fix(api): autocmds mess up nvim_get_option_value's dummy buffer

Problem: When the "filetype" key is set for nvim_get_option_value, autocommands
can crash Nvim by prematurely wiping the dummy buffer, or cause options intended
for it to instead be set for unrelated buffers if switched during OptionSet.

Solution: Don't crash. Also quash side-effects from setting the buffer options.

src/nvim/api/options.c

@@ -13,6 +13,7 @@
 #include "nvim/buffer.h"
 #include "nvim/buffer_defs.h"
 #include "nvim/globals.h"
+#include "nvim/memline.h"
 #include "nvim/memory.h"
 #include "nvim/memory_defs.h"
 #include "nvim/option.h"
@@ -99,8 +100,10 @@ static int validate_option_value_args(Dict(option) *opts, char *name, OptIndex *
 }
 
 /// Create a dummy buffer and run the FileType autocmd on it.
-static buf_T *do_ft_buf(char *filetype, aco_save_T *aco, Error *err)
+static buf_T *do_ft_buf(const char *filetype, aco_save_T *aco, bool *aco_used, Error *err)
+  FUNC_ATTR_NONNULL_ARG(2, 3, 4)
 {
+  *aco_used = false;
   if (filetype == NULL) {
     return NULL;
   }
@@ -112,19 +115,37 @@ static buf_T *do_ft_buf(char *filetype, aco_save_T *aco, Error *err)
     return NULL;
   }
 
+  // Open a memline for use by autocommands.
+  if (ml_open(ftbuf) == FAIL) {
+    api_set_error(err, kErrorTypeException, "Could not load internal buffer");
+    return ftbuf;
+  }
+
+  bufref_T bufref;
+  set_bufref(&bufref, ftbuf);
+
   // Set curwin/curbuf to buf and save a few things.
   aucmd_prepbuf(aco, ftbuf);
+  *aco_used = true;
+
+  set_option_direct(kOptBufhidden, STATIC_CSTR_AS_OPTVAL("hide"), OPT_LOCAL, SID_NONE);
+  set_option_direct(kOptBuftype, STATIC_CSTR_AS_OPTVAL("nofile"), OPT_LOCAL, SID_NONE);
+  assert(ftbuf->b_ml.ml_mfp->mf_fd < 0);  // ml_open() should not have opened swapfile already
+  ftbuf->b_p_swf = false;
+  ftbuf->b_p_ml = false;
+  ftbuf->b_p_ft = xstrdup(filetype);
 
   TRY_WRAP(err, {
-    set_option_value(kOptBufhidden, STATIC_CSTR_AS_OPTVAL("hide"), OPT_LOCAL);
-    set_option_value(kOptBuftype, STATIC_CSTR_AS_OPTVAL("nofile"), OPT_LOCAL);
-    set_option_value(kOptSwapfile, BOOLEAN_OPTVAL(false), OPT_LOCAL);
-    set_option_value(kOptModeline, BOOLEAN_OPTVAL(false), OPT_LOCAL);  // 'nomodeline'
-
-    ftbuf->b_p_ft = xstrdup(filetype);
     do_filetype_autocmd(ftbuf, false);
   });
 
+  if (!bufref_valid(&bufref)) {
+    if (!ERROR_SET(err)) {
+      api_set_error(err, kErrorTypeException, "Internal buffer was deleted");
+    }
+    return NULL;
+  }
+
   return ftbuf;
 }
 
@@ -161,13 +182,15 @@ Object nvim_get_option_value(String name, Dict(option) *opts, Error *err)
   }
 
   aco_save_T aco;
+  bool aco_used;
 
-  buf_T *ftbuf = do_ft_buf(filetype, &aco, err);
+  buf_T *ftbuf = do_ft_buf(filetype, &aco, &aco_used, err);
   if (ERROR_SET(err)) {
-    if (ftbuf != NULL) {
+    if (aco_used) {
       // restore curwin/curbuf and a few other things
       aucmd_restbuf(&aco);
-
+    }
+    if (ftbuf != NULL) {
       assert(curbuf != ftbuf);  // safety check
       wipe_buffer(ftbuf, false);
     }
@@ -183,9 +206,10 @@ Object nvim_get_option_value(String name, Dict(option) *opts, Error *err)
   OptVal value = get_option_value_for(opt_idx, opt_flags, scope, from, err);
 
   if (ftbuf != NULL) {
-    // restore curwin/curbuf and a few other things
+    if (aco_used) {
-    aucmd_restbuf(&aco);
+      // restore curwin/curbuf and a few other things
-
+      aucmd_restbuf(&aco);
+    }
     assert(curbuf != ftbuf);  // safety check
     wipe_buffer(ftbuf, false);
   }

test/functional/api/vim_spec.lua

@@ -1977,6 +1977,42 @@ describe('API', function()
       ]])
       eq(false, api.nvim_get_option_value('modified', {}))
     end)
+
+    it('errors if autocmds wipe the dummy buffer', function()
+      -- Wipe the dummy buffer. This will throw E813, but the buffer will still be wiped; check that
+      -- such errors from setting the filetype have priority.
+      command 'autocmd FileType * ++once bwipeout!'
+      eq(
+        'FileType Autocommands for "*": Vim(bwipeout):E813: Cannot close autocmd window',
+        pcall_err(api.nvim_get_option_value, 'formatexpr', { filetype = 'lua' })
+      )
+
+      -- Silence E813 to check that the error for wiping the dummy buffer is set.
+      command 'autocmd FileType * ++once silent! bwipeout!'
+      eq(
+        'Internal buffer was deleted',
+        pcall_err(api.nvim_get_option_value, 'formatexpr', { filetype = 'lua' })
+      )
+    end)
+
+    it('sets dummy buffer options without side-effects', function()
+      exec [[
+        let g:events = []
+        autocmd OptionSet * let g:events += [expand("<amatch>")]
+        autocmd FileType * ++once let g:bufhidden = &l:bufhidden
+                               \| let g:buftype = &l:buftype
+                               \| let g:swapfile = &l:swapfile
+                               \| let g:modeline = &l:modeline
+                               \| let g:bufloaded = bufloaded(bufnr())
+      ]]
+      api.nvim_get_option_value('formatexpr', { filetype = 'lua' })
+      eq({}, eval('g:events'))
+      eq('hide', eval('g:bufhidden'))
+      eq('nofile', eval('g:buftype'))
+      eq(0, eval('g:swapfile'))
+      eq(0, eval('g:modeline'))
+      eq(1, eval('g:bufloaded'))
+    end)
   end)
 
   describe('nvim_{get,set}_current_buf, nvim_list_bufs', function()