fix: Admin server dies silently

This is a workaround for Warp crashing with "thread blocked indefinitely in an STM transaction" when receiving eMFILE from accept.

The fix is to provide Warp with a custom accept function that handles eMFILE by logging it and keeping accepting connections.

Author: mkleczek

src/PostgREST/Admin.hs

@@ -2,15 +2,19 @@ module PostgREST.Admin
   ( runAdmin
   ) where
 
+import           Control.Exception         (ExceptionWithContext (ExceptionWithContext),
+                                            catchNoPropagate,
+                                            rethrowIO)
+import           Control.Monad.Extra       (whenJust)
 import qualified Data.Aeson                as JSON
+import           Foreign.C.Error           (Errno (..), eMFILE)
+import           GHC.IO.Exception
 import qualified Network.HTTP.Types.Status as HTTP
+import           Network.Socket            hiding (addrFamily)
+import           Network.Socket.ByteString
 import qualified Network.Wai               as Wai
 import qualified Network.Wai.Handler.Warp  as Warp
 
-import Control.Monad.Extra       (whenJust)
-import Network.Socket            hiding (addrFamily)
-import Network.Socket.ByteString
-
 import PostgREST.AppState    (AppState, getConfig)
 import PostgREST.Config      (AppConfig (..))
 import PostgREST.MediaType   (MediaType (..), toContentType)
@@ -35,9 +39,21 @@ runAdmin appState maybeAdminSocket getSocketREST settings = do
     observer = AppState.getObserver appState
     adminServerSettings config addr=
       settings
+        & Warp.setAccept safeAccept
         & Warp.setBeforeMainLoop (observer $ AdminStartObs addr)
         & maybe identity Warp.setPort (configAdminServerPort config)
 
+    isEMFILE = (Just eMFILE ==) . fmap Errno . ioe_errno
+    safeAccept sock =
+      accept sock `catchNoPropagate`
+        \ec@(ExceptionWithContext _ e) ->
+          if isEMFILE e then
+            -- Keep accepting on eMFILE
+            -- TODO log observation???
+            safeAccept sock
+          else
+            rethrowIO ec
+
     onError adminSock ex = do
       observer $ AdminServerCrashedObs ex
       NS.close adminSock -- we close the socket so request doesn't hang