fix(api): nvim_create_buf assert fails if autocmds set &swapfile

Problem: assertion failure in nvim_create_buf if buflist_new autocommands open
a swapfile when "scratch" is set.

Solution: block autocommands when setting up the buffer; fire them later
instead.

Note that, unlike buflist_new, I don't check if autocommands aborted script
processing; the buffer is already created and configured at that point, so might
as well return the handle anyway.

Rather than repeat try_{start,end} and {un}block_autocmds for each relevant
operation, just do it at the start and near the end. This means that, if
TermResponse fires from unblock_autocmds for whatever reason, it can see the
buffer in an already configured state if we didn't bail due to an error (plus
it's probably a bit cleaner this way).

src/nvim/api/vim.c

@@ -958,24 +958,22 @@ Buffer nvim_create_buf(Boolean listed, Boolean scratch, Error *err)
   FUNC_API_SINCE(6)
 {
   try_start();
+  // Block autocommands for now so they don't mess with the buffer before we
+  // finish configuring it.
+  block_autocmds();
+
   buf_T *buf = buflist_new(NULL, NULL, 0,
                            BLN_NOOPT | BLN_NEW | (listed ? BLN_LISTED : 0));
-  try_end(err);
   if (buf == NULL) {
+    unblock_autocmds();
     goto fail;
   }
 
   // Open the memline for the buffer. This will avoid spurious autocmds when
   // a later nvim_buf_set_lines call would have needed to "open" the buffer.
-  if (buf->b_ml.ml_mfp == NULL) {
-    try_start();
-    block_autocmds();
-    int status = ml_open(buf);
+  if (ml_open(buf) == FAIL) {
     unblock_autocmds();
-    try_end(err);
-    if (status == FAIL) {
-      goto fail;
-    }
+    goto fail;
   }
 
   // Set last_changedtick to avoid triggering a TextChanged autocommand right
@@ -985,7 +983,7 @@ Buffer nvim_create_buf(Boolean listed, Boolean scratch, Error *err)
   buf->b_last_changedtick_pum = buf_get_changedtick(buf);
 
   // Only strictly needed for scratch, but could just as well be consistent
-  // and do this now. buffer is created NOW, not when it latter first happen
+  // and do this now. Buffer is created NOW, not when it later first happens
   // to reach a window or aucmd_prepbuf() ..
   buf_copy_options(buf, BCO_ENTER | BCO_NOHELP);
 
@@ -996,10 +994,26 @@ Buffer nvim_create_buf(Boolean listed, Boolean scratch, Error *err)
     buf->b_p_swf = false;
     buf->b_p_ml = false;
   }
+
+  unblock_autocmds();
+
+  bufref_T bufref;
+  set_bufref(&bufref, buf);
+  if (apply_autocmds(EVENT_BUFNEW, NULL, NULL, false, buf)
+      && !bufref_valid(&bufref)) {
+    goto fail;
+  }
+  if (listed
+      && apply_autocmds(EVENT_BUFADD, NULL, NULL, false, buf)
+      && !bufref_valid(&bufref)) {
+    goto fail;
+  }
+
+  try_end(err);
   return buf->b_fnum;
 
 fail:
-  if (!ERROR_SET(err)) {
+  if (!try_end(err)) {
     api_set_error(err, kErrorTypeException, "Failed to create buffer");
   }
   return 0;

test/functional/api/vim_spec.lua

@@ -3144,6 +3144,47 @@ describe('API', function()
       api.nvim_create_buf(false, true)
       eq(1, eval('g:loaded'))
     end)
+
+    it('creating scratch buffer where autocommands set &swapfile works', function()
+      exec([[
+        autocmd BufNew * ++once execute expand("<abuf>") "buffer"
+                             \| file foobar
+                             \| setlocal swapfile
+      ]])
+      local new_buf = api.nvim_create_buf(false, true)
+      neq('', fn.swapname(new_buf))
+    end)
+
+    it('fires expected autocommands', function()
+      exec([=[
+        " Append the &buftype to check autocommands trigger *after* the buffer was configured to be
+        " scratch, if applicable.
+        autocmd BufNew * let fired += [["BufNew", expand("<abuf>")->str2nr(),
+                                      \ getbufvar(expand("<abuf>")->str2nr(), "&buftype")]]
+        autocmd BufAdd * let fired += [["BufAdd", expand("<abuf>")->str2nr(),
+                                      \ getbufvar(expand("<abuf>")->str2nr(), "&buftype")]]
+
+        " Don't want to see OptionSet; buffer options set from passing true for "scratch", etc.
+        " should be configured invisibly, and before autocommands.
+        autocmd OptionSet * let fired += [["OptionSet", expand("<amatch>")]]
+
+        let fired = []
+      ]=])
+      local new_buf = api.nvim_create_buf(false, false)
+      eq({ { 'BufNew', new_buf, '' } }, eval('g:fired'))
+
+      command('let fired = []')
+      new_buf = api.nvim_create_buf(false, true)
+      eq({ { 'BufNew', new_buf, 'nofile' } }, eval('g:fired'))
+
+      command('let fired = []')
+      new_buf = api.nvim_create_buf(true, false)
+      eq({ { 'BufNew', new_buf, '' }, { 'BufAdd', new_buf, '' } }, eval('g:fired'))
+
+      command('let fired = []')
+      new_buf = api.nvim_create_buf(true, true)
+      eq({ { 'BufNew', new_buf, 'nofile' }, { 'BufAdd', new_buf, 'nofile' } }, eval('g:fired'))
+    end)
   end)
 
   describe('nvim_get_runtime_file', function()