fix(filetype): empty 'filetype' skips :setfiletype #41958

Problem:
After `:restart` a restored buffer can have an empty 'filetype'. Lazy
LSP setup during the read replays FileType via `:doautoall`, which sets
`did_filetype` even though `filetype=''`.

    :e foo.md
    :restart
    :set ft?  " filetype=

Solution:
Set `did_filetype` only if 'filetype' is non-empty.

runtime/doc/options.txt

@@ -462,15 +462,16 @@ reads back as the function (from Lua) or a `"<Lua …>"` hint (Vimscript/RPC).
 Setting the filetype
 
 :setf[iletype] [FALLBACK] {filetype}			*:setf* *:setfiletype*
-			Set the 'filetype' option to {filetype}, but only if
-			not done yet in a sequence of (nested) autocommands.
+			Set the 'filetype' option to {filetype}, unless it was
+			already set (non-empty) in a sequence of (nested)
+			autocommands.
 			This is short for: >
 				:if !did_filetype()
 				:  setlocal filetype={filetype}
 				:endif
-<			This command is used in a filetype.vim file to avoid
-			setting the 'filetype' option twice, causing different
-			settings and syntax files to be loaded.
+<			Used in a filetype.vim file to avoid setting the
+			'filetype' option twice, causing different settings
+			and syntax files to be loaded.
 
 			When the optional FALLBACK argument is present, a
 			later :setfiletype command will override the

runtime/doc/vimfn.txt

@@ -1743,10 +1743,11 @@ dictwatcherdel({dict}, {pattern}, {callback})                 *dictwatcherdel()*
 
 did_filetype()                                                  *did_filetype()*
 		Returns |TRUE| when autocommands are being executed and the
-		FileType event has been triggered at least once.  Can be used
-		to avoid triggering the FileType event again in the scripts
-		that detect the file type. |FileType|
+		FileType event triggered at least once for a buffer with
+		a non-empty 'filetype'.  Can be used to avoid triggering
+		the FileType event again in the |ftdetect| scripts. |FileType|
 		Returns |FALSE| when `:setf FALLBACK` was used.
+
 		When editing another file, the counter is reset, thus this
 		really checks if the FileType event has been triggered for the
 		current buffer.  This allows an autocommand that starts

runtime/lua/vim/_meta/vimfn.gen.lua

@@ -1528,10 +1528,11 @@ function vim.fn.dictwatcheradd(dict, pattern, callback) end
 function vim.fn.dictwatcherdel(dict, pattern, callback) end
 
 --- Returns |TRUE| when autocommands are being executed and the
---- FileType event has been triggered at least once.  Can be used
---- to avoid triggering the FileType event again in the scripts
---- that detect the file type. |FileType|
+--- FileType event triggered at least once for a buffer with
+--- a non-empty 'filetype'.  Can be used to avoid triggering
+--- the FileType event again in the |ftdetect| scripts. |FileType|
 --- Returns |FALSE| when `:setf FALLBACK` was used.
+---
 --- When editing another file, the counter is reset, thus this
 --- really checks if the FileType event has been triggered for the
 --- current buffer.  This allows an autocommand that starts

src/nvim/autocmd.c

@@ -1710,8 +1710,8 @@ bool apply_autocmds_group(event_T event, char *fname, char *fname_io, bool force
   filechangeshell_busy = (event == EVENT_FILECHANGEDSHELL);
   nesting++;  // see matching decrement below
 
-  // Remember that FileType was triggered.  Used for did_filetype().
-  if (event == EVENT_FILETYPE) {
+  // Remember that FileType was triggered, if 'filetype' was actually set.  For :setf.
+  if (event == EVENT_FILETYPE && *curbuf->b_p_ft != NUL) {
     curbuf->b_did_filetype = true;
   }
 

src/nvim/eval.lua

@@ -1952,10 +1952,11 @@ M.funcs = {
   did_filetype = {
     desc = [=[
       Returns |TRUE| when autocommands are being executed and the
-      FileType event has been triggered at least once.  Can be used
-      to avoid triggering the FileType event again in the scripts
-      that detect the file type. |FileType|
+      FileType event triggered at least once for a buffer with
+      a non-empty 'filetype'.  Can be used to avoid triggering
+      the FileType event again in the |ftdetect| scripts. |FileType|
       Returns |FALSE| when `:setf FALLBACK` was used.
+
       When editing another file, the counter is reset, thus this
       really checks if the FileType event has been triggered for the
       current buffer.  This allows an autocommand that starts

test/functional/autocmd/filetype_spec.lua

@@ -16,4 +16,17 @@ describe('autocmd FileType', function()
     command('help help')
     t.eq(1, eval('g:foo'))
   end)
+
+  it("empty 'filetype' does not prevent FileType event #41711", function()
+    local file = t.tmpname(false) .. '.md'
+    t.write_file(file, '# hi\n')
+    command('filetype on')
+    -- Like vim.lsp.enable() lazy-loaded while the buffer is being read.
+    command('autocmd FileType * :')
+    command('autocmd BufReadPre * ++once doautoall FileType')
+    -- Run :edit in a nested event (mimics :restart session-restore).
+    command(('autocmd User X ++nested edit %s'):format(vim.fn.fnameescape(file)))
+    command('doautocmd User X')
+    t.eq('markdown', eval('&filetype'))
+  end)
 end)