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, 12 insertions, 15 deletions

diff --git a/test/v9fs.py b/test/v9fs.py
index fa35388f..30efe9fb 100644
--- a/test/v9fs.py
+++ b/test/v9fs.py
@@ -45,6 +45,7 @@ def worker(mountpoint, socket, uid, gid, original_namespace):
     tree = mountpoint
     assert {'os', 'index', 'pane', 'status', 'look', 'exec', 'log', 'screen', 'README'} <= set(os.listdir(tree))
     assert 'self' not in os.listdir(tree) and 'new' not in os.listdir(tree)
+    assert 'new' not in os.listdir(tree / 'pane'), 'a listing would make a pane per stat'
     # A direct connection provides independent evidence for VFS reads/writes.
     with Client(socket) as client:
         assert (tree / 'index').read_bytes() == client.read('/index')
@@ -55,26 +56,22 @@ def worker(mountpoint, socket, uid, gid, original_namespace):
         subprocess.run(['find', str(tree / 'pane'), '-ls'], check=True, capture_output=True, timeout=5)
         assert (tree / 'README').read_bytes() == client.read('/README')
         assert client.read('/index') == before, 'browsing created panes'
-        # mkdir through the kernel mount opens a pane. The editor names it
-        # after its serial, not after the name asked for, so the kernel's own
-        # revalidation of that name may fail; the index is the answer.
+        # Opening pane/new makes a pane and the read names it, so no trip
+        # through the index. Whether a repeated path reaches the server at all
+        # is the kernel's dentry cache's business, so the second pane comes
+        # over the wire, where the open is exact.
         def serials():
             return {int(row.split()[0]) for row in (tree / 'index').read_bytes().splitlines()}
 
-        def mkpane(name):
+        def mkpane():
             known = serials()
-            try:
-                (tree / 'pane' / name).mkdir()
-            except FileNotFoundError:
-                pass
-            made = serials() - known
-            assert len(made) == 1, (name, made)
-            return made.pop()
+            made = int((tree / 'pane' / 'new').read_bytes())
+            assert made not in known, 'the open of new made no pane'
+            return made
 
-        serial = mkpane('kernel-made')
-        another = mkpane('kernel-made-again')
-        assert serial != another, 'a second mkdir reused a pane'
-        assert not (tree / 'pane' / 'kernel-made').exists()
+        serial = mkpane()
+        another = int(client.read('/pane/new'))
+        assert serial != another, 'a second open of new reused a pane'
         (tree / 'pane' / str(another)).rmdir()
         assert another not in serials()
         pane = tree / 'pane' / str(serial)