1 files changed, 170 insertions, 2 deletions

diff --git a/src/tty/tty.zig b/src/tty/tty.zig
index df1e5d02..f5ae078a 100644
--- a/src/tty/tty.zig
+++ b/src/tty/tty.zig
@@ -132,7 +132,14 @@ fn readInput(loop: *Loop, tty: anytype, cache: *vaxis.GraphemeCache) !void {
         if (carried == buf.len) return error.InputSequenceTooLong;
         const received = try tty.read(buf[carried..]);
         if (received == 0) return;
-        const end = carried + received;
+        // The terminal's answers to frames (FrameAck) come out first,
+        // wherever they sit: never events, and never glued to a key (an
+        // Escape pressed just before one would read as Alt on it).
+        const end = FrameAck.take(buf[0 .. carried + received], loop);
+        if (end == 0) {
+            carried = 0;
+            continue;
+        }
         if (kitty_shm_probing.load(.acquire)) if (std.mem.indexOf(u8, buf[0..end], std.fmt.comptimePrint("\x1b_Gi={d};", .{kitty_shm_probe_id}))) |at| {
             kitty_shm_probing.store(false, .release);
             kitty_shm.store(std.mem.startsWith(u8, buf[at..end], std.fmt.comptimePrint("\x1b_Gi={d};OK", .{kitty_shm_probe_id})), .release);
@@ -153,7 +160,8 @@ fn readInput(loop: *Loop, tty: anytype, cache: *vaxis.GraphemeCache) !void {
         // its writes whole, so no sequence arrives cut right after its ESC.
         // Waiting on it cost every Escape press 50 ms on a legacy terminal
         // (tmux, most ssh sessions), and 1 s before kitty's flags were on.
-        var waited = carried == 0 and received == 1;
+        // (A terminal's answer to a frame is one write too: FrameAck.)
+        var waited = carried == 0 and end == 1;
         while (consumed < parse_end) {
             // A lone ESC that ends a longer read is the Escape key, or the
             // first byte of a sequence whose rest the read boundary held
@@ -162,6 +170,9 @@ fn readInput(loop: *Loop, tty: anytype, cache: *vaxis.GraphemeCache) !void {
             // kitty keyboard protocol Escape is `CSI 27 u`, so there the ESC
             // can only start a sequence and the wait can be long.
             const rest = buf[consumed..parse_end];
+            // A frame's answer cut short by the read waits for its rest.
+            if (parse_end == end and rest.len > 1 and rest.len < FrameAck.answer.len and
+                std.mem.startsWith(u8, FrameAck.answer, rest)) break;
             if (!waited and parse_end == end and rest[0] == 0x1b and (rest.len == 1 or (rest.len == 2 and rest[1] == 0x1b))) {
                 waited = true;
                 if (try inputFollows(tty, loop.io, if (loop.vaxis.caps.kitty_keyboard) 1000 else 50)) break;
@@ -191,6 +202,63 @@ fn readInput(loop: *Loop, tty: anytype, cache: *vaxis.GraphemeCache) !void {
     }
 }
 
+/// One frame in flight at most. Each frame the tty draws ends with a device
+/// status request (`CSI 5 n`), and no other frame is drawn until the
+/// terminal's `CSI 0 n` comes back: over ssh the frames a burst of scrolling
+/// or typing would queue behind each other (sshd's window holds megabytes)
+/// are never drawn, and the burst ends one frame and one round trip after
+/// its last input. On a local terminal the answer is back in well under a
+/// millisecond. A terminal that never answers turns this off for good after
+/// `timeout_ns`.
+const FrameAck = struct {
+    const request = "\x1b[5n";
+    const answer = "\x1b[0n";
+    const timeout_ns: u64 = std.time.ns_per_s;
+    var waiting = std.atomic.Value(bool).init(false);
+    var off = std.atomic.Value(bool).init(false);
+    /// When the request went out (the loop's thread only).
+    var sent_ns: u64 = 0;
+
+    /// Take every answer out of `bytes`, closing up the rest; the new
+    /// length. The loop is woken for the frame that waited on one.
+    fn take(bytes: []u8, loop: *Loop) usize {
+        var len = bytes.len;
+        var found = false;
+        while (std.mem.indexOf(u8, bytes[0..len], answer)) |at| {
+            std.mem.copyForwards(u8, bytes[at..], bytes[at + answer.len .. len]);
+            len -= answer.len;
+            found = true;
+        }
+        if (found and waiting.swap(false, .acq_rel)) loop.postEvent(.nop) catch {};
+        return len;
+    }
+
+    /// Whether a new frame must wait (and the loop has a wake at the
+    /// timeout); past the timeout the terminal is taken to never answer.
+    fn holds(now_ns: u64, io: std.Io) bool {
+        if (off.load(.acquire) or !waiting.load(.acquire)) return false;
+        const waited_ns = now_ns -| sent_ns;
+        if (waited_ns >= timeout_ns) {
+            off.store(true, .release);
+            waiting.store(false, .release);
+            return false;
+        }
+        requestWake(io, @intCast((timeout_ns - waited_ns + std.time.ns_per_ms - 1) / std.time.ns_per_ms));
+        return true;
+    }
+
+    /// After a frame that wrote something: ask for the answer.
+    fn ask(w: *std.Io.Writer, now_ns: u64) void {
+        if (off.load(.acquire)) return;
+        // Waiting before the request goes: a local terminal can answer
+        // before the write returns, and an answer nobody waited for is lost.
+        sent_ns = now_ns;
+        waiting.store(true, .release);
+        w.writeAll(request) catch return waiting.store(false, .release);
+        w.flush() catch return waiting.store(false, .release);
+    }
+};
+
 /// Whether more input is readable within `ms`, polled in short slices so
 /// that a cancel of the input thread is still seen. A test reader answers
 /// for itself.
@@ -521,6 +589,98 @@ test "an ESC read by itself is the Escape key at once" {
     vx.caps.kitty_keyboard = false;
 }
 
+test "a frame's answer is taken wherever the reads cut it, and never typed" {
+    if (comptime builtin.os.tag == .windows) return error.SkipZigTest;
+    const gpa = std.testing.allocator;
+    const io = std.testing.io;
+    var env = try std.testing.environ.createMap(gpa);
+    defer env.deinit();
+    var vx = try vaxis.init(io, gpa, &env, .{});
+    var output: std.Io.Writer.Allocating = .init(gpa);
+    defer output.deinit();
+    defer vx.deinit(gpa, &output.writer);
+    var tty: vaxis.Tty = undefined;
+    var loop: Loop = .init(io, &tty, &vx);
+    var cache: vaxis.GraphemeCache = .{};
+    const Reader = struct {
+        parts: [2][]const u8,
+        next: usize = 0,
+        fn getWinsize(_: *@This()) !vaxis.Winsize {
+            return .{ .rows = 24, .cols = 80, .x_pixel = 0, .y_pixel = 0 };
+        }
+        fn inputFollows(self: *@This(), _: u32) bool {
+            return self.next < self.parts.len;
+        }
+        fn read(self: *@This(), buf: []u8) !usize {
+            while (self.next < self.parts.len) {
+                const part = self.parts[self.next];
+                self.next += 1;
+                if (part.len == 0) continue;
+                @memcpy(buf[0..part.len], part);
+                return part.len;
+            }
+            return 0;
+        }
+    };
+    const all = "x" ++ FrameAck.answer ++ "y";
+    defer FrameAck.waiting.store(false, .release);
+    for (0..all.len) |cut| {
+        FrameAck.waiting.store(true, .release);
+        var reader: Reader = .{ .parts = .{ all[0..cut], all[cut..] } };
+        try readInput(&loop, &reader, &cache);
+        try std.testing.expect(!FrameAck.waiting.load(.acquire));
+        var typed: std.ArrayList(u8) = .empty;
+        defer typed.deinit(gpa);
+        var woke = false;
+        while (try loop.tryEvent()) |event| switch (event) {
+            .winsize => {},
+            .nop => woke = true,
+            .key_press => |key| try typed.appendSlice(gpa, key.text orelse return error.UnexpectedInputKey),
+            else => return error.UnexpectedInputEvent,
+        };
+        std.testing.expectEqualStrings("xy", typed.items) catch |err| {
+            std.debug.print("cut at {d}\n", .{cut});
+            return err;
+        };
+        try std.testing.expect(woke);
+    }
+    // An Escape pressed just before an answer, in the same read, is the
+    // Escape key, not Alt on the answer.
+    FrameAck.waiting.store(true, .release);
+    var glued: Reader = .{ .parts = .{ "\x1b" ++ FrameAck.answer, "j" } };
+    try readInput(&loop, &glued, &cache);
+    try std.testing.expectEqual(.winsize, std.meta.activeTag((try loop.tryEvent()).?));
+    try std.testing.expectEqual(.nop, std.meta.activeTag((try loop.tryEvent()).?));
+    const escape = (try loop.tryEvent()).?.key_press;
+    try std.testing.expectEqual(vaxis.Key.escape, escape.codepoint);
+    try std.testing.expect(!escape.mods.alt);
+    try std.testing.expectEqual(@as(u21, 'j'), (try loop.tryEvent()).?.key_press.codepoint);
+    try std.testing.expect(try loop.tryEvent() == null);
+}
+
+test "a terminal that never answers a frame stops being asked after one timeout" {
+    const io = std.testing.io;
+    defer {
+        FrameAck.off.store(false, .release);
+        FrameAck.waiting.store(false, .release);
+        FrameAck.sent_ns = 0;
+    }
+    var out: std.Io.Writer.Allocating = .init(std.testing.allocator);
+    defer out.deinit();
+    const t0: u64 = 5 * std.time.ns_per_s;
+    FrameAck.ask(&out.writer, t0);
+    try std.testing.expectEqualStrings(FrameAck.request, out.written());
+    // Unanswered: the next frame waits, until the timeout.
+    try std.testing.expect(FrameAck.holds(t0 + 10 * std.time.ns_per_ms, io));
+    try std.testing.expect(FrameAck.holds(t0 + 999 * std.time.ns_per_ms, io));
+    try std.testing.expect(!FrameAck.holds(t0 + FrameAck.timeout_ns, io));
+    // Then never again: no request, no wait.
+    out.clearRetainingCapacity();
+    FrameAck.ask(&out.writer, t0 + 2 * std.time.ns_per_s);
+    try std.testing.expectEqual(@as(usize, 0), out.written().len);
+    try std.testing.expect(!FrameAck.holds(t0 + 2 * std.time.ns_per_s, io));
+}
+
 test "terminal input cancellation joins blocked reads and queued EOF" {
     if (comptime builtin.os.tag == .windows) return error.SkipZigTest;
     const gpa = std.testing.allocator;
@@ -1438,6 +1598,12 @@ const Shell = struct {
         const s = of(ctx);
         const vx = s.vx;
         s.tracks = canonical.panelTracks();
+        // The last frame is not answered yet: this one stays owed, and the
+        // answer's wake draws the state as it is then.
+        if (FrameAck.holds(host_io.monotonicNs(), s.io)) {
+            s.core.present_skipped = true;
+            return;
+        }
         _ = s.frame.reset(.retain_capacity);
         const surface = panel_compositor.compose(
             s.frame.allocator(),
@@ -1523,7 +1689,9 @@ const Shell = struct {
         }
         if (surface.cursor) |cur| paintCursor(win, cur.x, cur.y, cur.bar);
         const tz_render = tracy.zone(@src(), "vx.render");
+        const before = s.tty.tty_writer.pos + s.tty.writer().end;
         vx.render(s.tty.writer()) catch {};
+        if (s.tty.tty_writer.pos + s.tty.writer().end != before) FrameAck.ask(s.tty.writer(), host_io.monotonicNs());
         tz_render.end();
     }