One rule for failing builtins: the write fails, one err, no msg

A builtin that failed through a ctl failed the write and logged its err,
but through look, exec, tagexec or a column's exec it only said so on the
message row, logged as a msg, and the write succeeded. Now every click
write runs its builtin as a ctl line does (ctl.captured): a failure fails
the write, with its words (EINVAL for malformed input, else EIO or what
the words name), logs one err and no msg or announcement. get's Modified
and a look miss are shown but logged once, as their err. A control
character's refusal names its reason, a failed click reads back nothing,
and look never reads back a pane closed since. The special case for a
cut-short Edit through exec goes (the rule covers it); fs.md's table of
exceptions becomes the one rule, as does the skill.

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

1 files changed, 14 insertions, 3 deletions

diff --git a/test/fs.py b/test/fs.py
index b7e8adcc..8d16d718 100644
--- a/test/fs.py
+++ b/test/fs.py
@@ -133,6 +133,16 @@ def execute(client, serial, command):
     client.write(f'/pane/{serial}/exec', f'{command}\n'.encode())
 
 
+def execute_refused(client, serial, command, words):
+    """A middle click on a builtin that fails: the write fails, saying why."""
+    try:
+        execute(client, serial, command)
+    except OSError as why:
+        assert words in str(why), (command, why)
+        return
+    raise AssertionError(f'{command!r} was taken')
+
+
 def walk_tree(client, path='/', skip=('/log', '/os', '/screen')):
     """Stat every entry below `path`, as `find` does, without opening anything."""
     seen = []
@@ -725,8 +735,9 @@ def test(binary, quic=False):
                 execute(remote, control, 'Mount peer ' + str(address))
                 opened = look(remote, '/n/peer/pane/1/body', source=control)
                 assert remote.read(f'/pane/{opened}/body') == client.read('/pane/1/body')
-                execute(remote, control, 'Mount peer ' + str(own_address))
-                execute(remote, control, 'Unmount peer')
+                # A builtin that fails through exec fails the write (one rule).
+                execute_refused(remote, control, 'Mount peer ' + str(own_address), 'already mounted')
+                execute_refused(remote, control, 'Unmount peer', 'mount in use')  # a pane is open on it
                 remote.write(f'/pane/{opened}/body', b'runtime mounted Save\n', truncate=True)
                 remote.write(f'/pane/{opened}/exec', b'Save\n')
                 assert client.read('/pane/1/body') == b'runtime mounted Save\n'
@@ -753,7 +764,7 @@ def test(binary, quic=False):
             with session(binary, root, name, tty=tty) as (old, address):
                 old.write('/pane/1/body', b'dumped state\n', truncate=True)
                 control = new_pane(old, b'')
-                execute(old, control, 'Restore ' + str(root / 'missing dump.zon'))
+                execute_refused(old, control, 'Restore ' + str(root / 'missing dump.zon'), 'no such file')
                 assert old.read('/pane/1/body') == b'dumped state\n'
                 execute(old, control, 'Mount own ' + str(address))
                 execute(old, control, 'Dump')