feat(ui): restore has('gui_running')

Problem:
has('gui_running') is still common in the wild and our answer has
changed over time, causing frustration.
https://github.com/vimpostor/vim-tpipeline/commit/95a6ccbe9f33bc42dd4cee45731d8bc3fbcd92d1

Solution:
Use stdin_tty/stdout_tty to decide if a UI is (not) a GUI.

8 files changed, 43 insertions, 9 deletions

diff --git a/runtime/doc/news.txt b/runtime/doc/news.txt
index 4c9cbf9189..90e17d3678 100644
--- a/runtime/doc/news.txt
+++ b/runtime/doc/news.txt
@@ -181,6 +181,9 @@ The following new APIs or features were added.
 
 • |nvim_list_uis()| reports all |ui-option| fields.
 
+• Vim's `has('gui_running')` is now supported as a way for plugins to check if
+  a GUI (not the |TUI|) is attached to Nvim. |has()|
+
 ==============================================================================
 CHANGED FEATURES                                                 *news-changes*
 
diff --git a/src/nvim/api/ui.c b/src/nvim/api/ui.c
index 00fd2781ff..a8f5d2e070 100644
--- a/src/nvim/api/ui.c
+++ b/src/nvim/api/ui.c
@@ -352,7 +352,6 @@ static void ui_set_option(UI *ui, bool init, String name, Object value, Error *e
     });
 
     stdin_fd = (int)value.data.integer;
-    ui->stdin_fd = (int)value.data.integer;
     return;
   }
 
diff --git a/src/nvim/eval/funcs.c b/src/nvim/eval/funcs.c
index 1baf96e281..d97f7b6d35 100644
--- a/src/nvim/eval/funcs.c
+++ b/src/nvim/eval/funcs.c
@@ -3228,7 +3228,9 @@ static void f_has(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
   }
 
   if (!n) {
-    if (STRNICMP(name, "patch", 5) == 0) {
+    if (STRNICMP(name, "gui_running", 11) == 0) {
+      n = ui_gui_attached();
+    } else if (STRNICMP(name, "patch", 5) == 0) {
       if (name[5] == '-'
           && strlen(name) >= 11
           && ascii_isdigit(name[6])
diff --git a/src/nvim/ui.c b/src/nvim/ui.c
index 6c95579b47..ce1a57350a 100644
--- a/src/nvim/ui.c
+++ b/src/nvim/ui.c
@@ -134,6 +134,7 @@ void ui_free_all_mem(void)
 }
 #endif
 
+/// Returns true if any `rgb=true` UI is attached.
 bool ui_rgb_attached(void)
 {
   if (!headless_mode && p_tgc) {
@@ -147,6 +148,18 @@ bool ui_rgb_attached(void)
   return false;
 }
 
+/// Returns true if a GUI is attached.
+bool ui_gui_attached(void)
+{
+  for (size_t i = 0; i < ui_count; i++) {
+    bool tui = uis[i]->stdin_tty || uis[i]->stdout_tty;
+    if (!tui) {
+      return true;
+    }
+  }
+  return false;
+}
+
 /// Returns true if any UI requested `override=true`.
 bool ui_override(void)
 {
@@ -599,11 +612,10 @@ Array ui_array(void)
     PUT(info, "rgb", BOOLEAN_OBJ(ui->rgb));
     PUT(info, "override", BOOLEAN_OBJ(ui->override));
 
-    // TUI fields.
+    // TUI fields. (`stdin_fd` is intentionally omitted.)
     PUT(info, "term_name", STRING_OBJ(cstr_to_string(ui->term_name)));
     PUT(info, "term_background", STRING_OBJ(cstr_to_string(ui->term_background)));
     PUT(info, "term_colors", INTEGER_OBJ(ui->term_colors));
-    PUT(info, "stdin_fd", INTEGER_OBJ(ui->stdin_fd));
     PUT(info, "stdin_tty", BOOLEAN_OBJ(ui->stdin_tty));
     PUT(info, "stdout_tty", BOOLEAN_OBJ(ui->stdout_tty));
 
diff --git a/src/nvim/ui.h b/src/nvim/ui.h
index c1377f56b4..dc0ccc73ea 100644
--- a/src/nvim/ui.h
+++ b/src/nvim/ui.h
@@ -107,7 +107,6 @@ struct ui_t {
   char *term_name;
   char *term_background;
   int term_colors;
-  int stdin_fd;
   bool stdin_tty;
   bool stdout_tty;
 
diff --git a/test/functional/terminal/tui_spec.lua b/test/functional/terminal/tui_spec.lua
index 9db80e0db2..76ea6256a1 100644
--- a/test/functional/terminal/tui_spec.lua
+++ b/test/functional/terminal/tui_spec.lua
@@ -1399,6 +1399,8 @@ describe('TUI', function()
   end)
 
   it('in nvim_list_uis()', function()
+    -- $TERM in :terminal.
+    local exp_term = is_os('bsd') and 'builtin_xterm' or 'xterm-256color'
     local expected = {
       {
          chan = 1,
@@ -1418,7 +1420,7 @@ describe('TUI', function()
          stdout_tty = true,
          term_background = '',
          term_colors = 256,
-         term_name = 'xterm-256color',  -- $TERM in :terminal.
+         term_name = exp_term,
          width = 50
        },
     }
diff --git a/test/functional/vimscript/has_spec.lua b/test/functional/vimscript/has_spec.lua
index 2e26d603b3..78a761d370 100644
--- a/test/functional/vimscript/has_spec.lua
+++ b/test/functional/vimscript/has_spec.lua
@@ -1,8 +1,11 @@
 local helpers = require('test.functional.helpers')(after_each)
-local eq = helpers.eq
+local Screen = require('test.functional.ui.screen')
 local clear = helpers.clear
+local connect = helpers.connect
+local eq = helpers.eq
 local funcs = helpers.funcs
 local is_os = helpers.is_os
+local nvim_prog = helpers.nvim_prog
 
 describe('has()', function()
   before_each(clear)
@@ -69,8 +72,22 @@ describe('has()', function()
     end
   end)
 
+  it('"gui_running"', function()
+    eq(0, funcs.has('gui_running'))
+    local tui = Screen.new(50,15)
+    local gui_session = connect(funcs.serverstart())
+    local gui = Screen.new(50,15)
+    eq(0, funcs.has('gui_running'))
+    tui:attach({ext_linegrid=true, rgb=true, stdin_tty=true, stdout_tty=true})
+    gui:attach({ext_multigrid=true, rgb=true}, gui_session)
+    eq(1, funcs.has('gui_running'))
+    tui:detach()
+    eq(1, funcs.has('gui_running'))
+    gui:detach()
+    eq(0, funcs.has('gui_running'))
+  end)
+
   it('does not change v:shell_error', function()
-    local nvim_prog = helpers.nvim_prog
     funcs.system({nvim_prog, '-es', '+73cquit'})
     funcs.has('python3') -- use a call whose implementation shells out
     eq(73, funcs.eval('v:shell_error'))
diff --git a/test/helpers.lua b/test/helpers.lua
index d45536b42b..117b6b4aaa 100644
--- a/test/helpers.lua
+++ b/test/helpers.lua
@@ -312,7 +312,7 @@ function module.is_os(s)
     or s == 'bsd') then
     error('unknown platform: '..tostring(s))
   end
-  return ((s == 'win' and module.sysname():find('windows'))
+  return not not ((s == 'win' and module.sysname():find('windows'))
     or (s == 'mac' and module.sysname() == 'darwin')
     or (s == 'freebsd' and module.sysname() == 'freebsd')
     or (s == 'openbsd' and module.sysname() == 'openbsd')