From f0d9daaaac162bc0c87d25be9613e6383a0c8359 Mon Sep 17 00:00:00 2001
From: Samuel Williams <samuel.williams@shopify.com>
Date: Thu, 24 Sep 2026 12:44:42 +1200
Subject: [PATCH 1/2] Cover partial and zero-progress writes in the legacy
 Select backend

---
 test/io/event/selector/select.rb | 51 ++++++++++++++++++++++++++++++++
 1 file changed, 51 insertions(+)

diff --git a/test/io/event/selector/select.rb b/test/io/event/selector/select.rb
index b1a2d58d..6639658b 100644
--- a/test/io/event/selector/select.rb
+++ b/test/io/event/selector/select.rb
@@ -88,6 +88,57 @@
 		end
 	end
 	
+	with "#io_write (legacy IO::Buffer)" do
+		before do
+			if defined?(IO::Buffer::VERSION) && IO::Buffer::VERSION >= 3
+				skip "Requires the legacy minimum-length write loop"
+			end
+		end
+		
+		it "advances the offset after a partial write until the minimum is reached" do
+			input, output = IO.pipe
+			buffer = IO::Buffer.new(8)
+			writes = []
+			results = [2, 2]
+			
+			# Force short writes without depending on the pipe's capacity:
+			mock(buffer) do |mock|
+				mock.replace(:write) do |io, length, offset|
+					writes << [io, length, offset]
+					results.shift or raise "Unexpected write!"
+				end
+			end
+			
+			expect(selector.io_write(Fiber.current, output, buffer, 4, 1)).to be == 4
+			expect(writes).to be == [[output, 0, 1], [output, 0, 3]]
+		ensure
+			input&.close
+			output&.close
+			buffer&.free
+		end
+		
+		it "stops on a zero-byte write and returns the bytes already written" do
+			input, output = IO.pipe
+			buffer = IO::Buffer.new(8)
+			writes = []
+			results = [2, 0]
+			
+			mock(buffer) do |mock|
+				mock.replace(:write) do |io, length, offset|
+					writes << [io, length, offset]
+					results.shift or raise "Unexpected write!"
+				end
+			end
+			
+			expect(selector.io_write(Fiber.current, output, buffer, 4, 1)).to be == 2
+			expect(writes).to be == [[output, 0, 1], [output, 0, 3]]
+		ensure
+			input&.close
+			output&.close
+			buffer&.free
+		end
+	end
+	
 	with "#select" do
 		it "dispatches priority events" do
 			server = TCPServer.new("127.0.0.1", 0)

From 63fbea56344340e056d53c4203dc86ce2dcb5aac Mon Sep 17 00:00:00 2001
From: Samuel Williams <samuel.williams@shopify.com>
Date: Thu, 24 Sep 2026 12:47:31 +1200
Subject: [PATCH 2/2] Skip legacy write tests from their test bodies on Ruby
 head

---
 test/io/event/selector/select.rb | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/test/io/event/selector/select.rb b/test/io/event/selector/select.rb
index 6639658b..a35ef92b 100644
--- a/test/io/event/selector/select.rb
+++ b/test/io/event/selector/select.rb
@@ -89,13 +89,9 @@
 	end
 	
 	with "#io_write (legacy IO::Buffer)" do
-		before do
-			if defined?(IO::Buffer::VERSION) && IO::Buffer::VERSION >= 3
-				skip "Requires the legacy minimum-length write loop"
-			end
-		end
-		
 		it "advances the offset after a partial write until the minimum is reached" do
+			skip "Requires the legacy minimum-length write loop" if defined?(IO::Buffer::VERSION) && IO::Buffer::VERSION >= 3
+			
 			input, output = IO.pipe
 			buffer = IO::Buffer.new(8)
 			writes = []
@@ -118,6 +114,8 @@
 		end
 		
 		it "stops on a zero-byte write and returns the bytes already written" do
+			skip "Requires the legacy minimum-length write loop" if defined?(IO::Buffer::VERSION) && IO::Buffer::VERSION >= 3
+			
 			input, output = IO.pipe
 			buffer = IO::Buffer.new(8)
 			writes = []