From bf90f954435d34f91f3c535366a912f02416040c Mon Sep 17 00:00:00 2001
From: Laurence Isla <lau.isla.c@gmail.com>
Date: Wed, 20 Aug 2025 02:33:04 -0500
Subject: [PATCH] fix: logging the Haskell type instead of the listener error
 message directly

Previously: Just "connection error..."
Now: connection error...
---
 CHANGELOG.md                 |  1 +
 src/PostgREST/Observation.hs | 20 ++++++++++----------
 test/io/test_io.py           | 18 ++++++++++++++++++
 3 files changed, 29 insertions(+), 10 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8d3e392a8..6f0411acf 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -19,6 +19,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 - Fix OpenAPI specification incorrectly exposing GET methods for volatile functions by @joelonsql in #4174
 - Fix empty spread embeddings return unexpected SQL error by @taimoorzaeem in #3887
 - Fix `/metrics` endpoint not responding with `Content-Type` header by @taimoorzaeem in #4271
+- Fix logging the Haskell type instead of the listener error message directly by @laurenceisla in #3588
 
 ### Changed
 
diff --git a/src/PostgREST/Observation.hs b/src/PostgREST/Observation.hs
index 296de0f48..6e0b35516 100644
--- a/src/PostgREST/Observation.hs
+++ b/src/PostgREST/Observation.hs
@@ -110,11 +110,8 @@ 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 ->
@@ -163,8 +160,11 @@ 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
diff --git a/test/io/test_io.py b/test/io/test_io.py
index a2b1152f8..3c24c2256 100644
--- a/test/io/test_io.py
+++ b/test/io/test_io.py
@@ -1937,3 +1937,21 @@ 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
+        )