refactor: encapsulate how main application liveness check is done

This change makes Admin module independent from how liveness check is performed. It moves reachMainApp function to App module and makes Admin.runAdmin, instead of action to get the main socket, take an IO action checking liveness as a parameter.

Thanks to this, any change in how liveness check is performed and, more importantly, what data it uses, does not require changing multiple modules (Admin and App).

src/PostgREST/App.hs

@@ -72,6 +72,7 @@ import qualified Data.Text                 as T
 import qualified Network.HTTP.Types        as HTTP
 import           Network.HTTP.Types.Header (hVary)
 import qualified Network.Socket            as NS
+import           Network.Socket.ByteString (send)
 import           PostgREST.Unix            (createAndBindDomainSocket)
 import           System.Posix.Types        (FileMode)
 
@@ -89,7 +90,7 @@ run appState = do
         readIORef mainSocketRef >>= foldMap NS.close
   Unix.installSignalHandlers observer closeSockets (AppState.schemaCacheLoader appState) (AppState.readInDbConfig False appState)
 
-  Admin.runAdmin appState adminSocket (readIORef mainSocketRef) (serverSettings conf)
+  Admin.runAdmin appState adminSocket (checkMainAppLive (readIORef mainSocketRef)) (serverSettings conf)
 
   Listener.runListener appState
 
@@ -291,3 +292,22 @@ initAdminServerSocket AppConfig{..} =
   initSocket
     configAdminServerUnixSocket configAdminServerUnixSocketMode
     configAdminServerHost configAdminServerPort
+
+checkMainAppLive :: IO (Maybe NS.Socket) -> IO Bool
+checkMainAppLive getMainSocket =
+  getMainSocket >>= maybe (pure False) (fmap isRight . reachMainApp)
+
+-- Try to connect to the main app socket
+-- Note that it doesn't even send a valid HTTP request, we just want to check that the main app is accepting connections
+reachMainApp :: NS.Socket -> IO (Either IOException ())
+reachMainApp appSock = do
+  sockAddr <- NS.getSocketName appSock
+  sock <- NS.socket (addrFamily sockAddr) NS.Stream NS.defaultProtocol
+  try $ do
+    NS.connect sock sockAddr
+    NS.withSocketsDo $ bracket (pure sock) NS.close sendEmpty
+  where
+    sendEmpty sock = void $ send sock mempty
+    addrFamily (NS.SockAddrInet _ _) = NS.AF_INET
+    addrFamily (NS.SockAddrInet6 {}) = NS.AF_INET6
+    addrFamily (NS.SockAddrUnix _)   = NS.AF_UNIX