eval: Add s flag, use p_fs by default, error out on unknown flag

3 files changed, 27 insertions, 13 deletions

diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 727199f742..7060cc4186 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -7916,10 +7916,11 @@ writefile({list}, {fname} [, {flags}])
 			:call writefile(["foo"], "event.log", "a")
 			:call writefile(["bar"], "event.log", "a")
 <
-		When {flags} contains "S" fsync() call is not used. This means 
-		that writefile() will finish faster, but writes may be left in 
-		OS buffers and not yet written to disk. Such changes will 
-		disappear if system crashes before OS does writing.
+		When {flags} contains "S" fsync() call is not used, with "s" 
+		it is used, 'fsync' option applies by default. No fsync() 
+		means that writefile() will finish faster, but writes may be 
+		left in OS buffers and not yet written to disk. Such changes 
+		will disappear if system crashes before OS does writing.
 
 		All NL characters are replaced with a NUL character.
 		Inserting CR characters needs to be done before passing {list}
diff --git a/src/nvim/eval.c b/src/nvim/eval.c
index 8a3e3f3e22..7ab07fe6a2 100644
--- a/src/nvim/eval.c
+++ b/src/nvim/eval.c
@@ -17421,20 +17421,24 @@ static void f_writefile(typval_T *argvars, typval_T *rettv, FunPtr fptr)
 
   bool binary = false;
   bool append = false;
-  bool do_fsync = true;
+  bool do_fsync = !!p_fs;
   if (argvars[2].v_type != VAR_UNKNOWN) {
     const char *const flags = tv_get_string_chk(&argvars[2]);
     if (flags == NULL) {
       return;
     }
-    if (strchr(flags, 'b')) {
-      binary = true;
-    }
-    if (strchr(flags, 'a')) {
-      append = true;
-    }
-    if (strchr(flags, 'S')) {
-      do_fsync = false;
+    for (const char *p = flags; *p; p++) {
+      switch (*p) {
+        case 'b': { binary = true; break; }
+        case 'a': { append = true; break; }
+        case 's': { do_fsync = true; break; }
+        case 'S': { do_fsync = false; break; }
+        default: {
+          // Using %s, p and not %c, *p to preserve multibyte characters
+          emsgf(_("E5060: Unknown flag: %s"), p);
+          return;
+        }
+      }
     }
   }
 
diff --git a/test/functional/eval/writefile_spec.lua b/test/functional/eval/writefile_spec.lua
index 3052c616e0..2f84114b9b 100644
--- a/test/functional/eval/writefile_spec.lua
+++ b/test/functional/eval/writefile_spec.lua
@@ -80,6 +80,13 @@ describe('writefile()', function()
     eq('a\0\0\0b', read_file(fname))
   end)
 
+  it('writes with s and S', function()
+    eq(0, funcs.writefile({'\na\nb\n'}, fname, 'bs'))
+    eq('\0a\0b\0', read_file(fname))
+    eq(0, funcs.writefile({'a\n\n\nb'}, fname, 'bS'))
+    eq('a\0\0\0b', read_file(fname))
+  end)
+
   it('correctly overwrites file', function()
     eq(0, funcs.writefile({'\na\nb\n'}, fname, 'b'))
     eq('\0a\0b\0', read_file(fname))
@@ -115,6 +122,8 @@ describe('writefile()', function()
       eq('\nE729: using Funcref as a String',
          redir_exec(('call writefile(%s)'):format(args:format('function("tr")'))))
     end
+    eq('\nE5060: Unknown flag: «»',
+       redir_exec(('call writefile([], "%s", "bs«»")'):format(fname)))
     eq('TEST', read_file(fname))
   end)