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, 7 insertions, 19 deletions

diff --git a/src/ninep/testing.zig b/src/ninep/testing.zig
index ad1d84e4..44329eee 100644
--- a/src/ninep/testing.zig
+++ b/src/ninep/testing.zig
@@ -69,26 +69,14 @@ pub fn look_up(p: *Pardes, dir: u64, name: []const u8) Answer {
     return call(p, .{ .tag = 3, .op = .lookup, .node = dir, .data = name });
 }
 
-/// Creates a pane in /pane, as a client's mkdir does, and returns its serial.
+/// Makes a pane the way a client does, by walking to /pane/new, and answers
+/// the serial of the pane the walk landed on.
+/// The open is what makes the pane; the handle it answers is the serial, which
+/// is also what a read of that fid reports.
 pub fn newPane(p: *Pardes) !u32 {
-    const made = mkdir(p, "scratch");
-    if (made.reply.status != .ok) return error.NoPane;
-    const target = tree.Node.target(made.reply.attr.node) orelse return error.NoPane;
-    return switch (target) {
-        .pane => |t| t.serial,
-        .top => error.NoPane,
-    };
-}
-
-pub fn mkdir(p: *Pardes, name: []const u8) Answer {
-    return call(p, .{
-        .tag = 9,
-        .op = .open,
-        .node = @intFromEnum(tree.TopFile.pane),
-        .data = name,
-        .create = true,
-        .perm = 0x8000_0000 | 0o755,
-    });
+    const made = call(p, .{ .tag = 8, .op = .open, .node = @intFromEnum(tree.TopFile.new) });
+    if (made.reply.status != .ok or made.reply.handle == 0) return error.NoPane;
+    return made.reply.handle;
 }
 
 pub fn rmdir(p: *Pardes, node: u64) Answer {