A refused or failed write is an err record in the log, naming the file and the reason

Through a mount every refusal reads as Invalid argument, so a shell user
could not tell a missing regexp match from a bad ctl word. Every write or
truncation the tree refuses or that fails now adds err <serial|-> <file>:
<why> to /log. No per-pane readable error file: acme's errors only takes
text, and one stream is simpler to watch.

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

1 files changed, 3 insertions, 1 deletions

diff --git a/test/fs.py b/test/fs.py
index 816f416c..c56c6298 100644
--- a/test/fs.py
+++ b/test/fs.py
@@ -200,7 +200,7 @@ def discovery(binary, embedded=False):
             assert newest(client) == second, 'a stat of new made a pane'
             # What the editor said is in the stream too.
             def pane_event():
-                while (record := client.read_fid(log, frozen)).startswith(b'msg '):
+                while (record := client.read_fid(log, frozen)).startswith((b'msg ', b'err ')):
                     pass
                 return record
             assert pane_event() == f'new {first} {root}/+New\n'.encode()
@@ -356,6 +356,8 @@ def discovery(binary, embedded=False):
                 except OSError as refused:
                     assert why in str(refused), refused
             assert client.read(f'/pane/{scratch}/body') == b'HOWDY world\nsecond line\n'
+            # Through a mount both read as EINVAL; the log says which and why.
+            assert f'err {scratch} addr: no match for regexp\n'.encode() in client.read('/log')
             client.remove(f'/pane/{scratch}')
 
     print('9P discovery: listing/stat/find are inert; new, remove, look, exec, name, sel, log, ctl lock, focus, the ctl split and commands behave')