Run Bunny::Channel#on_error in a separate thread

This callback already runs on a thread that's
different from the application thread, so
this behavior change should not affect any
apps.

On the upside, this allows the callback use
`Bunny::Session` methods, including protocol
operation ones, without blocking the I/O loop.

References ruby-amqp/hutch#414

Author: michaelklishin

ChangeLog.md

@@ -1,6 +1,10 @@
 ## Changes between Bunny 3.0.0 and 3.1.0 (unrelesed)
 
-(no changes yet)
+### `on_error` Callback Runs in a Separate Thread
+
+`Channel#on_error` callbacks are now invoked in a separate thread
+when triggered by the reader loop. This allows callbacks to perform
+blocking operations such as `Channel#reopen` without deadlocking.
 
 
 ## Changes between Bunny 2.24.0 and 3.0.0 (March 31, 2026)

lib/bunny/channel.rb

@@ -2250,7 +2250,9 @@ def handle_method(method)
 
         # basic.ack, basic.reject, basic.nack. MK.
         if channel_level_exception_after_operation_that_has_no_response?(method)
-          @on_error.call(self, method) if @on_error
+          # Runs outside the reader loop so that the callback can perform
+          # blocking operations such as channel.reopen.
+          Thread.new { @on_error.call(self, method) } if @on_error
         else
           @last_channel_error = instantiate_channel_level_exception(method)
           @continuations.push(method)

spec/higher_level_api/integration/channel_reopen_spec.rb

@@ -1,4 +1,5 @@
 require "spec_helper"
+require "bunny/concurrent/condition"
 
 describe Bunny::Channel, "#reopen" do
   let(:connection) do
@@ -11,6 +12,25 @@
     connection.close if connection.open?
   end
 
+  it "can reopen a channel directly inside on_error" do
+    ch = connection.create_channel
+    done = Bunny::Concurrent::Condition.new
+
+    ch.on_error do |closed_ch, amq_close|
+      closed_ch.reopen
+      done.notify
+    end
+
+    ch.ack(82, false)
+    done.wait
+    expect(ch).to be_open
+
+    q = ch.queue("bunny.test.channel-reopen.on-error.#{rand}", exclusive: true)
+    ch.default_exchange.publish("hello", routing_key: q.name)
+    sleep 0.25
+    expect(q.message_count).to eq 1
+  end
+
   it "reopens a channel after a server-initiated closure" do
     ch = connection.create_channel
     q  = ch.queue("bunny.test.channel-reopen.#{rand}", exclusive: true)