From 18f7169dccc2cb0e0fad502eee13dd5cbdea7929 Mon Sep 17 00:00:00 2001
From: David Turner <david.turner@elastic.co>
Date: Thu, 17 Jul 2025 09:59:11 +0100
Subject: [PATCH 1/2] Log failure in `internalSend`

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..04984f1619404 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(),
+                    messageDescription,
+                    channel
+                ),
+                ex
+            );
+            assert false : ex;
             channel.setCloseException(ex);
             Releasables.closeExpectNoException(() -> listener.onFailure(ex), () -> CloseableChannel.closeChannel(channel));
             throw ex;

From ddcd68ce29cb33c0aaf9845041d6fcdcf9e9adbe Mon Sep 17 00:00:00 2001
From: David Turner <david.turner@elastic.co>
Date: Thu, 17 Jul 2025 10:35:53 +0100
Subject: [PATCH 2/2] Permit exception if called directly from test thread

---
 .../java/org/elasticsearch/transport/OutboundHandler.java     | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/server/src/main/java/org/elasticsearch/transport/OutboundHandler.java b/server/src/main/java/org/elasticsearch/transport/OutboundHandler.java
index 04984f1619404..b0408ac3c60cc 100644
--- a/server/src/main/java/org/elasticsearch/transport/OutboundHandler.java
+++ b/server/src/main/java/org/elasticsearch/transport/OutboundHandler.java
@@ -437,12 +437,12 @@ private void maybeLogSlowMessage(boolean success) {
                 Strings.format(
                     "unexpected exception calling sendMessage for transport message [%s] of size [%d] on [%s]",
                     messageDescription.get(),
-                    messageDescription,
+                    messageSize,
                     channel
                 ),
                 ex
             );
-            assert false : ex;
+            assert Thread.currentThread().getName().startsWith("TEST-") : ex;
             channel.setCloseException(ex);
             Releasables.closeExpectNoException(() -> listener.onFailure(ex), () -> CloseableChannel.closeChannel(channel));
             throw ex;