fix(api): nvim_get_option_value dummy buffer crashes

Problem: nvim_get_option_value with "filetype" set can crash if autocommands
open the dummy buffer in more windows, or if &bufhidden == "wipe".

Solution: Attempt to close all dummy buffer windows before wiping. Promote the
dummy buffer to a normal buffer if that fails.

(cherry picked from commit 7e2e116343)

src/nvim/api/options.c

@@ -19,6 +19,7 @@
 #include "nvim/option.h"
 #include "nvim/types_defs.h"
 #include "nvim/vim_defs.h"
+#include "nvim/window.h"
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "api/options.c.generated.h"
@@ -151,6 +152,27 @@ static buf_T *do_ft_buf(const char *filetype, aco_save_T *aco, bool *aco_used, E
   return ftbuf;
 }
 
+static void wipe_ft_buf(buf_T *buf)
+  FUNC_ATTR_NONNULL_ALL
+{
+  block_autocmds();
+
+  bufref_T bufref;
+  set_bufref(&bufref, buf);
+
+  close_windows(buf, false);
+  // Autocommands are blocked, but 'bufhidden' may have wiped it already.
+  // Also can't wipe if the buffer is somehow still in a window or current.
+  if (bufref_valid(&bufref) && buf != curbuf && buf->b_nwindows == 0) {
+    wipe_buffer(buf, false);
+  }
+  if (bufref_valid(&bufref)) {
+    buf->b_flags &= ~BF_DUMMY;  // Couldn't wipe; keep it instead.
+  }
+
+  unblock_autocmds();
+}
+
 /// Gets the value of an option. The behavior of this function matches that of
 /// |:set|: the local value of an option is returned if it exists; otherwise,
 /// the global value is returned. Local values always correspond to the current
@@ -193,10 +215,8 @@ Object nvim_get_option_value(String name, Dict(option) *opts, Error *err)
       aucmd_restbuf(&aco);
     }
     if (ftbuf != NULL) {
-      assert(curbuf != ftbuf);  // safety check
-      wipe_buffer(ftbuf, false);
+      wipe_ft_buf(ftbuf);
     }
-
     return (Object)OBJECT_INIT;
   }
 
@@ -212,8 +232,7 @@ Object nvim_get_option_value(String name, Dict(option) *opts, Error *err)
       // restore curwin/curbuf and a few other things
       aucmd_restbuf(&aco);
     }
-    assert(curbuf != ftbuf);  // safety check
-    wipe_buffer(ftbuf, false);
+    wipe_ft_buf(ftbuf);
   }
 
   if (ERROR_SET(err)) {

test/functional/api/vim_spec.lua

@@ -2008,6 +2008,46 @@ describe('API', function()
       )
     end)
 
+    it('does not crash if autocmds open dummy buffer in other windows', function()
+      exec [[
+        autocmd FileType * ++once let g:dummy_buf = bufnr() | split
+
+        " Autocommands should be blocked while Nvim attempts to wipe the buffer.
+        let g:wipe_events = []
+        autocmd WinClosed * if winbufnr(expand('<amatch>')) == g:dummy_buf
+                         \| let g:wipe_events += ['WinClosed']
+                         \| endif
+        autocmd BufWipeout * if expand('<abuf>') == g:dummy_buf
+                         \| let g:wipe_events += ['BufWipeout']
+                         \| endif
+      ]]
+      api.nvim_get_option_value('formatexpr', { filetype = 'lua' })
+      eq(0, eval('bufexists(g:dummy_buf)'))
+      eq({}, eval('win_findbuf(g:dummy_buf)'))
+      eq({}, eval('g:wipe_events'))
+
+      -- Be an ABSOLUTE nuisance and make it the only window to prevent it from wiping.
+      -- Do it this way to avoid E813 from :only trying to close the autocmd window.
+      command('autocmd FileType * ++once let g:dummy_buf = bufnr() | split | wincmd w | quit')
+      api.nvim_get_option_value('formatexpr', { filetype = 'lua' })
+      eq(1, eval('bufexists(g:dummy_buf)'))
+
+      -- Ensure the buffer does not remain as a dummy by checking that we can switch to it.
+      local old_win = api.nvim_get_current_win()
+      command('execute g:dummy_buf "sbuffer"')
+      eq(eval('g:dummy_buf'), api.nvim_get_current_buf())
+      neq(old_win, api.nvim_get_current_win())
+      eq({}, eval('g:wipe_events'))
+    end)
+
+    it('does not crash if dummy buffer wiped after autocommands', function()
+      -- Autocommands are blocked while Nvim attempts to wipe the buffer, but check something like
+      -- &bufhidden = "wipe" causing a premature wipe doesn't crash.
+      command('autocmd FileType * ++once setlocal bufhidden=wipe | split')
+      api.nvim_get_option_value('formatexpr', { filetype = 'lua' })
+      assert_alive()
+    end)
+
     it('sets dummy buffer options without side-effects', function()
       exec [[
         let g:events = []