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, 12 insertions, 0 deletions

diff --git a/src/Messages.zig b/src/Messages.zig
index 8b6c795d..7d284e6b 100644
--- a/src/Messages.zig
+++ b/src/Messages.zig
@@ -445,10 +445,22 @@ pub fn messageLog(m: *const Messages, i: usize) ?*const LoggedMessage {
 }
 
 /// `<operation>: <the error in words>`: FileNotFound reads "file not found".
+/// What a route that would open a pane says when every slot is taken.
+pub const no_pane_slots = "no pane slots: {d} max";
+
 pub fn reportError(p: *Pardes, id: usize, operation: []const u8, err: anyerror) void {
     var buf: [256]u8 = undefined;
     var w = std.Io.Writer.fixed(&buf);
     w.print("{s}: ", .{operation}) catch {};
+    // Every route that opens a pane says the cap the same way.
+    if (err == error.NoPaneSlots) {
+        w.print(no_pane_slots, .{pardes.MAX_PANES}) catch {};
+        // A 9P look or exec that would open the pane fails with it too.
+        p.fs.no_pane_slot = true;
+        @memcpy(p.fs.ename[0..w.buffered().len], w.buffered());
+        p.fs.no_pane_slot_len = @intCast(w.buffered().len);
+        return reportFailure(p, id, w.buffered());
+    }
     for (@errorName(err), 0..) |c, i| {
         if (std.ascii.isUpper(c) and i > 0) w.writeByte(' ') catch break;
         w.writeByte(std.ascii.toLower(c)) catch break;