Bead bf-gqf: PTY spawn and signal forwarding - Already Complete

Task

Implement src/pty.rs: PTY spawn and signal forwarding

Findings

The PTY implementation in src/pty.rs was already complete at the time of this bead's execution. All required functionality is implemented:

PtySpawner::spawn (lines 56-97)

Opens PTY pair via nix::pty::openpty
Forks process using nix::unistd::fork
In child: calls login_tty to make slave the controlling terminal, then execvp the command
In parent: returns PtySpawner with master fd and child pid

Signal forwarding in relay() (lines 101-249)

SIGWINCH handler (lines 14-17): Sets atomic flag when window size changes
SIGINT handler (lines 19-22): Sets atomic flag when Ctrl-C pressed
SIGINT forwarding (lines 122-127): When flag set, calls kill(child_pid, SIGINT)
SIGWINCH propagation (lines 130-136): Reads winsize from stdin, applies to PTY via TIOCSWINSZ ioctl
I/O relay: Poll loop copies data between stdin→PTY master and PTY master→stdout
Exit code handling: Waits for child and returns exit code or signal+128

Key invariants verified

✅ Invariant #3: SIGINT is forwarded to child process (not just terminates claude-print) ✅ Signal handlers are async-signal-safe (only touch AtomicBool) ✅ Window size propagated from controlling terminal to PTY ✅ Proper cleanup with waitpid

Tests verified

cargo test --lib pty

All 7 tests passed:

spawn_bin_true_exits_zero - verifies fork/exec works
master_fd_carries_child_stdout - verifies PTY I/O
relay_echo_exits_zero_and_produces_output - verifies full relay loop
relay_surfaces_nonzero_exit_code - verifies exit code propagation

Conclusion

No implementation work was required. The PTY spawn and signal forwarding functionality is fully implemented and tested per AGENTS.md specification.

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