fillchars: make checks more strict and improve tests

runtime/doc/vim_diff.txt

@@ -180,7 +180,8 @@ Options:
   'cpoptions' flags: |cpo-_|
   'display' flag `msgsep` to minimize scrolling when showing messages
   'guicursor' works in the terminal
-  'fillchars' flag `msgsep` (see 'display' above)
+  'fillchars' flags: `msgsep` (see 'display' above)
+	      and `eob` for |EndOfBuffer| marker
   'inccommand' shows interactive results for |:substitute|-like commands
   'scrollback'
   'statusline' supports unlimited alignment sections

src/nvim/option.c

@@ -3438,16 +3438,20 @@ static char_u *set_chars_option(char_u **varp)
             && p[len] == ':'
             && p[len + 1] != NUL) {
           s = p + len + 1;
-          c1 = mb_ptr2char_adv((const char_u **)&s);
+
-          if (mb_char2cells(c1) > 1) {
+          // TODO(bfredl): use schar_T representation and utfc_ptr2len
+          int c1len = utf_ptr2len(s);
+          c1 = mb_cptr2char_adv((const char_u **)&s);
+          if (mb_char2cells(c1) > 1 || (c1len == 1 && c1 > 127)) {
             continue;
           }
           if (tab[i].cp == &lcs_tab2) {
             if (*s == NUL) {
               continue;
             }
-            c2 = mb_ptr2char_adv((const char_u **)&s);
+            int c2len = utf_ptr2len(s);
-            if (mb_char2cells(c2) > 1) {
+            c2 = mb_cptr2char_adv((const char_u **)&s);
+            if (mb_char2cells(c2) > 1 || (c2len == 1 && c2 > 127)) {
               continue;
             }
           }

test/functional/options/fillchars_spec.lua

@@ -1,6 +1,9 @@
 local helpers = require('test.functional.helpers')(after_each)
 local Screen = require('test.functional.ui.screen')
-local clear, execute = helpers.clear, helpers.execute
+local clear, command = helpers.clear, helpers.command
+local eval = helpers.eval
+local eq = helpers.eq
+local exc_exec = helpers.exc_exec
 
 describe("'fillchars'", function()
   local screen
@@ -15,8 +18,15 @@ describe("'fillchars'", function()
     screen:detach()
   end)
 
+  local function shouldfail(val,errval)
+    errval = errval or val
+    eq('Vim(set):E474: Invalid argument: fillchars='..errval,
+       exc_exec('set fillchars='..val))
+  end
+
   describe('"eob" flag', function()
-    it('renders empty lines at the end of the buffer with eob', function()
+    it("uses '~' by default", function()
+      eq('', eval('&fillchars'))
       screen:expect([[
         ^                         |
         ~                        |
@@ -24,22 +34,33 @@ describe("'fillchars'", function()
         ~                        |
                                  |
       ]])
-      execute('set fillchars+=eob:\\ ')
+    end)
+    it('supports whitespace', function()
+      command('set fillchars=eob:\\ ')
       screen:expect([[
         ^                         |
                                  |
                                  |
                                  |
-        :set fillchars+=eob:\    |
+                                 |
       ]])
-      execute('set fillchars+=eob:ñ')
+    end)
+    it('supports multibyte char', function()
+      command('set fillchars=eob:ñ')
       screen:expect([[
         ^                         |
         ñ                        |
         ñ                        |
         ñ                        |
-        :set fillchars+=eob:ñ    |
+                                 |
       ]])
     end)
+    it('handles invalid values', function()
+      shouldfail('eob:') -- empty string
+      shouldfail('eob:馬') -- doublewidth char
+      shouldfail('eob:å̲') -- composing chars
+      shouldfail('eob:xy') -- two ascii chars
+      shouldfail('eob:\255', 'eob:<ff>') -- invalid UTF-8
+    end)
   end)
 end)