add(logs): Include a HINT when the LISTEN channel breaks due to a Postgres bug

The HINT shows a SQL command that solves the issue.

src/PostgREST/Observation.hs

@@ -12,6 +12,7 @@ module PostgREST.Observation
   , observationMessage
   , ObservationHandler
   , showOnSingleLine
+  , isDbListenerBug
   ) where
 
 import qualified Data.ByteString.Lazy       as LBS
@@ -46,6 +47,7 @@ data Observation
   | DBListenStart Text
   | DBListenFail Text (Either SQL.ConnectionError (Either SomeException ()))
   | DBListenRetry Int
+  | DBListenBugHint -- https://github.com/PostgREST/postgrest/issues/3147
   | DBListenerGotSCacheMsg ByteString
   | DBListenerGotConfigMsg ByteString
   | QueryObs MainQuery Status
@@ -114,6 +116,8 @@ observationMessage = \case
       either showListenerConnError showListenerException listenErr
   DBListenRetry delay ->
     "Retrying listening for database notifications in " <> (show delay::Text) <> " seconds..."
+  DBListenBugHint ->
+    "HINT:  This is likely a bug in the notification queue, try executing the following to solve it: select pg_notification_queue_usage();"
   DBListenerGotSCacheMsg channel ->
     "Received a schema cache reload message on the " <> show channel <> " channel"
   DBListenerGotConfigMsg channel ->
@@ -171,3 +175,7 @@ observationMessage = \case
 
 showOnSingleLine :: Char -> Text -> Text
 showOnSingleLine split txt = T.intercalate " " $ T.filter (/= split) <$> T.lines txt -- the errors from hasql-notifications come intercalated with "\t\n"
+
+isDbListenerBug :: Either SomeException () -> Bool
+isDbListenerBug (Left e) = "could not access status of transaction" `T.isInfixOf` show e
+isDbListenerBug _        = False