patch 8.0.1305: writefile() never calls fsync()

Problem:    Writefile() never calls fsync().
Solution:   Follow the 'fsync' option with override to enable or disable.

src/evalfunc.c

@@ -13348,6 +13348,9 @@ f_writefile(typval_T *argvars, typval_T *rettv)
 {
     int		binary = FALSE;
     int		append = FALSE;
+#ifdef HAVE_FSYNC
+    int		do_fsync = p_fs;
+#endif
     char_u	*fname;
     FILE	*fd;
     int		ret = 0;
@@ -13380,6 +13383,12 @@ f_writefile(typval_T *argvars, typval_T *rettv)
 	    binary = TRUE;
 	if (vim_strchr(arg2, 'a') != NULL)
 	    append = TRUE;
+#ifdef HAVE_FSYNC
+	if (vim_strchr(arg2, 's') != NULL)
+	    do_fsync = TRUE;
+	else if (vim_strchr(arg2, 'S') != NULL)
+	    do_fsync = FALSE;
+#endif
     }
 
     fname = get_tv_string_chk(&argvars[1]);
@@ -13398,6 +13407,10 @@ f_writefile(typval_T *argvars, typval_T *rettv)
     {
 	if (write_list(fd, list, binary) == FAIL)
 	    ret = -1;
+#ifdef HAVE_FSYNC
+	else if (do_fsync && fsync(fileno(fd)) != 0)
+	    EMSG(_(e_fsync));
+#endif
 	fclose(fd);
     }
 

src/fileio.c

@@ -4771,7 +4771,7 @@ restore_backup:
 	 */
 	if (p_fs && fsync(fd) != 0 && !device)
 	{
-	    errmsg = (char_u *)_("E667: Fsync failed");
+	    errmsg = (char_u *)_(e_fsync);
 	    end = 0;
 	}
 #endif

src/globals.h

@@ -1449,6 +1449,9 @@ EXTERN char_u e_isadir2[]	INIT(= N_("E17: \"%s\" is a directory"));
 #ifdef FEAT_LIBCALL
 EXTERN char_u e_libcall[]	INIT(= N_("E364: Library call failed for \"%s()\""));
 #endif
+#ifdef HAVE_FSYNC
+EXTERN char_u e_fsync[]		INIT(= N_("E667: Fsync failed"));
+#endif
 #if defined(DYNAMIC_PERL) \
 	|| defined(DYNAMIC_PYTHON) || defined(DYNAMIC_PYTHON3) \
 	|| defined(DYNAMIC_RUBY) \

src/testdir/test_writefile.vim

@@ -93,3 +93,10 @@ func Test_nowrite_quit_split()
   endif
   bwipe Xfile
 endfunc
+
+func Test_writefile_sync_arg()
+  " This doesn't check if fsync() works, only that the argument is accepted.
+  call writefile(['one'], 'Xtest', 's')
+  call writefile(['two'], 'Xtest', 'S')
+  call delete('Xtest')
+endfunc

src/version.c

@@ -766,6 +766,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1305,
 /**/
     1304,
 /**/