A command line runs when its newline comes, or at its open's close, never because a write was short: a long line cut by a mount is one line

A write shorter than its message, and not a multiple of 4096, was taken
as whole, its tail a line. 9ns cuts a FUSE write of up to 512 KiB into
65512-byte Twrites, the last one short, so a 600 KiB `Edit ,c/.../`
arrived cut and its tail ran as an unknown control message; the 1 MiB
refusal failed the same way. Each open now holds a partial line until
its newline, or its release, which ends the last line; an Edit block is
whole when it closes, as before. A name write with a NUL still fails
its write at once. selfmount writes a 600 KiB Edit line and a 1.1 MiB
line through a real 9ns mount; building.typ says how lines are cut.

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

1 files changed, 6 insertions, 9 deletions

diff --git a/test/fs.py b/test/fs.py
index 52ac61c8..0e1e44df 100644
--- a/test/fs.py
+++ b/test/fs.py
@@ -1034,20 +1034,17 @@ def test(binary, quic=False):
             with Client(address, msize=256) as small:
                 assert set(small.list('/os' + str(listing))) == {
                     f'entry-{number:02}' for number in range(24)}
-            # 64 KiB frames: a ctl line with no newline, whole in its one
-            # Twrite, runs with it however long -- the cutoff for a write
-            # that may go on is the frame the client asked for, less 24.
+            # 64 KiB frames: a ctl line with no newline is held whatever the
+            # size of its write -- a mount cuts lines anywhere -- and runs at
+            # the close, its failure then in the log.
             with Client(address, msize=65536) as big:
                 assert big.msize == 65536
                 ctl = big.open('/pane/1/ctl', 1)
                 line = b'bogus' + b'x' * 20000
-                try:
-                    big.rpc(118, struct.pack('<IQI', ctl, 0, len(line)) + line)
-                except OSError:
-                    pass  # refused with its write (a line over 1024 bytes), not held
-                else:
-                    raise AssertionError('a whole 20000-byte line waited for a newline')
+                big.rpc(118, struct.pack('<IQI', ctl, 0, len(line)) + line)
+                assert b'ctl: unknown control message' not in client.read('/log')
                 big.close(ctl)
+                assert b'ctl: unknown control message' in client.read('/log')
 
             frozen = client.open('/screen')
             before = bytearray(client.read_fid(frozen, count=31))