Make a pane by opening /pane/new, and a Plan 9 idiom pass

The Tcreate that replaced acme's /new was a step away from the idiom dressed
up as a step toward it. A pane is named by a server-assigned serial, so the
create ignored the client's name: `mkdir /pane/foo` succeeded and left you
/pane/12. A mkdir that does not make the directory you named is worse than the
read-with-side-effect it replaced, and it broke in the shell workflow that
motivated the change. `create` is out of the declared features, so Tcreate is
EPERM again; Tremove stays, since `rm` to close a pane is unambiguously right.

/pane/new is now opened, not created: the open makes the pane, the read of
that fid answers its serial, two reads agree, and closing it leaves the pane.
That is /net/tcp/clone's mechanism (kernel/network/ip/devip.c, in ipopen),
not acme's, and the difference is deliberate. acme allocates during the walk
and lands inside the new window, so /dev/new/body works in one step, and it
can afford to list `new` because a Plan 9 directory read carries every entry's
stat and nothing walks. A kernel or FUSE mount walks and stats each name a
listing gave it, so allocate-on-walk would make a pane per `ls -l`. Allocating
on open keeps `new` listed -- a stat is not an open -- at the cost of the
one-step new/body. `new` stays unreachable from an editor path, because that
resolution serves Look hover previews.

The idiom pass behind it, read out of the Plan 9 tree at
~/05-genizah/principia-softwarica rather than recalled:

Rerror carries a string, not an errno (man 5 error: `ename[s]`), and acme
names every refusal. The five refusals pardes shares with acme now say what
they mean; the generic sites keep their bare errno rather than invent strings
acme does not have. body and tag declare DMAPPEND, which they had always
behaved as (acme(4): "always appended; the file offset is ignored"), checked
first against Linux's fs/9p, which never maps the bit. excl stays unset
everywhere, because acme sets DMEXCL on nothing. Blocking reads, per-object
addr scope and the readable pane ctl were already right. Real stat sizes and
qid versions stay: acme reports length 0 and version 0 for everything, and
Linux clients need better.

One bug fell out of it. open reset the addr range, so `echo '#0,#5' >addr;
cat addr` answered `0 0` and `cp addr dot` copied zeros. acme(4) makes the
contract explicit -- "a regular expression may be evaluated by writing it to
addr and reading it back" -- and acme gets away with resetting on the 0-to-1
open only because its clients hold the fid across both. A shell cannot: that
is two opens. The register is cleared by truncating it now.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

1 files changed, 5 insertions, 0 deletions

diff --git a/features.txt b/features.txt
index bee079a7..a09607ea 100644
--- a/features.txt
+++ b/features.txt
@@ -68,3 +68,8 @@ bias, so only a definite refusal reaps.
 After all of the above, do an optimization pass on startup time for the gui and tty platforms: measure first, then optimize. build.zig already has a `perf` step
 (gesture latency and bounded terminal stress, with --json and a --base baseline to compare against), so a startup measurement belongs there rather than in a new
 harness, and the baseline files are how a regression gets caught later.
+
+9p create semantics: doing an action on read is not how plan9 frames it. The canonical pattern is the clone file -- /net/tcp/clone -- where *opening* allocates the
+object and reading the fid only tells you which one you got; acme's new/ctl is the same shape, and pardes's old /new already keyed its side effect on open. The
+Tcreate that replaced it has a real wart: a pane is named by a server-assigned serial, so the create ignores the client's name and `mkdir /pane/foo` leaves you
+/pane/12. Replace it with /pane/clone (open allocates, read answers the serial) and keep Tremove, which is unambiguously right.