b91f705a

pty: add openForScenario — pty pair without a child

a73x 2026-04-19 13:22

Commit message

pty: add openForScenario — pty pair without a child

Uses openpty(3) instead of forkpty(3). master_fd is non-blocking
(same as spawn). child_pid is sentinel -1 so isChildAlive returns
true and deinit skips reaping. Used by scenario mode where the
main loop needs a live pty invariant but no shell.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/pty.zig

Old		New
@@ -85,6 +85,7 @@ pub const Pty = struct {
85	}	85	}
86		86
87	pub fn isChildAlive(self: *Pty) bool {	87	pub fn isChildAlive(self: *Pty) bool {
		88	if (self.child_pid == -1) return true; // scenario mode: always alive
88	if (self.child_reaped) return false;	89	if (self.child_reaped) return false;
89	var status: c_int = 0;	90	var status: c_int = 0;
90	const rc = c.waitpid(self.child_pid, &status, c.WNOHANG);	91	const rc = c.waitpid(self.child_pid, &status, c.WNOHANG);
@@ -98,11 +99,43 @@ pub const Pty = struct {
98		99
99	pub fn deinit(self: *Pty) void {	100	pub fn deinit(self: *Pty) void {
100	std.posix.close(self.master_fd);	101	std.posix.close(self.master_fd);
101	if (!self.child_reaped) {	102	if (self.child_pid != -1 and !self.child_reaped) {
102	_ = std.c.kill(self.child_pid, std.c.SIG.TERM);	103	_ = std.c.kill(self.child_pid, std.c.SIG.TERM);
103	_ = std.c.waitpid(self.child_pid, null, 0);	104	_ = std.c.waitpid(self.child_pid, null, 0);
104	}	105	}
105	}	106	}
		107
		108	/// Open a Pty without spawning a child. Master and slave fds are real,
		109	/// the pty is usable for TIOCSWINSZ, but `isChildAlive` returns true
		110	/// forever so a scenario-mode main loop stays scheduled. No fork().
		111	pub fn openForScenario(cols: u16, rows: u16) !Pty {
		112	var master: c_int = undefined;
		113	var slave: c_int = undefined;
		114	var winsize = c.struct_winsize{
		115	.ws_row = rows,
		116	.ws_col = cols,
		117	.ws_xpixel = 0,
		118	.ws_ypixel = 0,
		119	};
		120
		121	// openpty(3): allocates a pty pair without forking.
		122	if (c.openpty(&master, &slave, null, null, &winsize) < 0) {
		123	return error.OpenptyFailed;
		124	}
		125	// Slave fd isn't used by anyone in scenario mode; close to avoid leak.
		126	_ = c.close(slave);
		127
		128	// Match spawn's O_NONBLOCK on master.
		129	const flags = try std.posix.fcntl(master, std.posix.F.GETFL, 0);
		130	const nonblock_bit: usize = @as(u32, @bitCast(std.posix.O{ .NONBLOCK = true }));
		131	_ = try std.posix.fcntl(master, std.posix.F.SETFL, flags \| nonblock_bit);
		132
		133	return .{
		134	.master_fd = master,
		135	.child_pid = -1, // sentinel: no child
		136	.child_reaped = true, // there's nothing to reap
		137	};
		138	}
106	};	139	};
107		140
108	test "Pty.write and read echoes through shell" {	141	test "Pty.write and read echoes through shell" {
@@ -168,6 +201,14 @@ test "Pty.resize sets winsize via ioctl" {
168	try std.testing.expectEqual(@as(c_ushort, 40), ws.ws_row);	201	try std.testing.expectEqual(@as(c_ushort, 40), ws.ws_row);
169	}	202	}
170		203
		204	test "openForScenario: returns a pty with master fd and fake child" {
		205	var p = try Pty.openForScenario(80, 24);
		206	defer p.deinit();
		207	try std.testing.expect(p.master_fd >= 0);
		208	try std.testing.expectEqual(@as(std.posix.pid_t, -1), p.child_pid);
		209	try std.testing.expect(p.isChildAlive());
		210	}
		211
171	test "Pty.isChildAlive returns true while shell runs, false after exit" {	212	test "Pty.isChildAlive returns true while shell runs, false after exit" {
172	var pty = try Pty.spawn(.{	213	var pty = try Pty.spawn(.{
173	.cols = 80,	214	.cols = 80,