From 5356f4e973364c3ee39cb042bbc2a63a1c7fe4cb Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20K=C5=82eczek?= <michal@kleczek.org>
Date: Tue, 27 Jan 2026 11:04:51 +0100
Subject: [PATCH] add: Log actual host and port of listener connection

Diagnosing problems with listener channel notifications not being handled properly by PostgREST connected to read replicas is difficult. Issues might be related to lost connections and listener not being connected to the right host after failover or database server restarts.
This patch adds logging of actual host:port used by libpq connection opened by the listener. It should make it easier to find out if PostgREST is connected to the right host.
---
 CHANGELOG.md                 |  1 +
 src/PostgREST/Listener.hs    | 11 ++++++++---
 src/PostgREST/Observation.hs |  6 +++---
 test/io/test_io.py           | 17 +++++++++++++++++
 4 files changed, 29 insertions(+), 6 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index ee2cad413..4864b9c8a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,7 @@ All notable changes to this project will be documented in this file. From versio
 - Log error when `db-schemas` config contains schema `pg_catalog` or `information_schema` by @taimoorzaeem in #4359
 - Add a `HINT` when the LISTEN channel stops working due to a PostgreSQL bug by @laurenceisla in #4581
 - Add string slicing operator for `jwt-role-claim-key` by @taimoorzaeem in #4599
+- Log the actual host and port of listener database connection by @mkleczek in #4617
 
 ### Fixed
 
diff --git a/src/PostgREST/Listener.hs b/src/PostgREST/Listener.hs
index b8680a743..13eae3c86 100644
--- a/src/PostgREST/Listener.hs
+++ b/src/PostgREST/Listener.hs
@@ -17,8 +17,11 @@ import           PostgREST.Version     (prettyVersion)
 import qualified PostgREST.AppState as AppState
 import qualified PostgREST.Config   as Config
 
-import Data.Either.Combinators (whenRight)
-import Protolude
+import           Control.Arrow             ((&&&))
+import           Data.Bitraversable        (bisequence)
+import           Data.Either.Combinators   (whenRight)
+import qualified Database.PostgreSQL.LibPQ as LibPQ
+import           Protolude
 
 -- | Starts the Listener in a thread
 runListener :: AppState -> IO ()
@@ -70,7 +73,9 @@ retryingListen appState = do
             -- reset the delay
             AppState.putNextListenerDelay appState 1
 
-          observer $ DBListenStart dbChannel
+          (pqHost, pqPort) <- SQL.withLibPQConnection db $ bisequence . (LibPQ.host &&& LibPQ.port)
+          observer $ DBListenStart pqHost pqPort dbChannel
+
           SQL.waitForNotifications handleNotification db
 
         Left err -> do
diff --git a/src/PostgREST/Observation.hs b/src/PostgREST/Observation.hs
index 23adac727..03a8e6d95 100644
--- a/src/PostgREST/Observation.hs
+++ b/src/PostgREST/Observation.hs
@@ -44,7 +44,7 @@ data Observation
   | SchemaCacheSummaryObs Text
   | SchemaCacheLoadedObs Double
   | ConnectionRetryObs Int
-  | DBListenStart Text
+  | DBListenStart (Maybe ByteString) (Maybe ByteString) Text -- host, port, channel
   | DBListenFail Text (Either SQL.ConnectionError (Either SomeException ()))
   | DBListenRetry Int
   | DBListenBugHint -- https://github.com/PostgREST/postgrest/issues/3147
@@ -110,8 +110,8 @@ observationMessage = \case
     "Attempting to reconnect to the database in " <> (show delay::Text) <> " seconds..."
   QueryPgVersionError usageErr ->
     "Failed to query the PostgreSQL version. " <> jsonMessage usageErr
-  DBListenStart channel -> do
-    "Listening for database notifications on the " <> show channel <> " channel"
+  DBListenStart host port channel -> do
+    "Listener connected to " <> show (fold $ host <> fmap (":" <>) port) <> " and listening for database notifications on the " <> show channel <> " channel"
   DBListenFail channel listenErr ->
     "Failed listening for database notifications on the " <> show channel <> " channel. " <>
       either showListenerConnError showListenerException listenErr
diff --git a/test/io/test_io.py b/test/io/test_io.py
index df284f345..46f6414a9 100644
--- a/test/io/test_io.py
+++ b/test/io/test_io.py
@@ -1640,6 +1640,23 @@ def test_log_listener_connection_errors(defaultenv):
         )
 
 
+def test_log_listener_connection_start(defaultenv):
+    "The logs should show the listener connection start message in a single line"
+
+    env = {
+        **defaultenv,
+        "PGRST_DB_CHANNEL_ENABLED": "true",
+    }
+
+    with run(env=env, no_startup_stdout=False, wait_for_readiness=True) as postgrest:
+        output = postgrest.read_stdout(nlines=5)
+        assert any(
+            f'Listener connected to "{defaultenv["PGHOST"]}:5432" and listening for database notifications on the "pgrst" channel'
+            in line
+            for line in output
+        )
+
+
 def test_db_pre_config_with_pg_reserved_words(defaultenv):
     "The db-pre-config should not fail unexpectedly when function name is a postgres reserved word"