A pty/ctl exec that cannot start its shell keeps the one running

Each host closed the running shell before it forked the new one, so an exec whose shell failed left the pane with none and later runs answered error shell gone. A shell not there is now refused up front, and every host starts the new shell first, replacing the old only once the close-on-exec pipe says it ran; a failure there is only said (restartFailed).

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

1 files changed, 8 insertions, 0 deletions

diff --git a/src/ninep/pty.zig b/src/ninep/pty.zig
index aa0f499a..bf5f432c 100644
--- a/src/ninep/pty.zig
+++ b/src/ninep/pty.zig
@@ -67,6 +67,14 @@ pub fn writeCtl(p: *Pardes, req: Req, id: usize) Reply {
                 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");
             };
+            // Nor is a shell that is not there, or a script whose
+            // interpreter is not: refused, and the running shell kept.
+            if (comptime pardes.hosted) if (!apply and std.mem.eql(u8, line, "exec")) if (p.panes[id]) |pane| {
+                const bin = pane.shell orelse p.shellBin();
+                var why: [320]u8 = undefined;
+                if (@import("../host_io.zig").Shell.refusal(bin[0..@min(bin.len, 200)], &why)) |refused|
+                    return tree.failText(req.tag, E.NOENT, std.fmt.bufPrint(&p.fs.ename, "exec: {s}", .{refused}) catch "exec: no such shell");
+            };
             if (!verb(p, id, line, apply)) return tree.failText(req.tag, E.INVAL, if (outOfRange(line)) e_winsize_range else e_bad_pty_ctl);
         }
     }