlover <fr33domlover@riseup.net>.
  -
  - ♡ Copying is an act of love. Please copy, reuse and share.
  -
@@ -30,7 +30,6 @@ module Vervis.Application
     )
 where
 
-import Control.Concurrent (forkIO)
 import Control.Monad.Logger                 (liftLoc, runLoggingT, logInfo, logError)
 import Database.Persist.Postgresql          (createPostgresqlPool, pgConnStr,
                                              pgPoolSize, runSqlPool)
@@ -54,6 +53,10 @@ import Yesod.Mail.Send (runMailer)
 
 import qualified Data.Text as T (unpack)
 
+import Control.Concurrent.Local (forkCheck)
+
+import Vervis.ActorKey (generateActorKey, actorKeyRotator)
+
 -- Import all relevant handler modules here.
 -- Don't forget to add new modules to your cabal file!
 import Vervis.Handler.Common
@@ -229,10 +232,10 @@ appMain = do
     app <- makeApplication foundation
 
     -- Run SSH server
-    forkIO $ sshServer foundation
+    forkCheck $ sshServer foundation
 
     -- Run mailer if SMTP is enabled
-    forkIO $ mailer foundation
+    forkCheck $ mailer foundation
 
     -- Run the application with Warp
     runSettings (warpSettings foundation) app

vervis.cabal

@@ -39,6 +39,7 @@ flag library-only
 
 library
   exposed-modules:     Control.Applicative.Local
+                       Control.Concurrent.Local
                        Darcs.Local.Repository
                        Data.Attoparsec.ByteString.Local
                        Data.Binary.Local