From 326cbd97b7b7ef754303c6ad09f7bf0c0d49af3e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Joel=20Junstr=C3=B6m?= <joel.junstrom@gmail.com>
Date: Thu, 21 May 2026 11:12:23 +0200
Subject: [PATCH] - server/base: tolerate connection close failures in #restart

ActionCable::Server::Base#restart iterates @connections and calls
the iteration and propagates out. Since restart is wired to Rails'
before_class_unload reloader callback, that raise tears down the
unload callback chain and 500s the request that triggered the
reload.

We hit this in development under async-cable, whose Socket#transmit
pushes to a Thread::Queue and raises ClosedQueueError when the
queue is already closed (stale connection, second restart in
flight, client disconnect racing with restart):

  async-cable/lib/async/cable/socket.rb:52:in `Thread::Queue#push`
  async-cable/lib/async/cable/socket.rb:52:in `transmit`
  action_cable/connection/base.rb:117:in `transmit`
  action_cable/connection/base.rb:126:in `close`
  action_cable/server/base.rb:75:in `block in restart`
  action_cable/server/base.rb:74:in `each`
  action_cable/server/base.rb:74:in `restart`

The root cause is adapter-specific, but the fix belongs at the
restart boundary too: it runs from a reloader callback and
shouldn't propagate per-connection failures regardless of which
adapter is serving the socket.

We catch and log around each close so one bad connection cannot
abort the iteration.

The remaining connections still close and the worker pool, executor,
and pub-sub adapter still shut down.
---
 lib/action_cable/server/base.rb |  2 +
 test/server/base_test.rb        | 66 +++++++++++++++++++++++++++++++++
 2 files changed, 68 insertions(+)

diff --git a/lib/action_cable/server/base.rb b/lib/action_cable/server/base.rb
index 3da7643..f0f7360 100644
--- a/lib/action_cable/server/base.rb
+++ b/lib/action_cable/server/base.rb
@@ -73,6 +73,8 @@ def disconnect(identifiers)
       def restart
         connections.each do |connection|
           connection.close(reason: ActionCable::INTERNAL[:disconnect_reasons][:server_restart])
+        rescue => e
+          logger&.error "Failed to close connection during restart: [#{e.class} - #{e.message}]: #{e.backtrace.first(5).join(" | ")}"
         end
 
         @mutex.synchronize do
diff --git a/test/server/base_test.rb b/test/server/base_test.rb
index d46debe..7d17125 100644
--- a/test/server/base_test.rb
+++ b/test/server/base_test.rb
@@ -15,6 +15,12 @@ def close
     end
   end
 
+  class RaisingConnection
+    def close(*)
+      raise ClosedQueueError, "queue closed"
+    end
+  end
+
   test "#restart closes all open connections" do
     conn = FakeConnection.new
     @server.add_connection(conn)
@@ -35,4 +41,64 @@ def close
       @server.restart
     end
   end
+
+  test "#restart does not propagate exceptions raised by connection#close" do
+    @server.add_connection(RaisingConnection.new)
+
+    assert_nothing_raised do
+      @server.restart
+    end
+  end
+
+  test "#restart still closes remaining connections when one #close raises" do
+    @server.add_connection(RaisingConnection.new)
+    survivor = FakeConnection.new
+    @server.add_connection(survivor)
+
+    assert_called(survivor, :close) do
+      @server.restart
+    end
+  end
+
+  test "#restart still shuts down worker pool when connection#close raises" do
+    @server.add_connection(RaisingConnection.new)
+
+    assert_called(@server.worker_pool, :halt) do
+      @server.restart
+    end
+  end
+
+  test "#restart still shuts down pub/sub when connection#close raises" do
+    @server.add_connection(RaisingConnection.new)
+
+    assert_called(@server.pubsub, :shutdown) do
+      @server.restart
+    end
+  end
+
+  test "#restart is safe to call twice when a connection#close raises" do
+    @server.add_connection(RaisingConnection.new)
+
+    @server.restart
+    assert_nothing_raised do
+      @server.restart
+    end
+  end
+
+  test "#restart logs an error including the exception class when connection#close raises" do
+    @server.add_connection(RaisingConnection.new)
+
+    log = StringIO.new
+    old_logger = @server.config.logger
+    @server.config.logger = Logger.new(log)
+
+    begin
+      @server.restart
+
+      log.rewind
+      assert_match(/ClosedQueueError.*queue closed/, log.read)
+    ensure
+      @server.config.logger = old_logger
+    end
+  end
 end