Install default SIGINT handler

lib/async.rb

@@ -11,3 +11,4 @@
 require_relative "kernel/async"
 require_relative "kernel/sync"
 require_relative "kernel/barrier"
+require_relative "async/sigint"

lib/async/sigint.rb

@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+# Released under the MIT License.
+# Copyright, 2026, by Samuel Williams.
+
+module Async
+	# Installs a compatibility SIGINT handler when Ruby's default SIGINT handling does not respect `Thread.handle_interrupt`.
+	#
+	# See <https://bugs.ruby-lang.org/issues/22133> for more context.
+	module SIGINT
+		# Whether this Ruby needs the compatibility SIGINT handler.
+		def self.required?
+			true
+		end
+
+		# Install the compatibility SIGINT handler, if needed.
+		def self.install
+			return unless required?
+
+			previous = ::Signal.trap(:INT, "DEFAULT")
+
+			if previous == "DEFAULT"
+				::Signal.trap(:INT) do
+					::Thread.main.raise(::Interrupt)
+				end
+			else
+				::Signal.trap(:INT, previous)
+			end
+		end
+
+		self.install
+	end
+
+	private_constant :SIGINT
+end

releases.md

@@ -1,5 +1,9 @@
 # Releases
 
+## Unreleased
+
+  - Install a compatibility SIGINT handler on affected CRuby versions when SIGINT is still using the default handler, so `Thread.handle_interrupt` can defer interrupts.
+
 ## v2.41.0
 
   - **Fixed**: Protect initial task from Interrupt exceptions.

test/async/sigint.rb

@@ -0,0 +1,92 @@
+# frozen_string_literal: true
+
+# Released under the MIT License.
+# Copyright, 2026, by Samuel Williams.
+
+require "open3"
+require "rbconfig"
+
+describe "Async::SIGINT" do
+	let(:ruby_binary) {RbConfig.ruby}
+	let(:load_path) {File.expand_path("../../lib", __dir__)}
+
+	def capture_ruby(*arguments)
+		Open3.capture3(ruby_binary, "-I#{load_path}", *arguments)
+	end
+
+	it "requires the SIGINT compatibility handler" do
+		script = <<~RUBY
+			require "async/sigint"
+
+			sigint = Async.const_get(:SIGINT)
+
+			puts sigint.required?
+		RUBY
+
+		output, error, status = capture_ruby("-e", script)
+
+		expect(error).to be == ""
+		expect(status).to be(:success?)
+		expect(output.lines(chomp: true)).to be == ["true"]
+	end
+
+	it "does not replace an existing SIGINT handler" do
+		script = <<~RUBY
+			handled = Thread::Queue.new
+
+			Signal.trap(:INT) do
+				handled.push(true)
+			end
+
+			require "async/sigint"
+
+			Process.kill(:INT, Process.pid)
+
+			puts handled.pop
+		RUBY
+
+		output, error, status = capture_ruby("-e", script)
+
+		expect(error).to be == ""
+		expect(status).to be(:success?)
+		expect(output.lines(chomp: true)).to be == ["true"]
+	end
+
+	it "defers SIGINT while signal exceptions are masked" do
+		script = <<~RUBY
+			require "async"
+
+			waiting = Thread::Queue.new
+			release = Thread::Queue.new
+			inner = false
+
+			Thread.new do
+				waiting.pop
+				Process.kill(:INT, Process.pid)
+				release.push(true)
+			end
+
+			begin
+				Thread.handle_interrupt(SignalException => :never) do
+					begin
+						waiting.push(true)
+						release.pop
+					rescue Interrupt
+						inner = true
+						raise
+					end
+				end
+			rescue Interrupt
+				puts "outer"
+			end
+
+			puts inner
+		RUBY
+
+		output, error, status = capture_ruby("-e", script)
+
+		expect(error).to be == ""
+		expect(status).to be(:success?)
+		expect(output.lines(chomp: true)).to be == ["outer", "false"]
+	end
+end