fix: schema cache retrying without backoff

Fixes https://github.com/PostgREST/postgrest/issues/3523.

Now if there's a failure when obtaining the pg version OR schema cache,
we do the same retrying process. This way we don't add two retries.

Refactors and renames the "connectionWorker" to "schemaCacheLoader".
This makes more sense since what we really want is the schema cache,
the version is the pre-requisite for ensuring our
schema cache queries work.

Additionally, we no longer log ` Attempting to connect to the database...`
at startup unnecessarily. This is only logged whenever there's a retry attempt.

src/PostgREST/Observation.hs

@@ -29,7 +29,6 @@ data Observation
   | AppStartObs ByteString
   | AppServerPortObs NS.PortNumber
   | AppServerUnixObs FilePath
-  | DBConnectAttemptObs
   | ExitUnsupportedPgVersion PgVersion PgVersion
   | ExitDBNoRecoveryObs
   | ExitDBFatalError ObsFatalError SQL.UsageError
@@ -39,7 +38,6 @@ data Observation
   | SchemaCacheSummaryObs Text
   | SchemaCacheLoadedObs Double
   | ConnectionRetryObs Int
-  | ConnectionPgVersionErrorObs SQL.UsageError
   | DBListenStart Text
   | DBListenFail Text (Either SQL.ConnectionError (Either SomeException ()))
   | DBListenRetry Int
@@ -50,6 +48,7 @@ data Observation
   | ConfigSucceededObs
   | QueryRoleSettingsErrorObs SQL.UsageError
   | QueryErrorCodeHighObs SQL.UsageError
+  | QueryPgVersionError SQL.UsageError
   | PoolAcqTimeoutObs SQL.UsageError
   | HasqlPoolObs SQL.Observation
   | PoolRequest
@@ -69,8 +68,6 @@ observationMessage = \case
     "Listening on port " <> show port
   AppServerUnixObs sock ->
     "Listening on unix socket " <> show sock
-  DBConnectAttemptObs ->
-    "Attempting to connect to the database..."
   DBConnectedObs ver ->
     "Successfully connected to " <> ver
   ExitUnsupportedPgVersion pgVer minPgVer ->
@@ -78,7 +75,7 @@ observationMessage = \case
   ExitDBNoRecoveryObs ->
     "Automatic recovery disabled, exiting."
   ExitDBFatalError ServerAuthError usageErr ->
-    jsonMessage usageErr
+    "Failed to establish a connection. " <> jsonMessage usageErr
   ExitDBFatalError ServerPgrstBug usageErr ->
     "This is probably a bug in PostgREST, please report it at https://github.com/PostgREST/postgrest/issues. " <> jsonMessage usageErr
   ExitDBFatalError ServerError42P05 usageErr ->
@@ -95,8 +92,8 @@ observationMessage = \case
     "Schema cache loaded in " <> showMillis resultTime <> " milliseconds"
   ConnectionRetryObs delay ->
     "Attempting to reconnect to the database in " <> (show delay::Text) <> " seconds..."
-  ConnectionPgVersionErrorObs usageErr ->
-    jsonMessage usageErr
+  QueryPgVersionError usageErr ->
+    "Failed to query the PostgreSQL version. " <> jsonMessage usageErr
   DBListenStart channel -> do
     "Listening for notifications on the " <> show channel <> " channel"
   DBListenFail channel listenErr ->