A shell that cannot start fails Tty and pty/ctl exec with why, before either answers

A shell whose exec failed (a script's missing interpreter) was reported
started: Tty and pty/ctl exec succeeded, then the pane died. The child now
reports a failed chdir or exec through a close-on-exec pipe the parent
reads before acknowledging the shell; the host's spawn fails with the
reason (`shell not found`, ENOENT), which fails the waiting write. A Tty
whose first shell never ran leaves no pane; a terminal restarted by
pty/ctl exec keeps its pane.

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

1 files changed, 22 insertions, 0 deletions

diff --git a/test/fs.py b/test/fs.py
index 3c10c53b..20a9bbff 100644
--- a/test/fs.py
+++ b/test/fs.py
@@ -515,6 +515,28 @@ def new_terminals_named_once(binary):
             for serial in made:
                 assert f'\nrename {serial} ' not in '\n' + log, (serial, log)
                 assert f'\nnew {serial} {root}' in '\n' + log, (serial, log)
+            # A shell that cannot start (its interpreter is not there): Tty
+            # fails the write with why and leaves no pane; pty/ctl exec
+            # fails with why and keeps its terminal.
+            bad = root / 'badsh'
+            bad.write_bytes(b'#!/nonexistent/interp\n')
+            bad.chmod(0o755)
+            panes = client.read('/index')
+            try:
+                client.write('/pane/1/ctl', f'Tty {bad}\n'.encode())
+                raise AssertionError('Tty of a shell that cannot start was taken')
+            except OSError as refused:
+                assert 'shell' in str(refused), refused
+            time.sleep(.3)
+            assert client.read('/index').count(b'\n') == panes.count(b'\n'), client.read('/index')
+            client.write('/ctl', f'Shell {bad}\n'.encode())
+            try:
+                client.write(f'/pane/{made[0]}/pty/ctl', b'exec\n')
+                raise AssertionError('pty/ctl exec of a shell that cannot start was taken')
+            except OSError as refused:
+                assert 'shell' in str(refused), refused
+            client.write('/ctl', b'Shell\n')
+            assert str(made[0]).encode() in b' '.join(r.split()[0] for r in client.read('/index').splitlines())
 
 
 def workflow_canary(binary):