A write of a multiple of 4096 bytes holds the partial line it ends with, as one that fills its Twrite does: a burst through a mount no longer runs its cut lines as two commands

selfmount's 1000-line burst reported 943 lines, on main too. The log
ring was not losing records: a follower opened before the burst heard
exactly 943 and no `lost`, and the burst itself exited 1 with
`err 1 exec: wrong #args in control message "Msg"`. `seq ... > exec`
through the mount arrives as stdio's 4096-byte writes, each shorter than
a Twrite. The rule that such a write is whole ran each one's cut last
line at once, so both halves of the line ran as commands (hence `exit
127` records). At 12288 bytes the cut left a bare `Msg`, which was
refused and failed the write and the rest of the burst.

A write of a multiple of 4096 bytes is where a writer's buffer (stdio, a
page cache) filled, so it may go on: its unended tail now waits for the
next write or the close, like one that fills its Twrite. `printf Save >
exec` stays whole at once. fs.md says so, the 9P fuzzer's model of served
lines follows it, and a unit test cuts a `Msg` at 4096 bytes.

A follower does not lose records silently: a ring that outruns one
already reads it `lost N` first (documented, tested). selfmount now
follows the log from before the burst, so it checks every line heard
once, in order, or a loss said, not a fresh open of a 64 KiB ring. It
passed 3 in 3 and joins the gates with the new 9ns.

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

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