A root focus file names the pane with the keyboard and moves it; a pane's ctl says whether it is current

Scripts had no way to ask which pane has the keyboard or to give it one.
rio answers both through a window's wctl: its read ends with current or
notcurrent, and a write of current takes the keyboard (rio(4)). A pane's
ctl line now ends the same way, and since there is one keyboard for the
whole tree, /focus reads the focused pane's serial and takes a serial to
move the keyboard there, off any header that had it; a serial no pane has
fails with "no such window". fs-bench's stale readdir check is fixed too.

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

1 files changed, 14 insertions, 1 deletions

diff --git a/src/ninep/tree.zig b/src/ninep/tree.zig
index f93ab203..b0de446e 100644
--- a/src/ninep/tree.zig
+++ b/src/ninep/tree.zig
@@ -174,6 +174,7 @@ pub const TopFile = enum(u4) {
     listeners,
     pane,
     new,
+    focus,
 
     pub fn fileName(f: TopFile) []const u8 {
         return if (f == .root) "/" else @tagName(f);
@@ -182,7 +183,7 @@ pub const TopFile = enum(u4) {
     pub fn mode(f: TopFile) u16 {
         return switch (f) {
             .root, .pane => 0o755,
-            .look, .exec, .log => 0o666,
+            .look, .exec, .log, .focus => 0o666,
             .README, .index, .status, .screen, .listeners, .new => 0o444,
         };
     }
@@ -444,6 +445,11 @@ fn topSize(p: *Pardes, f: TopFile) u64 {
         .look, .exec => ctl.resultsLen(p),
         .log => events.logLen(&p.fs.log),
         .listeners => screen.listenersLen(p),
+        .focus => focus: {
+            const pn = p.panes[p.active] orelse break :focus 0;
+            var digits: [16]u8 = undefined;
+            break :focus (std.fmt.bufPrint(&digits, "{d}\n", .{pn.serial}) catch unreachable).len;
+        },
     };
 }
 
@@ -516,6 +522,10 @@ fn readdir(p: *Pardes, req: Req, target: Target) Reply {
                     if (skip > 0) skip -= 1 else stageDirent(out, p.gpa, @intFromEnum(t), t.dir(), t.fileName());
                 }
                 if (skip > 0) skip -= 1 else stageDirent(out, p.gpa, fs.os_root, true, "os");
+                // After the first ten, which older clients list by position.
+                inline for (.{TopFile.focus}) |t| {
+                    if (skip > 0) skip -= 1 else stageDirent(out, p.gpa, @intFromEnum(t), t.dir(), t.fileName());
+                }
                 sources.stage(p, out, "", &skip);
             },
             .pane => {
@@ -705,6 +715,7 @@ fn read(p: *Pardes, req: Req, target: Target) Reply {
             .log => events.readLog(p, req),
             .screen => screen.readSnapshot(p, req, null),
             .listeners => screen.readListeners(p, req),
+            .focus => ctl.readFocus(p, req),
             // The serial the open handed this fid, so that two reads of one
             // fid answer the same pane: the read observes, the open acted.
             .new => serial: {
@@ -737,6 +748,7 @@ fn write(p: *Pardes, req: Req, target: Target) Reply {
         .top => |f| return switch (f) {
             .look, .exec => ctl.command(p, req, null, f == .exec),
             .log => events.writeLog(p, req),
+            .focus => ctl.writeFocus(p, req),
             else => Reply.fail(req.tag, E.PERM),
         },
         .pane => |t| {
@@ -894,6 +906,7 @@ test "readdir lists the root and a pane directory without creating anything" {
     }
     try testing.expectEqual(@intFromEnum(TopFile.pane), top[8].node);
     try testing.expectEqual(fs.os_root, top[9].node);
+    try testing.expectEqualStrings("focus", top[10].name);
     var idbuf: [16]u8 = undefined;
     try testing.expect(th.nameAt(top, try std.fmt.bufPrint(&idbuf, "{d}", .{serial})) == null);
     try testing.expect(th.nameAt(top, "self") == null);