formula_assertions: add pty_spawn_output

[cho-m]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
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To simplify some PTY usage in Homebrew/core.

Defaulting winsize to 80x24 as traditional default (also used by Terminal.app).

[Copilot]

Pull request overview

This PR adds a new pty_spawn_output method to the Homebrew::Assertions module to simplify PTY (pseudo-terminal) usage in Homebrew/core. The method spawns commands in a PTY environment, which is useful for testing interactive applications that behave differently when not connected to a terminal.

• Adds pty_spawn_output method with support for window sizing, stdin data, timeouts, and exit status assertions
• Provides comprehensive test coverage for the core functionality
• Uses traditional 80x24 terminal dimensions as the default

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 6 comments.

File	Description
Library/Homebrew/formula_assertions.rb	Implements the pty_spawn_output method with PTY spawning, thread-based output capture, configurable window size, stdin handling, and timeout support
Library/Homebrew/test/formula_assertions_spec.rb	Adds RSpec tests verifying exit code assertions, window size configuration, and timeout behavior

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[Copilot]

Calling Process.wait(pid) without ensuring the process has terminated could cause the method to hang. If the timeout fires but the process is still running, consider using Process.kill to terminate the process before waiting, or use Process.wait with the Process::WNOHANG flag to avoid blocking.

Suggested change

	Timeout.timeout(timeout) { stdout_thread.join }
	rescue Timeout::Error
	# Ignore error and let ensure clean up
	ensure
	stdout.close
	stdin.close
	Process.wait(pid)
	begin
	Timeout.timeout(timeout) { stdout_thread.join }
	rescue Timeout::Error
	# Ignore error and let ensure clean up, but make sure the process is signaled
	begin
	Process.kill("TERM", pid)
	rescue Errno::ESRCH
	# Process already exited
	end
	ensure
	stdout.close
	stdin.close
	begin
	Process.wait(pid)
	rescue Errno::ECHILD
	# Process already reaped
	end
	end

[Copilot]

The stdin_data and stdin_delay parameters are not covered by any tests. Consider adding test cases that verify stdin data is correctly sent to the spawned process and that the delay works as expected.

[Copilot]

The ensure block closes stdout before ensuring the stdout_thread has terminated. If the timeout fires while the thread is still reading from stdout, closing it could cause race conditions. The thread should be explicitly terminated or joined before closing the streams to ensure clean shutdown.

Suggested change

	stdout.close
	stdin.close
	# Ensure the stdout_thread is not reading when closing the streams.
	if defined?(stdout_thread) && stdout_thread&.alive?
	stdout_thread.kill
	stdout_thread.join
	end
	begin
	stdout.close
	rescue IOError
	# stdout may already be closed
	end
	begin
	stdin.close
	rescue IOError
	# stdin may already be closed
	end

[Copilot]

Writing to stdin without handling potential exceptions could cause the method to fail unexpectedly. If the child process closes its stdin or terminates before the write completes, this could raise Errno::EPIPE or IOError. Consider wrapping the write operation in a rescue block to handle these cases gracefully.

Suggested change

	stdin.write(stdin_data)
	begin
	stdin.write(stdin_data)
	rescue Errno::EPIPE, IOError
	# Ignore errors when the child process closes stdin early.
	end

[p-linnane]

This is awesome. Usually these tests can be a pain, so would love to see them simplified like this.

[MikeMcQuaid]

Great idea! Can you open a homebrew/core PR using this (which will obviously be 🔴 for now) just to see how it'll look there?

[cho-m]

Great idea! Can you open a homebrew/core PR using this (which will obviously be 🔴 for now) just to see how it'll look there?

• WIP: brew PTY DSL examples homebrew-core#260696

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Still need to figure out why the stty size is not working on Linux. And check on some exception handling mentioned by Copilot.

[SMillerDev]

This is a great addition, thanks!