fix(api): make nil value in nvim_set_option_value clear local value (#16710)

For special options such as 'undolevels' and 'scrolloff', this sets the
local value to the special "unset" value (e.g. -12345 for 'undolevels').

src/nvim/api/vim.c

@@ -696,7 +696,17 @@ Object nvim_get_option_value(String name, Dict(option) *opts, Error *err)
     rv = INTEGER_OBJ(numval);
     break;
   case 2:
-    rv = BOOLEAN_OBJ(!!numval);
+    switch (numval) {
+      case 0:
+      case 1:
+        rv = BOOLEAN_OBJ(numval);
+        break;
+      default:
+        // Boolean options that return something other than 0 or 1 should return nil. Currently this
+        // only applies to 'autoread' which uses -1 as a local value to indicate "unset"
+        rv = NIL;
+        break;
+    }
     break;
   default:
     api_set_error(err, kErrorTypeValidation, "unknown option '%s'", name.data);
@@ -749,7 +759,7 @@ void nvim_set_option_value(String name, Object value, Dict(option) *opts, Error
     stringval = value.data.string.data;
     break;
   case kObjectTypeNil:
-    // Do nothing
+    scope |= OPT_CLEAR;
     break;
   default:
     api_set_error(err, kErrorTypeValidation, "invalid value for option");

src/nvim/option.c

@@ -5059,6 +5059,9 @@ int get_option_value_strict(char *name, int64_t *numval, char **stringval, int o
 /// @param[in]  number  New value for the number or boolean option.
 /// @param[in]  string  New value for string option.
 /// @param[in]  opt_flags  Flags: OPT_LOCAL, OPT_GLOBAL, or 0 (both).
+///                        If OPT_CLEAR is set, the value of the option
+///                        is cleared  (the exact semantics of this depend
+///                        on the option).
 ///
 /// @return NULL on success, error message on error.
 char *set_option_value(const char *const name, const long number, const char *const string,
@@ -5084,7 +5087,7 @@ char *set_option_value(const char *const name, const long number, const char *co
     }
     if (flags & P_STRING) {
       const char *s = string;
-      if (s == NULL) {
+      if (s == NULL || opt_flags & OPT_CLEAR) {
         s = "";
       }
       return set_string_option(opt_idx, s, opt_flags);
@@ -5106,10 +5109,23 @@ char *set_option_value(const char *const name, const long number, const char *co
             return NULL;  // do nothing as we hit an error
           }
         }
+        long numval = number;
+        if (opt_flags & OPT_CLEAR) {
+          if ((int *)varp == &curbuf->b_p_ar) {
+            numval = -1;
+          } else if ((long *)varp == &curbuf->b_p_ul) {
+            numval = NO_LOCAL_UNDOLEVEL;
+          } else if ((long *)varp == &curwin->w_p_so || (long *)varp == &curwin->w_p_siso) {
+            numval = -1;
+          } else {
+            char *s = NULL;
+            (void)get_option_value(name, &numval, (char_u **)&s, OPT_GLOBAL);
+          }
+        }
         if (flags & P_NUM) {
-          return set_num_option(opt_idx, varp, number, NULL, 0, opt_flags);
+          return set_num_option(opt_idx, varp, numval, NULL, 0, opt_flags);
         } else {
-          return set_bool_option(opt_idx, varp, (int)number, opt_flags);
+          return set_bool_option(opt_idx, varp, (int)numval, opt_flags);
         }
       }
     }

src/nvim/option.h

@@ -22,6 +22,7 @@ typedef enum {
   OPT_ONECOLUMN = 64,  ///< list options one per line
   OPT_NO_REDRAW = 128,  ///< ignore redraw flags on option
   OPT_SKIPRTP = 256,  ///< "skiprtp" in 'sessionoptions'
+  OPT_CLEAR = 512, ///< Clear local value of an option.
 } OptionFlags;
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS

test/functional/api/vim_spec.lua

@@ -974,6 +974,40 @@ describe('API', function()
       eq('hello', nvim('get_option_value', 'makeprg', {}))
       eq('', nvim('get_option_value', 'makeprg', {scope = 'local'}))
     end)
+
+    it('clears the local value of an option with nil', function()
+      -- Set global value
+      nvim('set_option_value', 'shiftwidth', 42, {})
+      eq(42, nvim('get_option_value', 'shiftwidth', {}))
+
+      -- Set local value
+      nvim('set_option_value', 'shiftwidth', 8, {scope = 'local'})
+      eq(8, nvim('get_option_value', 'shiftwidth', {}))
+      eq(8, nvim('get_option_value', 'shiftwidth', {scope = 'local'}))
+      eq(42, nvim('get_option_value', 'shiftwidth', {scope = 'global'}))
+
+      -- Clear value without scope
+      nvim('set_option_value', 'shiftwidth', NIL, {})
+      eq(42, nvim('get_option_value', 'shiftwidth', {}))
+      eq(42, nvim('get_option_value', 'shiftwidth', {scope = 'local'}))
+
+      -- Clear value with explicit scope
+      nvim('set_option_value', 'shiftwidth', 8, {scope = 'local'})
+      nvim('set_option_value', 'shiftwidth', NIL, {scope = 'local'})
+      eq(42, nvim('get_option_value', 'shiftwidth', {}))
+      eq(42, nvim('get_option_value', 'shiftwidth', {scope = 'local'}))
+
+      -- Now try with options with a special "local is unset" value (e.g. 'undolevels')
+      nvim('set_option_value', 'undolevels', 1000, {})
+      eq(1000, nvim('get_option_value', 'undolevels', {scope = 'local'}))
+      nvim('set_option_value', 'undolevels', NIL, {scope = 'local'})
+      eq(-123456, nvim('get_option_value', 'undolevels', {scope = 'local'}))
+
+      nvim('set_option_value', 'autoread', true, {})
+      eq(true, nvim('get_option_value', 'autoread', {scope = 'local'}))
+      nvim('set_option_value', 'autoread', NIL, {scope = 'local'})
+      eq(NIL, nvim('get_option_value', 'autoread', {scope = 'local'}))
+    end)
   end)
 
   describe('nvim_{get,set}_current_buf, nvim_list_bufs', function()