feat(ui): UI :detach command

Problem:
Cannot detach the current UI.

Solution:
- Introduce `:detach`.
- Introduce `Channel.detach`.

Co-authored-by: bfredl <bjorn.linse@gmail.com>

20 files changed, 244 insertions, 63 deletions

diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 324cb3e13a..2cf93e74ed 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -129,7 +129,7 @@ Each pull request must pass the automated builds on [Cirrus CI] and [GitHub Acti
   passes various linter checks.
 - CI for FreeBSD runs on [Cirrus CI].
 - To see CI results faster in your PR, you can temporarily set `TEST_FILE` in
-  [test.yml](https://github.com/neovim/neovim/blob/e35b9020b16985eee26e942f9a3f6b045bc3809b/.github/workflows/test.yml#L29).
+  [test.yml](https://github.com/neovim/neovim/blob/ad8e0cfc1dfd937c2577dc032e524c799a772693/.github/workflows/test.yml#L26).
 
 ### Coverity
 
diff --git a/runtime/doc/api.txt b/runtime/doc/api.txt
index 92f5a261ee..4f76e7e058 100644
--- a/runtime/doc/api.txt
+++ b/runtime/doc/api.txt
@@ -1208,8 +1208,8 @@ nvim_select_popupmenu_item({item}, {insert}, {finish}, {opts})
 
                                                       *nvim_set_client_info()*
 nvim_set_client_info({name}, {version}, {type}, {methods}, {attributes})
-    Self-identifies the client. Sets the `client` object returned by
-    |nvim_get_chan_info()|.
+    Self-identifies the client, and sets optional flags on the channel.
+    Defines the `client` object returned by |nvim_get_chan_info()|.
 
     Clients should call this just after connecting, to provide hints for
     debugging and orchestration. (Note: Something is better than nothing!
diff --git a/runtime/doc/gui.txt b/runtime/doc/gui.txt
index ecb4de09bb..780712df6c 100644
--- a/runtime/doc/gui.txt
+++ b/runtime/doc/gui.txt
@@ -47,6 +47,25 @@ can connect to any Nvim instance).
 Example: this sets "g:gui" to the value of the UI's "rgb" field:    >
         :autocmd UIEnter * let g:gui = filter(nvim_list_uis(),{k,v-> v.chan==v:event.chan})[0].rgb
 <
+------------------------------------------------------------------------------
+Stop or detach the current UI
+
+                                                *:detach*
+:detach
+                Detaches the current UI. Other UIs (if any) remain attached.
+                The server (typically `nvim --embed`) continues running as
+                a background process, and you can reattach to it later.
+                Before detaching, you may want to note the server address:
+                >vim
+                    :echo v:servername
+<
+                Note: The server closes the UI RPC channel, so :detach
+                inherently "works" for all UIs. But if a UI isn't expecting
+                the channel to be closed, it may be (incorrectly) reported as
+                an error.
+
+------------------------------------------------------------------------------
+GUI commands
 
                                                 *:winp* *:winpos* *E188*
 :winp[os]
diff --git a/runtime/doc/news.txt b/runtime/doc/news.txt
index 1f1ad84a02..90a020bb4d 100644
--- a/runtime/doc/news.txt
+++ b/runtime/doc/news.txt
@@ -400,6 +400,8 @@ TUI
 
 UI
 
+• |:detach| the current UI, let the Nvim server continue running as a background
+  process. Works with the builtin TUI, and all GUIs.
 • |vim.ui.open()| (by default bound to |gx|) accepts an `opt.cmd` parameter
   which controls the tool used to open the given path or URL. If you want to
   globally set this, you can override vim.ui.open using the same approach
diff --git a/src/nvim/api/ui.c b/src/nvim/api/ui.c
index f6f32a73ed..41a09999d0 100644
--- a/src/nvim/api/ui.c
+++ b/src/nvim/api/ui.c
@@ -189,6 +189,7 @@ void nvim_ui_attach(uint64_t channel_id, Integer width, Integer height, Dict opt
   ui->wildmenu_active = false;
 
   pmap_put(uint64_t)(&connected_uis, channel_id, ui);
+  current_ui = channel_id;
   ui_attach_impl(ui, channel_id);
 
   may_trigger_vim_suspend_resume(false);
@@ -214,6 +215,7 @@ void nvim_ui_set_focus(uint64_t channel_id, Boolean gained, Error *error)
   }
 
   if (gained) {
+    current_ui = channel_id;
     may_trigger_vim_suspend_resume(false);
   }
 
diff --git a/src/nvim/api/vim.c b/src/nvim/api/vim.c
index c103a56032..02aa73d98d 100644
--- a/src/nvim/api/vim.c
+++ b/src/nvim/api/vim.c
@@ -359,11 +359,11 @@ void nvim_feedkeys(String keys, String mode, Boolean escape_ks)
 /// @param keys to be typed
 /// @return Number of bytes actually written (can be fewer than
 ///         requested if the buffer becomes full).
-Integer nvim_input(String keys)
+Integer nvim_input(uint64_t channel_id, String keys)
   FUNC_API_SINCE(1) FUNC_API_FAST
 {
   may_trigger_vim_suspend_resume(false);
-  return (Integer)input_enqueue(keys);
+  return (Integer)input_enqueue(channel_id, keys);
 }
 
 /// Send mouse event from GUI.
@@ -1485,7 +1485,8 @@ Array nvim_get_api_info(uint64_t channel_id, Arena *arena)
   return rv;
 }
 
-/// Self-identifies the client. Sets the `client` object returned by |nvim_get_chan_info()|.
+/// Self-identifies the client, and sets optional flags on the channel. Defines the `client` object
+/// returned by |nvim_get_chan_info()|.
 ///
 /// Clients should call this just after connecting, to provide hints for debugging and
 /// orchestration. (Note: Something is better than nothing! Fields are optional, but at least set
diff --git a/src/nvim/channel.c b/src/nvim/channel.c
index 912d515f84..c964748a20 100644
--- a/src/nvim/channel.c
+++ b/src/nvim/channel.c
@@ -219,6 +219,7 @@ Channel *channel_alloc(ChannelStreamType type)
   chan->refcount = 1;
   chan->exit_status = -1;
   chan->streamtype = type;
+  chan->detach = false;
   assert(chan->id <= VARNUMBER_MAX);
   pmap_put(uint64_t)(&channels, chan->id, chan);
   return chan;
diff --git a/src/nvim/channel.h b/src/nvim/channel.h
index 2327216826..81d67ceb66 100644
--- a/src/nvim/channel.h
+++ b/src/nvim/channel.h
@@ -31,6 +31,9 @@ struct Channel {
   } stream;
 
   bool is_rpc;
+  bool detach;  ///< Prevents self-exit on channel-close. Normally, Nvim self-exits if its primary
+                ///< RPC channel is closed, unless detach=true. Note: currently, detach=false does
+                ///< not FORCE self-exit.
   RpcState rpc;
   Terminal *term;
 
diff --git a/src/nvim/event/proc.c b/src/nvim/event/proc.c
index 37cb102d11..2955693687 100644
--- a/src/nvim/event/proc.c
+++ b/src/nvim/event/proc.c
@@ -315,10 +315,8 @@ static void decref(Proc *proc)
 static void proc_close(Proc *proc)
   FUNC_ATTR_NONNULL_ARG(1)
 {
-  if (proc_is_tearing_down && (proc->detach || proc->type == kProcTypePty)
-      && proc->closed) {
-    // If a detached/pty process dies while tearing down it might get closed
-    // twice.
+  if (proc_is_tearing_down && proc->closed && (proc->detach || proc->type == kProcTypePty)) {
+    // If a detached/pty process dies while tearing down it might get closed twice.
     return;
   }
   assert(!proc->closed);
@@ -427,20 +425,22 @@ static void exit_event(void **argv)
   }
 }
 
-void exit_from_channel(int status)
+/// Performs self-exit because the primary RPC channel was closed.
+void exit_on_closed_chan(int status)
 {
+  DLOG("self-exit triggered by closed RPC channel...");
   multiqueue_put(main_loop.fast_events, exit_event, (void *)(intptr_t)status);
 }
 
 static void on_proc_exit(Proc *proc)
 {
   Loop *loop = proc->loop;
-  ILOG("exited: pid=%d status=%d stoptime=%" PRIu64, proc->pid, proc->status,
-       proc->stopped_time);
+  ILOG("child exited: pid=%d status=%d" PRIu64, proc->pid, proc->status);
 
-  if (ui_client_channel_id) {
-    exit_from_channel(proc->status);
-  }
+  // XXX: This assumes the TUI never spawns any other processes...?
+  // if (ui_client_channel_id) {
+  //   exit_on_closed_chan(proc->status);
+  // }
 
   // Process has terminated, but there could still be data to be read from the
   // OS. We are still in the libuv loop, so we cannot call code that polls for
diff --git a/src/nvim/event/stream.c b/src/nvim/event/stream.c
index 9c155b55ea..3e32813e1c 100644
--- a/src/nvim/event/stream.c
+++ b/src/nvim/event/stream.c
@@ -104,9 +104,10 @@ void stream_may_close(Stream *stream, bool rstream)
   if (stream->closed) {
     return;
   }
-  assert(!stream->closed);
   DLOG("closing Stream: %p", (void *)stream);
   stream->closed = true;
+  // TODO(justinmk): stream->close_cb is never actually invoked. Either remove it, or see if it can
+  // be used somewhere...
   stream->close_cb = NULL;
   stream->close_cb_data = NULL;
 
diff --git a/src/nvim/ex_cmds.lua b/src/nvim/ex_cmds.lua
index 0cdc397e9c..d118c808d3 100644
--- a/src/nvim/ex_cmds.lua
+++ b/src/nvim/ex_cmds.lua
@@ -733,6 +733,12 @@ M.cmds = {
     func = 'ex_delfunction',
   },
   {
+    command = 'detach',
+    flags = bit.bor(BANG, FILES, CMDARG, ARGOPT, TRLBAR, CMDWIN, LOCK_OK),
+    addr_type = 'ADDR_NONE',
+    func = 'ex_detach',
+  },
+  {
     command = 'display',
     flags = bit.bor(EXTRA, NOTRLCOM, TRLBAR, SBOXOK, CMDWIN, LOCK_OK),
     addr_type = 'ADDR_NONE',
diff --git a/src/nvim/ex_docmd.c b/src/nvim/ex_docmd.c
index ceb17a995d..3b78092daf 100644
--- a/src/nvim/ex_docmd.c
+++ b/src/nvim/ex_docmd.c
@@ -14,6 +14,7 @@
 #include "auto/config.h"
 #include "nvim/api/private/defs.h"
 #include "nvim/api/private/helpers.h"
+#include "nvim/api/ui.h"
 #include "nvim/arglist.h"
 #include "nvim/ascii_defs.h"
 #include "nvim/autocmd.h"
@@ -21,6 +22,7 @@
 #include "nvim/buffer.h"
 #include "nvim/buffer_defs.h"
 #include "nvim/change.h"
+#include "nvim/channel.h"
 #include "nvim/charset.h"
 #include "nvim/cmdexpand.h"
 #include "nvim/cmdexpand_defs.h"
@@ -67,6 +69,7 @@
 #include "nvim/message.h"
 #include "nvim/mouse.h"
 #include "nvim/move.h"
+#include "nvim/msgpack_rpc/server.h"
 #include "nvim/normal.h"
 #include "nvim/normal_defs.h"
 #include "nvim/ops.h"
@@ -5530,6 +5533,56 @@ static void ex_tabs(exarg_T *eap)
   }
 }
 
+/// ":detach"
+///
+/// Detaches the current UI.
+///
+/// ":detach!" with bang (!) detaches all UIs _except_ the current UI.
+static void ex_detach(exarg_T *eap)
+{
+  // come on pooky let's burn this mf down
+  if (eap && eap->forceit) {
+    emsg("bang (!) not supported yet");
+  } else {
+    // 1. (TODO) Send "detach" UI-event (notification only).
+    // 2. Perform server-side `nvim_ui_detach`.
+    // 3. Close server-side channel without self-exit.
+
+    if (!current_ui) {
+      emsg("UI not attached");
+      return;
+    }
+
+    Channel *chan = find_channel(current_ui);
+    if (!chan) {
+      emsg(e_invchan);
+      return;
+    }
+    chan->detach = true;  // Prevent self-exit on channel-close.
+
+    // Server-side UI detach. Doesn't close the channel.
+    Error err2 = ERROR_INIT;
+    nvim_ui_detach(chan->id, &err2);
+    if (ERROR_SET(&err2)) {
+      emsg(err2.msg);  // UI disappeared already?
+      api_clear_error(&err2);
+      return;
+    }
+
+    // Server-side channel close.
+    const char *err = NULL;
+    bool rv = channel_close(chan->id, kChannelPartAll, &err);
+    if (!rv && err) {
+      emsg(err);  // UI disappeared already?
+      return;
+    }
+    // XXX: Can't do this, channel_decref() is async...
+    // assert(!find_channel(chan->id));
+
+    ILOG("detach current_ui=%" PRId64, chan->id);
+  }
+}
+
 /// ":mode":
 /// If no argument given, get the screen size and redraw.
 static void ex_mode(exarg_T *eap)
diff --git a/src/nvim/globals.h b/src/nvim/globals.h
index a473180349..ea349352a8 100644
--- a/src/nvim/globals.h
+++ b/src/nvim/globals.h
@@ -301,6 +301,8 @@ EXTERN bool garbage_collect_at_exit INIT( = false);
 EXTERN sctx_T current_sctx INIT( = { 0, 0, 0 });
 // ID of the current channel making a client API call
 EXTERN uint64_t current_channel_id INIT( = 0);
+/// Last channel that invoked 'nvim_input` or got FocusGained.
+EXTERN uint64_t current_ui INIT( = 0);
 
 EXTERN bool did_source_packages INIT( = false);
 
diff --git a/src/nvim/msgpack_rpc/channel.c b/src/nvim/msgpack_rpc/channel.c
index e38bc1896d..fa50afd83b 100644
--- a/src/nvim/msgpack_rpc/channel.c
+++ b/src/nvim/msgpack_rpc/channel.c
@@ -224,8 +224,7 @@ static size_t receive_msgpack(RStream *stream, const char *rbuf, size_t c, void
   if (eof) {
     channel_close(channel->id, kChannelPartRpc, NULL);
     char buf[256];
-    snprintf(buf, sizeof(buf), "ch %" PRIu64 " was closed by the client",
-             channel->id);
+    snprintf(buf, sizeof(buf), "ch %" PRIu64 " was closed by the peer", channel->id);
     chan_close_with_error(channel, buf, LOGLVL_INF);
   }
 
@@ -293,7 +292,7 @@ static void parse_msgpack(Channel *channel)
 
       Object res = p->result;
       if (p->result.type != kObjectTypeArray) {
-        chan_close_with_error(channel, "msgpack-rpc request args has to be an array", LOGLVL_ERR);
+        chan_close_with_error(channel, "msgpack-rpc request args must be an array", LOGLVL_ERR);
         return;
       }
       Array arg = res.data.array;
@@ -487,13 +486,16 @@ void rpc_close(Channel *channel)
   channel->rpc.closed = true;
   channel_decref(channel);
 
-  if (channel->streamtype == kChannelStreamStdio
-      || (channel->id == ui_client_channel_id && channel->streamtype != kChannelStreamProc)) {
-    if (channel->streamtype == kChannelStreamStdio) {
-      // Avoid hanging when there are no other UIs and a prompt is triggered on exit.
-      remote_ui_disconnect(channel->id);
+  if (ui_client_channel_id && channel->id == ui_client_channel_id) {
+    assert(!channel->detach);  // `Channel.detach` is not currently used by the UI client.
+    exit_on_closed_chan(0);
+  } else if (channel->streamtype == kChannelStreamStdio) {
+    // Avoid hanging when there are no other UIs and a prompt is triggered on exit.
+    remote_ui_disconnect(channel->id);
+
+    if (!channel->detach) {
+      exit_on_closed_chan(0);
     }
-    exit_from_channel(0);
   }
 }
 
diff --git a/src/nvim/os/input.c b/src/nvim/os/input.c
index 3259fb500b..f510c67753 100644
--- a/src/nvim/os/input.c
+++ b/src/nvim/os/input.c
@@ -274,8 +274,10 @@ void input_enqueue_raw(const char *data, size_t size)
   input_write_pos += to_write;
 }
 
-size_t input_enqueue(String keys)
+size_t input_enqueue(uint64_t chan_id, String keys)
 {
+  current_ui = chan_id;
+
   const char *ptr = keys.data;
   const char *end = ptr + keys.size;
 
diff --git a/src/nvim/ui_client.c b/src/nvim/ui_client.c
index af946d799a..d42c0a7fdd 100644
--- a/src/nvim/ui_client.c
+++ b/src/nvim/ui_client.c
@@ -61,9 +61,14 @@ uint64_t ui_client_start_server(int argc, char **argv)
   CallbackReader on_err = CALLBACK_READER_INIT;
   on_err.fwd_err = true;
 
+#ifdef MSWIN
+  bool detach = false;  // TODO(justinmk): detach=true breaks `tt.setup_child_nvim` tests on Windows.
+#else
+  bool detach = true;
+#endif
   Channel *channel = channel_job_start(args, get_vim_var_str(VV_PROGPATH),
                                        CALLBACK_READER_INIT, on_err, CALLBACK_NONE,
-                                       false, true, true, false, kChannelStdinPipe,
+                                       false, true, true, detach, kChannelStdinPipe,
                                        NULL, 0, 0, NULL, &exit_status);
   if (!channel) {
     return 0;
diff --git a/test/functional/core/job_spec.lua b/test/functional/core/job_spec.lua
index e833b5127d..75e09f3455 100644
--- a/test/functional/core/job_spec.lua
+++ b/test/functional/core/job_spec.lua
@@ -1265,12 +1265,16 @@ describe('jobs', function()
     ]])
 
     feed(':q<CR>')
-    screen:expect([[
-                                                        |
-      [Process exited 0]^                                |
-                                                        |*4
-      {3:-- TERMINAL --}                                    |
-    ]])
+    if is_os('freebsd') then
+      screen:expect { any = vim.pesc('[Process exited 0]') }
+    else
+      screen:expect([[
+                                                          |
+        [Process exited 0]^                                |
+                                                          |*4
+        {3:-- TERMINAL --}                                    |
+      ]])
+    end
   end)
 end)
 
diff --git a/test/functional/editor/put_spec.lua b/test/functional/editor/put_spec.lua
index 79f9d97bc5..aeb9c75e42 100644
--- a/test/functional/editor/put_spec.lua
+++ b/test/functional/editor/put_spec.lua
@@ -931,6 +931,8 @@ describe('put command', function()
     end)
 
     it('should ring the bell when deleting if not appropriate', function()
+      t.skip(t.is_os('bsd'), 'crashes on freebsd')
+
       command('goto 2')
       feed('i<bs><esc>')
       expect([[
diff --git a/test/functional/terminal/tui_spec.lua b/test/functional/terminal/tui_spec.lua
index 147a8675cb..32c4ac613f 100644
--- a/test/functional/terminal/tui_spec.lua
+++ b/test/functional/terminal/tui_spec.lua
@@ -33,6 +33,95 @@ local assert_log = t.assert_log
 
 local testlog = 'Xtest-tui-log'
 
+describe('TUI :detach', function()
+  before_each(function()
+    os.remove(testlog)
+  end)
+  teardown(function()
+    os.remove(testlog)
+  end)
+
+  it('does not stop server', function()
+    local server_super = n.clear()
+    local client_super = n.new_session(true)
+    finally(function()
+      server_super:close()
+      client_super:close()
+    end)
+
+    local child_server = new_pipename()
+    local screen = tt.setup_child_nvim({
+      '--listen',
+      child_server,
+      '-u',
+      'NONE',
+      '-i',
+      'NONE',
+      '--cmd',
+      'colorscheme vim',
+      '--cmd',
+      nvim_set .. ' notermguicolors laststatus=2 background=dark',
+    }, {
+      env = {
+        NVIM_LOG_FILE = testlog,
+      },
+    })
+
+    tt.feed_data('iHello, World')
+    screen:expect {
+      grid = [[
+      Hello, World^                                      |
+      {4:~                                                 }|*3
+      {MATCH:No Name}
+      {3:-- INSERT --}                                      |
+      {3:-- TERMINAL --}                                    |
+    ]],
+    }
+
+    local child_session = n.connect(child_server)
+    finally(function()
+      child_session:request('nvim_command', 'qall!')
+    end)
+    local status, child_uis = child_session:request('nvim_list_uis')
+    assert(status)
+    eq(1, #child_uis)
+
+    tt.feed_data('\027\027:detach\013')
+    -- Note: "Process exited" message is misleading; tt.setup_child_nvim() sees the foreground
+    -- process (client) exited, and doesn't know the server is still running?
+    screen:expect {
+      any = [[Process exited 0]],
+    }
+
+    child_uis --[[@type any[] ]] = ({ child_session:request('nvim_list_uis') })[2]
+    eq(0, #child_uis)
+
+    -- NOTE: The tt.setup_child_nvim() screen just wraps :terminal, it's not connected to the child.
+    -- To use it again, we need to detach the old one.
+    screen:detach()
+
+    -- Edit some text on the headless server.
+    status = (child_session:request('nvim_input', 'ddiWe did it, pooky.<Esc><Esc>'))
+    assert(status)
+
+    -- Test reattach by connecting a new TUI.
+    local screen_reattached = tt.setup_child_nvim({
+      '--remote-ui',
+      '--server',
+      child_server,
+    })
+    screen_reattached:expect {
+      grid = [[
+      We did it, pooky^.                                 |
+      {4:~                                                 }|*3
+      {5:[No Name] [+]                                     }|
+                                                        |
+      {3:-- TERMINAL --}                                    |
+    ]],
+    }
+  end)
+end)
+
 if t.skip(is_os('win')) then
   return
 end
@@ -3384,9 +3473,9 @@ describe('TUI client', function()
 
     set_session(client_super)
     local screen_client = tt.setup_child_nvim({
+      '--remote-ui',
       '--server',
       server_pipe,
-      '--remote-ui',
     })
 
     screen_client:expect {
@@ -3428,9 +3517,9 @@ describe('TUI client', function()
 
     set_session(client_super)
     local screen_client = tt.setup_child_nvim({
+      '--remote-ui',
       '--server',
       server_pipe,
-      '--remote-ui',
     })
 
     screen_client:expect {
@@ -3457,7 +3546,7 @@ describe('TUI client', function()
 
     eq(0, api.nvim_get_vvar('shell_error'))
     -- exits on input eof #22244
-    fn.system({ nvim_prog, '--server', server_pipe, '--remote-ui' })
+    fn.system({ nvim_prog, '--remote-ui', '--server', server_pipe })
     eq(1, api.nvim_get_vvar('shell_error'))
 
     client_super:close()
@@ -3470,9 +3559,9 @@ describe('TUI client', function()
   it('throws error when no server exists', function()
     clear()
     local screen = tt.setup_child_nvim({
+      '--remote-ui',
       '--server',
       '127.0.0.1:2436546',
-      '--remote-ui',
     }, { cols = 60 })
 
     screen:expect([[
@@ -3485,10 +3574,13 @@ describe('TUI client', function()
   end)
 
   local function test_remote_tui_quit(status)
-    local server_super = n.new_session(false)
+    local server_super = n.clear()
     local client_super = n.new_session(true)
+    finally(function()
+      server_super:close()
+      client_super:close()
+    end)
 
-    set_session(server_super)
     local server_pipe = new_pipename()
     local screen_server = tt.setup_child_nvim({
       '--listen',
@@ -3535,9 +3627,9 @@ describe('TUI client', function()
 
     set_session(client_super)
     local screen_client = tt.setup_child_nvim({
+      '--remote-ui',
       '--server',
       server_pipe,
-      '--remote-ui',
     })
 
     screen_client:expect {
@@ -3554,26 +3646,8 @@ describe('TUI client', function()
     set_session(server_super)
     feed_data(status and ':' .. status .. 'cquit!\n' or ':quit!\n')
     status = status and status or 0
-    screen_server:expect {
-      grid = [[
-                                                        |
-      [Process exited ]] .. status .. [[]^ {MATCH:%s+}|
-                                                        |*4
-      {3:-- TERMINAL --}                                    |
-    ]],
-    }
-    -- assert that client has exited
-    screen_client:expect {
-      grid = [[
-                                                        |
-      [Process exited ]] .. status .. [[]^ {MATCH:%s+}|
-                                                        |*4
-      {3:-- TERMINAL --}                                    |
-    ]],
-    }
-
-    server_super:close()
-    client_super:close()
+    screen_server:expect({ any = 'Process exited ' .. status })
+    screen_client:expect({ any = 'Process exited ' .. status })
   end
 
   describe('exits when server quits', function()
diff --git a/test/functional/testnvim.lua b/test/functional/testnvim.lua
index 59cb593cf7..9b7ece3c21 100644
--- a/test/functional/testnvim.lua
+++ b/test/functional/testnvim.lua
@@ -908,8 +908,10 @@ function M.is_asan()
   return version:match('-fsanitize=[a-z,]*address')
 end
 
--- Returns a valid, platform-independent Nvim listen address.
--- Useful for communicating with child instances.
+--- Returns a valid, platform-independent Nvim listen address.
+--- Useful for communicating with child instances.
+---
+--- @return string
 function M.new_pipename()
   -- HACK: Start a server temporarily, get the name, then stop it.
   local pipename = M.eval('serverstart()')