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]>

2 files changed, 37 insertions, 1 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) };
diff --git a/src/ninep/tree.zig b/src/ninep/tree.zig
index 3be75960..c4ade1f7 100644
--- a/src/ninep/tree.zig
+++ b/src/ninep/tree.zig
@@ -664,7 +664,7 @@ fn open(p: *Pardes, req: Req, target: Target) Reply {
             // pane it made, so reading it twice answers the same one and
             // closing it leaves the pane alone.
             .new => {
-                const slot = p.freeSlot() orelse return Reply.fail(req.tag, E.NFILE);
+                const slot = p.freeSlot() orelse return failText(req.tag, E.NFILE, std.fmt.bufPrint(&p.fs.ename, "pane/new: " ++ pardes.Messages.no_pane_slots, .{pardes.MAX_PANES}) catch "no pane slots");
                 p.newScratchFor(p.active, .fs);
                 const made = p.panes[slot] orelse return Reply.fail(req.tag, E.NFILE);
                 return .{ .tag = req.tag, .handle = made.serial };
@@ -1284,3 +1284,32 @@ test "editor paths resolve to the same nodes the wire serves" {
     try testing.expect(resolveSelf(p, "pane/new/body") == null);
     try testing.expectEqual(before, p.next_serial);
 }
+
+test "at the pane cap, pane/new, look and New each say so, and look reads back empty" {
+    const p = try th.withFile(testing.allocator, "x\n");
+    defer p.deinit();
+    const new = @intFromEnum(TopFile.new);
+    while (p.freeSlot() != null) try testing.expectEqual(Status.ok, call(p, .{ .tag = 1, .op = .open, .node = new }).reply.status);
+    var want: [64]u8 = undefined;
+    const said = try std.fmt.bufPrint(&want, "no pane slots: {d} max", .{pardes.MAX_PANES});
+    const refused = call(p, .{ .tag = 2, .op = .open, .node = new });
+    try testing.expectEqual(E.NFILE, refused.errno());
+    try testing.expect(std.mem.endsWith(u8, refused.reply.ename, said));
+    try testing.expect(th.logHas(p, said));
+    // A look that would open a pane fails its write, and says the same.
+    var tmp = testing.tmpDir(.{});
+    defer tmp.cleanup();
+    try tmp.dir.writeFile(testing.io, .{ .sub_path = "f.txt", .data = "f\n" });
+    var dir_buf: [4096]u8 = undefined;
+    const dir = dir_buf[0..try tmp.dir.realPath(testing.io, &dir_buf)];
+    var line: [4200]u8 = undefined;
+    const look_node = @intFromEnum(TopFile.look);
+    const looked = th.wr(p, look_node, try std.fmt.bufPrint(&line, "{s}/f.txt\n", .{dir}));
+    try testing.expect(looked.errno() != 0);
+    try testing.expect(std.mem.indexOf(u8, looked.reply.ename, said) != null);
+    try testing.expectEqualStrings("", th.rd(p, look_node, 0, 64).bytes);
+    // New from a pane's ctl fails as well.
+    const ctl_node = Node.of(p.panes[0].?.serial, .ctl);
+    const made = th.wr(p, ctl_node, "New\n");
+    try testing.expect(std.mem.indexOf(u8, made.reply.ename, said) != null);
+}