feat(options): per-buffer 'busy' status #34493

Problem:
Plugins cannot mark a buffer as "busy".

Solution:
- Add a buffer-local 'busy' option.
- Show a busy indicator in the default 'statusline'.

src/nvim/buffer_defs.h

@@ -12,6 +12,7 @@
 #include "nvim/option_defs.h"
 #include "nvim/os/fs_defs.h"
 #include "nvim/statusline_defs.h"
+#include "nvim/types_defs.h"
 #include "nvim/undo_defs.h"
 
 /// Reference to a buffer that stores the value of buf_free_count.
@@ -522,6 +523,7 @@ struct file_buffer {
   int b_p_bomb;                 ///< 'bomb'
   char *b_p_bh;                 ///< 'bufhidden'
   char *b_p_bt;                 ///< 'buftype'
+  OptInt b_p_busy;              ///< 'busy'
   int b_has_qf_entry;           ///< quickfix exists for buffer
   int b_p_bl;                   ///< 'buflisted'
   OptInt b_p_channel;           ///< 'channel'

src/nvim/option.c

@@ -4681,6 +4681,8 @@ void *get_varp_from(vimoption_T *p, buf_T *buf, win_T *win)
     return &(buf->b_p_bt);
   case kOptBuflisted:
     return &(buf->b_p_bl);
+  case kOptBusy:
+    return &(buf->b_p_busy);
   case kOptChannel:
     return &(buf->b_p_channel);
   case kOptCopyindent:

src/nvim/option_vars.h

@@ -280,6 +280,7 @@ EXTERN char *p_bsk;             ///< 'backupskip'
 EXTERN char *p_breakat;         ///< 'breakat'
 EXTERN char *p_bh;              ///< 'bufhidden'
 EXTERN char *p_bt;              ///< 'buftype'
+EXTERN OptInt p_busy;           ///< 'busy'
 EXTERN char *p_cmp;             ///< 'casemap'
 EXTERN unsigned cmp_flags;
 EXTERN char *p_enc;             ///< 'encoding'

src/nvim/options.lua

@@ -950,6 +950,21 @@ local options = {
       type = 'string',
       varname = 'p_bt',
     },
+    {
+      defaults = 0,
+      desc = [=[
+        Sets a buffer "busy" status. Indicated in the default statusline.
+        When busy status is larger then 0 busy flag is shown in statusline.
+        The semantics of "busy" are arbitrary, typically decided by the plugin that owns the buffer.
+      ]=],
+      full_name = 'busy',
+      redraw = { 'statuslines' },
+      noglob = true,
+      scope = { 'buf' },
+      short_desc = N_('buffer is busy'),
+      type = 'number',
+      varname = 'p_busy',
+    },
     {
       abbreviation = 'cmp',
       defaults = 'internal,keepascii',
@@ -8640,6 +8655,7 @@ local options = {
         '%=',
         "%{% &showcmdloc == 'statusline' ? '%-10.S ' : '' %}",
         "%{% exists('b:keymap_name') ? '<'..b:keymap_name..'> ' : '' %}",
+        "%{% &busy > 0 ? '◐ ' : '' %}",
         "%{% &ruler ? ( &rulerformat == '' ? '%-14.(%l,%c%V%) %P' : &rulerformat ) : '' %}",
       }),
       desc = [=[