pty/ctl exec fails the write when the shell cannot start; a directory removed under a shell leaves its name

`exec` on pty/ctl restarted a shell in a directory that was gone, the
write succeeding and the pane dead; and the pane was then renamed
`<dir> (deleted)` from /proc, so a restart failed even once the directory
was back. `exec` is now refused up front, ENOENT, where the directory is
gone; a shell the host cannot start fails the waiting write (late
failure, as a Save's); and a `(deleted)` cwd is no name to take.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

2 files changed, 25 insertions, 0 deletions

diff --git a/src/ninep/ctl.zig b/src/ninep/ctl.zig
index eea033ed..2f94dd3c 100644
--- a/src/ninep/ctl.zig
+++ b/src/ninep/ctl.zig
@@ -1949,6 +1949,25 @@ test "a command line from a missing directory fails ENOENT and makes no pane; a
     try testing.expect(th.logHas(p, "shell: directory missing"));
 }
 
+test "pty/ctl exec in a directory that is gone fails ENOENT; a shell that cannot start fails the write" {
+    const p = try th.withTerm(testing.allocator);
+    defer p.deinit();
+    const Starting = struct {
+        fn spawn(_: ?*anyopaque, _: u8, _: []const u8) void {}
+    };
+    p.host = .{ .vtable = &.{ .spawn = Starting.spawn } };
+    defer p.host = .{};
+    const serial = serialOf(p);
+    p.setCwd(p.paneBySerial(serial).?, "/tmp/pardes-no-such-dir-zz");
+    const refused = wr(p, Node.of(serial, .pty_ctl), "exec\n");
+    try testing.expectEqual(E.NOENT, refused.errno());
+    try testing.expectStringStartsWith(refused.reply.ename, "exec: /tmp/pardes-no-such-dir-zz: no such directory");
+    // The host's own failure is the waiting write's (late_failure).
+    p.fs.late_failure_len = 0;
+    p.shellFailed(@intCast(p.paneBySerial(serial).?), error.AccessDenied);
+    try testing.expectEqualStrings("shell: access denied", p.fs.late_failure[0..p.fs.late_failure_len]);
+}
+
 test "size is monotonic: growing is never refused, and a size once taken is taken again" {
     const p = try Pardes.init(testing.allocator, .{ .tty_only = true, .cols = 160, .rows = 60 });
     defer p.deinit();
diff --git a/src/ninep/pty.zig b/src/ninep/pty.zig
index 68f98554..aa0f499a 100644
--- a/src/ninep/pty.zig
+++ b/src/ninep/pty.zig
@@ -61,6 +61,12 @@ pub fn writeCtl(p: *Pardes, req: Req, id: usize) Reply {
         while (it.next()) |raw| {
             const line = std.mem.trim(u8, raw, " \t\r");
             if (line.len == 0) continue;
+            // A shell restarted where the directory is gone would not start:
+            // refused before anything runs, as Tty there is.
+            if (!apply and std.mem.eql(u8, line, "exec")) if (p.panes[id]) |pane| if (pardes.exec.missingDir(p, pane.cwdSlice())) {
+                const dir = pane.cwdSlice();
+                return tree.failText(req.tag, E.NOENT, std.fmt.bufPrint(&p.fs.ename, "exec: {s}: no such directory", .{dir[0..@min(dir.len, 256)]}) catch "exec: no such directory");
+            };
             if (!verb(p, id, line, apply)) return tree.failText(req.tag, E.INVAL, if (outOfRange(line)) e_winsize_range else e_bad_pty_ctl);
         }
     }