A command line cut across writes runs once whole, and look/exec answer per open

A mount cuts a write at its message size, and each piece ran on its own: a
line cut at the boundary ran as shell commands and a 50 KB Edit block
broke. An open of look, exec, tagexec, any ctl or a column's exec now keeps
an unfinished last line, or an Edit block still open, until its next write
or its release, and never runs a fragment. The same open record holds what
its last write touched, which a read on it answers (as /net/tcp/clone
does); an open that never wrote reads the session's last as of its open.
A release that runs a held line waits for quiet and counts as a change;
a clicked line with a control character is still refused at its write.
Tested in unit tests, fs.py (python client, 8 KiB) and selfmount.py (the
kernel mount, a 1000-line seq burst and a 50 KB Edit block).

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

1 files changed, 24 insertions, 0 deletions

diff --git a/test/fs.py b/test/fs.py
index 0e38f950..bdfc3fcb 100644
--- a/test/fs.py
+++ b/test/fs.py
@@ -531,8 +531,32 @@ def signal_cleans_up(binary):
                 assert not address.exists() and not detached.exists(), (sig, list(root.iterdir()))
 
 
+def cut_writes(binary):
+    """A write bigger than a 9P message comes as several, cut anywhere: each
+    command line runs once, whole, an Edit block once it ends, and a last
+    line with no newline when the open closes. Never a fragment."""
+    with tempfile.TemporaryDirectory(prefix='pardes-cut-') as directory:
+        with session(binary, Path(directory), 'cut') as (client, _):
+            burst = b''.join(b'Msg burst-%04d\n' % i for i in range(1000))
+            assert len(burst) > client.msize
+            client.write('/exec', burst)
+            log = client.read('/log').decode()
+            said = [line.split(' ', 2)[2] for line in log.splitlines() if line.startswith('msg ') and 'burst-' in line]
+            assert said == ['burst-%04d' % i for i in range(1000)], said[:3] + said[-3:]
+            assert '\nrun ' not in log and 'exit 127' not in log, log[-400:]
+            client.write('/exec', b'Msg no-newline-at-end')
+            assert 'msg ' in client.read('/log').decode().split('no-newline-at-end')[0][-40:]
+            serial = new_pane(client, b'')
+            block = b'Edit ,a\n' + b''.join(b'line %05d of the block\n' % i for i in range(2500)) + b'.\n'
+            assert len(block) > 50000
+            client.write(f'/pane/{serial}/ctl', block)
+            body = client.read(f'/pane/{serial}/body').decode().splitlines()
+            assert body == ['line %05d of the block' % i for i in range(2500)], (len(body), body[:2])
+
+
 def test(binary, quic=False):
     started = time.monotonic()
+    cut_writes(binary)
     run_file(binary)
     exit_answers(binary)
     unattended_messages_expire(binary)