sl0ptty: M0 — libghostty-vt vendored, one pane, compositor, diff renderer

Design contract in DESIGN.md. libghostty-vt vendored fresh from ghostty
main@bedb6f2 (zig 0.16), built to a 14MB static lib.

M0: pty spawn (no libutil), a lib-vt terminal per pane, a screen cell buffer,
a diff emitter that repaints only what changed, raw mode, SIGWINCH, and a
~120Hz frame cap that coalesces pty reads.

Also the seed of the headless harness: --headless dumps the composited screen
without a tty, so composition is testable in milliseconds.

tests/live_m0.py covers what headless cannot: raw mode, the diff emitter,
resize propagation, and the quit path. 6/6 green.

1 files changed, 86 insertions, 0 deletions

diff --git a/src/pty.c b/src/pty.c
new file mode 100644
index 0000000..5ce166e
--- /dev/null
+++ b/src/pty.c
@@ -0,0 +1,86 @@
+/* pty spawn. Hand-rolled rather than forkpty() so we need no libutil and
+ * behave identically under musl. */
+#define _GNU_SOURCE
+#include "sl0ptty.h"
+
+#include <errno.h>
+#include <fcntl.h>
+#include <signal.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/ioctl.h>
+#include <sys/wait.h>
+#include <termios.h>
+#include <unistd.h>
+
+int pty_spawn(pty_t *p, const char *const argv[], uint16_t cols, uint16_t rows,
+              const char *cwd) {
+  int master = posix_openpt(O_RDWR | O_NOCTTY);
+  if (master < 0) return -1;
+  if (grantpt(master) < 0 || unlockpt(master) < 0) {
+    close(master);
+    return -1;
+  }
+
+  char slave_name[128];
+  if (ptsname_r(master, slave_name, sizeof slave_name) != 0) {
+    close(master);
+    return -1;
+  }
+
+  struct winsize ws = {.ws_col = cols, .ws_row = rows};
+  ioctl(master, TIOCSWINSZ, &ws);
+
+  pid_t pid = fork();
+  if (pid < 0) {
+    close(master);
+    return -1;
+  }
+
+  if (pid == 0) {
+    /* child: new session, pty becomes the controlling terminal */
+    setsid();
+    int slave = open(slave_name, O_RDWR);
+    if (slave < 0) _exit(127);
+    ioctl(slave, TIOCSCTTY, 0);
+    dup2(slave, STDIN_FILENO);
+    dup2(slave, STDOUT_FILENO);
+    dup2(slave, STDERR_FILENO);
+    if (slave > STDERR_FILENO) close(slave);
+    close(master);
+
+    if (cwd) { if (chdir(cwd) != 0) { /* keep going in the old cwd */ } }
+
+    /* We are opinionated about the outer terminal (D11), and we present the
+     * same contract inward. xterm-ghostty is what libghostty-vt implements. */
+    setenv("TERM", "xterm-ghostty", 1);
+    setenv("SL0PTTY", "1", 1);
+    unsetenv("ZELLIJ");
+
+    signal(SIGPIPE, SIG_DFL);
+    execvp(argv[0], (char *const *)argv);
+    _exit(127);
+  }
+
+  /* parent */
+  int flags = fcntl(master, F_GETFL, 0);
+  fcntl(master, F_SETFL, flags | O_NONBLOCK);
+  p->fd = master;
+  p->pid = pid;
+  return 0;
+}
+
+int pty_resize(pty_t *p, uint16_t cols, uint16_t rows) {
+  struct winsize ws = {.ws_col = cols, .ws_row = rows};
+  return ioctl(p->fd, TIOCSWINSZ, &ws);
+}
+
+void pty_close(pty_t *p) {
+  if (p->fd >= 0) close(p->fd);
+  p->fd = -1;
+  if (p->pid > 0) {
+    kill(p->pid, SIGHUP);
+    waitpid(p->pid, NULL, WNOHANG);
+  }
+  p->pid = -1;
+}