From 5a02a787e60571683bde4cdf6806121788bb220e Mon Sep 17 00:00:00 2001
From: David Turner <david.turner@elastic.co>
Date: Mon, 21 Jul 2025 08:16:16 +0100
Subject: [PATCH] Log failure in `internalSend` (#131418)

An exception here should be impossible, but we don't assert that, nor do
we emit a log message to prove it didn't happen in a production
environment. This commit adds the missing log and assert.
---
 .../org/elasticsearch/transport/OutboundHandler.java   | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/server/src/main/java/org/elasticsearch/transport/OutboundHandler.java b/server/src/main/java/org/elasticsearch/transport/OutboundHandler.java
index cab15fffa3fd0..b0408ac3c60cc 100644
--- a/server/src/main/java/org/elasticsearch/transport/OutboundHandler.java
+++ b/server/src/main/java/org/elasticsearch/transport/OutboundHandler.java
@@ -433,6 +433,16 @@ private void maybeLogSlowMessage(boolean success) {
                 }
             });
         } catch (RuntimeException ex) {
+            logger.error(
+                Strings.format(
+                    "unexpected exception calling sendMessage for transport message [%s] of size [%d] on [%s]",
+                    messageDescription.get(),
+                    messageSize,
+                    channel
+                ),
+                ex
+            );
+            assert Thread.currentThread().getName().startsWith("TEST-") : ex;
             channel.setCloseException(ex);
             Releasables.closeExpectNoException(() -> listener.onFailure(ex), () -> CloseableChannel.closeChannel(channel));
             throw ex;