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fix(bufwrite): cursor flicker on :write

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PROBLEM:
Cursor briefly flickers in cmdline when the "written" message is printed
(very noticeable in Neovide with cursor animation).

SOLUTION:
- Mark UI "busy" (cursor hidden) while emitting the message, as done for
  the search message (cb2ca54331).
- Note: ff68fd6b8a moved the message from `filemess()` into
  `buf_write..msg_progress`.
- Fix a bug in `tui.c:flush_buf` which manifested after this change. See
  ANALYSIS below. https://github.com/libuv/libuv/issues/5182

ANALYSIS:
After this change...

    ui_busy_start();
    set_keep_msg(msg_progress(IObuff, msg_id, "success", 0, true, true), 0);
    ui_busy_stop();

...ASAN analyzer fails on tui_spec.lua test "with non-tty (pipe) stdout/stderr":

    2026-07-04T14:09:28.9521023Z = ==32405==ERROR: AddressSanitizer: ABRT on unknown address 0x03e900007e95
    ...
    4 0x7f5506a288fe in abort stdlib/abort.c:79:7
    5 0x559afd7113a2 in uv__epoll_ctl_flush
    build/src/libuv/src/unix/linux.c:1335:7
    6 0x559afd710a81 in uv__io_poll
    build/src/libuv/src/unix/linux.c:1448:9
    7 0x559afd6f67a7 in uv_run
    build/src/libuv/src/unix/core.c:460:5
    8 0x559afd2a72cc in flush_buf
    nvim/tui/tui.c:2642:5
    9 0x559afd2bce9f in tui_flush
    nvim/tui/tui.c:1747:3
    10 0x559afd2f39a0 in ui_client_event_flush
    nvim/auto/ui_events_client.generated.h:64:3
    11 0x559afcb8e014 in parse_msgpack
    nvim/msgpack_rpc/channel.c:255:11
    12 0x559afcb840e7 in receive_msgpack
    nvim/msgpack_rpc/channel.c:217:5
    13 0x559afc53b2af in read_event
    nvim/event/rstream.c:180:23
    14 0x559afc53ad52 in invoke_read_cb
    nvim/event/rstream.c:233:3
    15 0x559afc5382d6 in read_cb
    nvim/event/rstream.c:135:3
    16 0x559afd707b25 in uv__read
    build/src/libuv/src/unix/stream.c:1145:7
    17 0x559afd70744a in uv__stream_io
    build/src/libuv/src/unix/stream.c:1208:5
    18 0x559afd6f727e in uv__io_cb
    build/src/libuv/src/unix/core.c:930:5
    19 0x559afd710d7a in uv__io_poll
    build/src/libuv/src/unix/linux.c:1546:11
    20 0x559afd6f67a7 in uv_run
    build/src/libuv/src/unix/core.c:460:5
    21 0x559afc526e2d in loop_uv_run
    nvim/event/loop.c:59:3
    22 0x559afc526aa4 in loop_poll_events
    nvim/event/loop.c:80:26
    23 0x559afd2fdbb7 in ui_client_run
    nvim/ui_client.c:172:5
    24 0x559afc920d7b in main /home/runner/work/neovim/neovim/src/nvim/main.c:355:5
    25 0x7f5506a2a1c9 in __libc_start_call_main
    main.h:58:16
    26 0x7f5506a2a28a in __libc_start_main csu/../csu/libc-start.c:360:3
    27 0x559afbd6d254 in _start

The abort requires two rare conditions:
1. a `uv_write` whose buffer array *ends* in a zero-length buffer, which only
   happens by "flush while cursor-hidden".
2. an output fd that *epoll cannot watch*. The only CI test with such an fd is
   tui_spec.lua "with non-tty (pipe) stdout/stderr", which runs a TUI with
   `stdout > /dev/null`, then runs `:w testF`, which triggers the "written"
   message.

Our change guarantees trailing-empty flushes (`:w` fires busy + `ui_flush()`
while busy) in the only test that runs a TUI on `/dev/null`.

0. In a normal (non-busy) no-sync flush, post always contains `cursor_normal`.
1. Our change emits:
   ```
   busy_start → msg_progress("…written") → ui_flush → busy_stop
   ```
2. TUI flushes *while busy*, so `should_invisible() = true` and `flush_buf`
   skips the cursor-restore string (`bufs[2]` is zero-length).
3. libuv *cannot complete* a write whose tail is a zero-length buffer: EVERY
   trailing-empty write goes through epoll, even when every byte was already
   written. https://github.com/libuv/libuv/issues/5182
4. In `tui_spec.lua:3298`, the output fd is `/dev/null`, whose kernel
   `file_operations` has no `.poll` method (`drivers/char/mem.c`), so
   `EPOLL_CTL_ADD` fails with `EPERM`.

Trailing empty buffers are semantically pointless, and libuv punishes
them: forced async, one epoll round-trip *per trailing empty*, plus
a 0-byte `write()` syscall.

Helped-by: Shougo Matsushita <Shougo.Matsu@gmail.com>
Helped-by: Fred Sundvik <fsundvik@gmail.com>
This commit is contained in:
Justin M. Keyes
2024-06-22 12:14:23 +09:00
parent 1aced004ce
commit 3b8c19ea46
3 changed files with 37 additions and 5 deletions
Download Patch File Download Diff File Expand all files Collapse all files

3
src/nvim/bufwrite.c
View File

@@ -1710,7 +1710,10 @@ restore_backup:
xstrlcat(IObuff, shortmess(SHM_WRI) ? _(" [w]") : _(" written"), IOSIZE);
}
}
// Hide cursor while emitting "written" message, so cursor doesn't flicker in cmdline. #25974
ui_busy_start();
set_keep_msg(msg_progress(IObuff, msg_id, "success", 0, true, true), 0);
ui_busy_stop();
}
// When written everything correctly: reset 'modified'. Unless not

15
src/nvim/tui/tui.c
View File

@@ -2634,12 +2634,17 @@ static void flush_buf(TUIData *tui, FlushBufFinish finish)
fwrite(bufs[i].base, bufs[i].len, 1, tui->screenshot);
}
} else {
int ret
= uv_write(&req, (uv_stream_t *)&tui->output_handle, bufs, ARRAY_SIZE(bufs), NULL);
if (ret) {
ELOG("uv_write failed: %s", uv_strerror(ret));
unsigned nbufs = ARRAY_SIZE(bufs);
while (nbufs > 0 && bufs[nbufs - 1].len == 0) {
nbufs--; // Trim trailing zero-length buffers. https://github.com/libuv/libuv/issues/5182
}
if (nbufs > 0) {
int ret = uv_write(&req, (uv_stream_t *)&tui->output_handle, bufs, nbufs, NULL);
if (ret) {
ELOG("uv_write failed: %s", uv_strerror(ret));
}
uv_run(&tui->write_loop, UV_RUN_DEFAULT);
}
uv_run(&tui->write_loop, UV_RUN_DEFAULT);
}
tui->buf_to_flush = NULL;
tui->bufpos = 0;

24
test/functional/ui/messages_spec.lua
View File

@@ -1727,6 +1727,30 @@ describe('ui/builtin messages', function()
}
end)
it('no cursor flicker during :write message (marks UI busy) #25974', function()
local fname = 'Xtest_write_busy'
finally(function()
os.remove(fname)
end)
local busy_start, busy_stop = 0, 0
screen._handle_busy_start = (function(orig)
return function()
orig(screen)
busy_start = busy_start + 1
end
end)(screen._handle_busy_start)
screen._handle_busy_stop = (function(orig)
return function()
orig(screen)
busy_stop = busy_stop + 1
end
end)(screen._handle_busy_stop)
command('write ' .. fname)
screen:expect({ any = 'written' })
eq(true, busy_start >= 1) -- cursor was hidden while the message was emitted
eq(busy_start, busy_stop) -- balanced: cursor restored afterwards
end)
it(':hi Group output', function()
screen:try_resize(70, 7)
feed(':hi ErrorMsg<cr>')