Introduce ReconnectTo

Reconnect policies can now specify whether they want to attempt reconnection
with the original server given to `withConnection`, the last server we attempted
connection with, or a new server specified by the policy itself.

src/Network/GRPC/Client.hs

@@ -9,6 +9,7 @@ module Network.GRPC.Client (
 
     -- ** Reconnection policy
   , ReconnectPolicy(..)
+  , ReconnectTo(..)
   , exponentialBackoff
 
     -- ** Connection parameters

src/Network/GRPC/Client/Connection.hs

@@ -16,6 +16,7 @@ module Network.GRPC.Client.Connection (
   , SslKeyLog(..)
   , ConnParams(..)
   , ReconnectPolicy(..)
+  , ReconnectTo(..)
   , exponentialBackoff
     -- * Using the connection
   , connParams
@@ -29,7 +30,6 @@ import Control.Concurrent.STM
 import Control.Monad
 import Control.Monad.Catch
 import Data.Default
-import Data.Maybe
 import GHC.Stack
 import Network.HPACK qualified as HPACK
 import Network.HTTP2.Client qualified as HTTP2.Client
@@ -169,15 +169,27 @@ data ReconnectPolicy =
     -- | Reconnect to the (potentially different) server after the IO action
     -- returns
     --
-    -- If the 'Maybe' is 'Just', we'll attempt to reconnect to a server at the
-    -- new address. If 'Nothing', we'll attempt to connect to the original
-    -- server that 'withConnection' was given.
+    -- The 'ReconnectTo' can be used to implement a rudimentary redundancy
+    -- scheme. For example, you could decide to reconnect to a known fallback
+    -- server after connection to a main server fails a certain number of times.
     --
     -- This is a very general API: typically the IO action will call
     -- 'threadDelay' after some amount of time (which will typically involve
     -- some randomness), but it can be used to do things such as display a
     -- message to the user somewhere that the client is reconnecting.
-  | ReconnectAfter (Maybe Server) (IO ReconnectPolicy)
+  | ReconnectAfter ReconnectTo (IO ReconnectPolicy)
+
+-- | What server should we attempt to reconnect to?
+--
+-- * 'ReconnectToPrevious' will attempt to reconnect to the last server we
+--   attempted to connect to, whether or not that attempt was successful.
+-- * 'ReconnectToOriginal' will attempt to reconnect to the original server that
+--   'withConnection' was given.
+-- * 'ReconnectToNew' will attempt to connect to the newly specified server.
+data ReconnectTo =
+      ReconnectToPrevious
+    | ReconnectToOriginal
+    | ReconnectToNew Server
 
 -- | The default policy is 'DontReconnect'
 --
@@ -186,6 +198,9 @@ data ReconnectPolicy =
 instance Default ReconnectPolicy where
   def = DontReconnect
 
+instance Default ReconnectTo where
+  def = ReconnectToPrevious
+
 -- | Exponential backoff
 --
 -- If the exponent is @1@, the delay interval will be the same every step;
@@ -213,7 +228,7 @@ exponentialBackoff waitFor e = go
   where
     go :: (Double, Double) -> Word -> ReconnectPolicy
     go _        0 = DontReconnect
-    go (lo, hi) n = ReconnectAfter Nothing $ do
+    go (lo, hi) n = ReconnectAfter def $ do
         delay <- randomRIO (lo, hi)
         waitFor $ round $ delay * 1_000_000
         return $ go (lo * e, hi * e) (pred n)
@@ -431,9 +446,16 @@ stayConnected connParams initialServer connStateVar connOutOfScope = do
                 atomically $ writeTVar connStateVar $ ConnectionAbandoned err
               (False, DontReconnect) -> do
                 atomically $ writeTVar connStateVar $ ConnectionAbandoned err
-              (False, ReconnectAfter mNewServer f) -> do
+                atomically $ writeTVar connStateVar $ ConnectionAbandoned err
+              (False, ReconnectAfter to f) -> do
+                let
+                  nextServer =
+                    case to of
+                      ReconnectToPrevious -> server
+                      ReconnectToOriginal -> initialServer
+                      ReconnectToNew new  -> new
                 atomically $ writeTVar connStateVar $ ConnectionNotReady
-                loop (fromMaybe initialServer mNewServer) =<< f
+                loop nextServer =<< f
 
 -- | Insecure connection (no TLS)
 connectInsecure :: ConnParams -> Attempt -> Address -> IO ()

test-grapesy/Test/Driver/ClientServer.hs

@@ -533,7 +533,7 @@ runTestClient cfg firstTestFailure port clientRun = do
               -- This avoids a race condition between the server starting first
               -- and the client starting first.
             , connReconnectPolicy =
-                  Client.ReconnectAfter Nothing $ do
+                  Client.ReconnectAfter def $ do
                     threadDelay 100_000
                     return Client.DontReconnect
             }

test-grapesy/Test/Sanity/Disconnect.hs

@@ -199,16 +199,16 @@ test_serverDisconnect = withTemporaryFile $ \ipcFile -> do
             go :: Int -> Client.ReconnectPolicy
             go n
               | n == 5
-              = Client.ReconnectAfter Nothing $ do
+              = Client.ReconnectAfter def $ do
                   killRestarted <- startServer
                   port2 <- ipcRead
                   putMVar signalRestart killRestarted
                   return $
                     Client.ReconnectAfter
-                      (Just $ serverAddress port2)
+                      (Client.ReconnectToNew $ serverAddress port2)
                       (pure Client.DontReconnect)
               | otherwise
-              = Client.ReconnectAfter Nothing $ do
+              = Client.ReconnectAfter def $ do
                   threadDelay 10000
                   return $ go (n + 1)
 
@@ -296,6 +296,9 @@ echoHandler disconnectCounter call = trackDisconnects disconnectCounter $ do
   Auxiliary
 -------------------------------------------------------------------------------}
 
+-- We need to use this to properly simulate the execution environment crashing
+-- in an unrecoverable way. In particular, we don't want to give the program a
+-- chance to do any of its normal exception handling/cleanup behavior.
 foreign import ccall unsafe "exit" c_exit :: CInt -> IO ()
 
 data ClientStep = KeepGoing (Maybe (IO ())) ClientStep | Done