fix: logging the Haskell type instead of the listener error message directly

Previously: Just "connection error..."
Now: connection error...

CHANGELOG.md

@@ -5,6 +5,10 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 ## Unreleased
 
+### Fixed
+
+- Fix logging the Haskell type instead of the listener error message directly by @laurenceisla in #3588
+
 ## [13.0.5] - 2025-08-24
 
 ### Fixed

src/PostgREST/Observation.hs

@@ -108,11 +108,8 @@ observationMessage = \case
   DBListenStart channel -> do
     "Listening for database notifications on the " <> show channel <> " channel"
   DBListenFail channel listenErr ->
-    "Failed listening for database notifications on the " <> show channel <> " channel. " <> (
-      case listenErr of
-        Left err  -> show err
-        Right err -> showListenerError err
-    )
+    "Failed listening for database notifications on the " <> show channel <> " channel. " <>
+      either showListenerConnError showListenerException listenErr
   DBListenRetry delay ->
     "Retrying listening for database notifications in " <> (show delay::Text) <> " seconds..."
   DBListenerGotSCacheMsg channel ->
@@ -157,8 +154,11 @@ observationMessage = \case
 
     jsonMessage err = T.decodeUtf8 . LBS.toStrict . Error.errorPayload $ Error.PgError False err
 
-    showListenerError :: Either SomeException () -> Text
-    showListenerError (Right _) = "Failed getting notifications" -- should not happen as the listener will never finish (hasql-notifications uses `forever` internally) with a Right result
-    showListenerError (Left e)  =
-      let showOnSingleLine txt = T.intercalate " " $ T.filter (/= '\t') <$> T.lines txt in -- the errors from hasql-notifications come intercalated with "\t\n"
-      showOnSingleLine $ show e
+    showOnSingleLine txt = T.intercalate " " $ T.filter (/= '\t') <$> T.lines txt -- the errors from hasql-notifications come intercalated with "\t\n"
+
+    showListenerConnError :: SQL.ConnectionError -> Text
+    showListenerConnError = maybe "Connection error" (showOnSingleLine . T.decodeUtf8)
+
+    showListenerException :: Either SomeException () -> Text
+    showListenerException (Right _) = "Failed getting notifications" -- should not happen as the listener will never finish (hasql-notifications uses `forever` internally) with a Right result
+    showListenerException (Left e)  = showOnSingleLine $ show e

test/io/test_io.py

@@ -1930,3 +1930,21 @@ def test_schema_cache_error_observation(defaultenv):
             "Failed to load the schema cache using db-schemas=public and db-extra-search-path=x"
             in output[7]
         )
+
+
+def test_log_listener_connection_errors(defaultenv):
+    "The logs should show the listener connection error message in a single line"
+
+    env = {
+        **defaultenv,
+        "PGHOST": "no_host",
+        "PGRST_DB_CHANNEL_ENABLED": "true",
+    }
+
+    with run(env=env, no_startup_stdout=False, wait_for_readiness=False) as postgrest:
+        output = postgrest.read_stdout(nlines=5)
+        assert any(
+            'Failed listening for database notifications on the "pgrst" channel. could not translate host name "no_host" to address:'
+            in line
+            for line in output
+        )