fix(highlight): allow globals to be cleared

- and reduce heap allocations

Fixes #17420

src/nvim/highlight.c

@@ -904,15 +904,13 @@ HlAttrs dict2hlattrs(Dict(highlight) *dict, bool use_rgb, int *link_id, Error *e
     hlattrs.rgb_bg_color = bg;
     hlattrs.rgb_fg_color = fg;
     hlattrs.rgb_sp_color = sp;
-    hlattrs.cterm_bg_color =
+    hlattrs.cterm_bg_color = ctermbg == -1 ? 0 : ctermbg + 1;
-      ctermbg == -1 ? cterm_normal_bg_color : ctermbg + 1;
+    hlattrs.cterm_fg_color = ctermfg == -1 ? 0 : ctermfg + 1;
-    hlattrs.cterm_fg_color =
-      ctermfg == -1 ? cterm_normal_fg_color : ctermfg + 1;
     hlattrs.cterm_ae_attr = cterm_mask;
   } else {
     hlattrs.cterm_ae_attr = cterm_mask;
-    hlattrs.cterm_bg_color = bg == -1 ? cterm_normal_bg_color : bg + 1;
+    hlattrs.cterm_bg_color = bg == -1 ? 0 : bg + 1;
-    hlattrs.cterm_fg_color = fg == -1 ? cterm_normal_fg_color : fg + 1;
+    hlattrs.cterm_fg_color = fg == -1 ? 0 : fg + 1;
   }
 
   return hlattrs;

src/nvim/syntax.c

@@ -6758,16 +6758,26 @@ void set_hl_group(int id, HlAttrs attrs, Dict(highlight) *dict, int link_id)
     { NULL, -1, NIL },
   };
 
+  char hex_name[8];
+  char *name;
+
   for (int j = 0; cattrs[j].dest; j++) {
-    if (cattrs[j].val != -1) {
+    if (cattrs[j].val < 0) {
+      XFREE_CLEAR(*cattrs[j].dest);
+      continue;
+    }
+
+    if (cattrs[j].name.type == kObjectTypeString && cattrs[j].name.data.string.size) {
+      name = cattrs[j].name.data.string.data;
+    } else {
+      snprintf(hex_name, sizeof(hex_name), "#%06x", cattrs[j].val);
+      name = hex_name;
+    }
+
+    if (!*cattrs[j].dest
+        || STRCMP(*cattrs[j].dest, name) != 0) {
       xfree(*cattrs[j].dest);
-      if (cattrs[j].name.type == kObjectTypeString && cattrs[j].name.data.string.size) {
+      *cattrs[j].dest = xstrdup(name);
-        *cattrs[j].dest = xstrdup(cattrs[j].name.data.string.data);
-      } else {
-        char hex_name[8];
-        snprintf(hex_name, sizeof(hex_name), "#%06x", cattrs[j].val);
-        *cattrs[j].dest = xstrdup(hex_name);
-      }
     }
   }
 

test/functional/api/highlight_spec.lua

@@ -276,4 +276,26 @@ describe("API: set highlight", function()
     eq('Test_hl3       xxx guifg=bLue guibg=reD',
       exec_capture('highlight Test_hl3'))
   end)
+
+  it ("can modify a highlight in the global namespace", function()
+    meths.set_hl(0, 'Test_hl3', { bg = 'red', fg = 'blue'})
+    eq('Test_hl3       xxx guifg=blue guibg=red',
+      exec_capture('highlight Test_hl3'))
+
+    meths.set_hl(0, 'Test_hl3', { bg = 'red' })
+    eq('Test_hl3       xxx guibg=red',
+      exec_capture('highlight Test_hl3'))
+
+    meths.set_hl(0, 'Test_hl3', { ctermbg = 9, ctermfg = 12})
+    eq('Test_hl3       xxx ctermfg=12 ctermbg=9',
+      exec_capture('highlight Test_hl3'))
+
+    meths.set_hl(0, 'Test_hl3', { ctermbg = 9 })
+    eq('Test_hl3       xxx ctermbg=9',
+      exec_capture('highlight Test_hl3'))
+
+    meths.set_hl(0, 'Test_hl3', {})
+    eq('Test_hl3       xxx cleared',
+      exec_capture('highlight Test_hl3'))
+  end)
 end)