A desktop session holds 64 panes, and every route that finds none free says so

Sixteen panes was far too few for a real session. MAX_PANES is now 64 on a
desktop and stays 16 on the board; the 48 more slots cost about 3.7 KB each
in fixed tables (the core 1.33 to 1.45 MB, its surface 19 to 72 KB), and a
pane itself (84 KB and its terminal) is allocated only when it opens. At the
cap, pane/new fails with `pane/new: no pane slots: 64 max` and an err
record rather than a bare ENFILE, a 9P look or exec that would open a pane
fails its write with the same words and reads back empty, and New, Tty,
Tutor and Alt-n say it on the message row (their ctl writes fail with it).

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

1 files changed, 7 insertions, 0 deletions

diff --git a/src/ninep/ctl.zig b/src/ninep/ctl.zig
index 11de3c1c..325fc8c8 100644
--- a/src/ninep/ctl.zig
+++ b/src/ninep/ctl.zig
@@ -138,7 +138,14 @@ pub fn command(p: *Pardes, req: Req, serial: ?u32, exec: bool) Reply {
             if (!apply) continue;
             const id = if (serial) |s| p.paneBySerial(s) orelse break else p.active;
             if (p.panes[id] == null) return Reply.fail(req.tag, E.NOENT);
+            p.fs.no_pane_slot = false;
             run(p, id, exec, text);
+            // A pane it would have opened had no slot: the write fails, and
+            // a look reads back nothing.
+            if (p.fs.no_pane_slot) {
+                p.fs.results_len = 0;
+                return tree.failText(req.tag, E.IO, p.fs.ename[0..p.fs.no_pane_slot_len]);
+            }
         }
     }
     return .{ .tag = req.tag, .written = @intCast(req.data.len) };