patch 8.2.3509: undo file is not synced

Problem:    Undo file is not synced. (Sami Farin)
Solution:   Sync the undo file if 'fsync' is set. (Christian Brabandt,
            closes #8879, closes #8920)

runtime/doc/options.txt

@@ -3610,8 +3610,8 @@ A jump table for the options with a short description can be found at |Q_op|.
 	systems without an fsync() implementation, this variable is always
 	off.
 	Also see 'swapsync' for controlling fsync() on swap files.
-	'fsync' also applies to |writefile()|, unless a flag is used to
-	overrule it.
+	'fsync' also applies to |writefile()| (unless a flag is used to
+	overrule it) and when writing undo files (see |undo-persistence|).
 	This option cannot be set from a |modeline| or in the |sandbox|, for
 	security reasons.
 

src/undo.c

@@ -1786,6 +1786,11 @@ u_write_undo(
 	write_ok = FALSE;
 #endif
 
+#if defined(UNIX) && defined(HAVE_FSYNC)
+    if (p_fs && fflush(fp) == 0 && vim_fsync(fd) != 0)
+	write_ok = FALSE;
+#endif
+
 write_error:
     fclose(fp);
     if (!write_ok)

src/version.c

@@ -757,6 +757,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3509,
 /**/
     3508,
 /**/