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, 2 deletions

diff --git a/src/tty/tty.zig b/src/tty/tty.zig
index 3c8c90bc..a6f6bdfc 100644
--- a/src/tty/tty.zig
+++ b/src/tty/tty.zig
@@ -1253,9 +1253,15 @@ const Shell = struct {
 
     fn spawn(ctx: ?*anyopaque, pane: u8, cwd: []const u8) void {
         const s = of(ctx);
-        closePty(ctx, pane); // a shell still in the slot goes first, reaped
+        // The new shell starts before a running one goes: one that cannot
+        // start leaves the running one be (pty/ctl's exec).
+        const running = s.ptys[pane] != null;
+        const child = host_io.forkShell(s.core, pane, s.prompt_rcs, s.core.shellBin(), cwd, s.core.screen_h, s.core.screen_w, s.fs) catch |err| {
+            if (running) return s.core.restartFailed(pane, err);
+            return s.core.shellFailed(pane, err);
+        };
+        closePty(ctx, pane); // a shell still in the slot goes, reaped
         s.gens[pane] +%= 1;
-        const child = host_io.forkShell(s.core, pane, s.prompt_rcs, s.core.shellBin(), cwd, s.core.screen_h, s.core.screen_w, s.fs) catch |err| return s.core.shellFailed(pane, err);
         const command = if (s.core.panes[pane]) |pn| pn.command != null else false;
         s.ptys[pane] = .{ .file = child.file, .pid = child.pid, .reader = .{ .any_future = null, .result = {} }, .cmd = .{ .watched = command and host_io.watchExit(child.pid), .fd = child.file.handle } };
         var lbuf: [pardes.memory.limits.host_path_cap + 1]u8 = undefined;