Add custom layout creation.

src/Pty.zig

@@ -27,6 +27,8 @@ const c = struct {
     extern "c" fn ptsname_r(fd: c_int, buf: [*]u8, buflen: usize) c_int;
     extern "c" fn setsid() pid_t;
     extern "c" fn fork() pid_t;
+    extern "c" fn chdir(path: [*:0]const u8) c_int;
+    extern "c" fn readlink(path: [*:0]const u8, buf: [*]u8, bufsiz: usize) isize;
     extern "c" fn close(fd: fd_t) c_int;
     extern "c" fn dup2(old: fd_t, new: fd_t) c_int;
     extern "c" fn open(path: [*:0]const u8, flags: c_int, ...) c_int;
@@ -86,10 +88,15 @@ pub const Error = error{
 /// started by running e.g. /usr/bin/zsh with an argv[0] of "-zsh"; folding
 /// them together would pass "-zsh" as an ordinary argument instead, which
 /// shells either misparse or reject outright.
+///
+/// `dir` is the directory the child starts in. A null, or a directory that
+/// cannot be entered, leaves it wherever the app was started — a layout
+/// naming a path that no longer exists should still give you a usable shell.
 pub fn create(
     alloc: std.mem.Allocator,
     path: [:0]const u8,
     argv: []const [:0]const u8,
+    dir: ?[:0]const u8,
     size: Winsize,
 ) !Pty {
     const master = c.posix_openpt(O_RDWR | O_NOCTTY);
@@ -120,7 +127,7 @@ pub fn create(
     const pid = c.fork();
     if (pid < 0) return Error.ForkFailed;
     if (pid == 0) {
-        childExec(master, slave_path_z, path, argv_z, envp_z);
+        childExec(master, slave_path_z, path, argv_z, envp_z, dir);
         // childExec only returns on failure, and a forked child has no
         // sensible way to report that back to us.
         c._exit(127);
@@ -136,9 +143,17 @@ fn childExec(
     path: [:0]const u8,
     argv: [:null]const ?[*:0]const u8,
     envp: [:null]const ?[*:0]const u8,
+    dir: ?[:0]const u8,
 ) void {
     _ = c.close(master);
 
+    // Deliberately unchecked: a layout pointing at a directory that has since
+    // been moved or deleted should still open a shell, in the directory the
+    // app was started from, rather than a pane that dies on arrival.
+    if (dir) |d| if (d.len > 0) {
+        _ = c.chdir(d.ptr);
+    };
+
     // A new session detaches us from the parent's controlling terminal so
     // that we can claim the slave as our own below.
     if (c.setsid() < 0) return;
@@ -211,6 +226,28 @@ fn freeEnv(alloc: std.mem.Allocator, envp: [:null]?[*:0]const u8) void {
     alloc.free(envp);
 }
 
+/// The child's current working directory, copied into `buf`, or null if it
+/// can't be read.
+///
+/// This is the shell's directory as of right now, not the one it started in,
+/// which is what makes "save this tab as a layout" record where you actually
+/// are rather than where you began. Read straight out of procfs: there is no
+/// portable API for it, and this app is Linux-only regardless.
+pub fn cwd(self: Pty, buf: []u8) ?[]const u8 {
+    var link_buf: [64]u8 = undefined;
+    const link = std.fmt.bufPrintZ(&link_buf, "/proc/{d}/cwd", .{self.pid}) catch return null;
+
+    const n = c.readlink(link.ptr, buf.ptr, buf.len);
+    if (n <= 0) return null;
+
+    const len: usize = @intCast(n);
+    // readlink truncates rather than failing, and a truncated path is worse
+    // than none: it would silently point somewhere else.
+    if (len >= buf.len) return null;
+
+    return buf[0..len];
+}
+
 /// Tell the child its window changed size. This both updates the kernel's
 /// idea of the terminal size and delivers SIGWINCH to the foreground group.
 pub fn setSize(self: Pty, size: Winsize) void {