lightningd: don't simply ignore defaults on flags, deprecate.

Changelog-Deprecated: Plugins: `default` no longer accepted on `flag` type parameters (it was silently ignored, so just don't set it).
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

contrib/pyln-client/pyln/client/plugin.py

@@ -912,7 +912,7 @@ class Plugin(object):
                 m["long_description"] = method.long_desc
 
         manifest = {
-            'options': list(self.options.values()),
+            'options': list({k: v for k, v in d.items() if v is not None} for d in self.options.values()),
             'rpcmethods': methods,
             'subscriptions': list(self.subscriptions.keys()),
             'hooks': hooks,

doc/PLUGINS.md

@@ -207,11 +207,12 @@ There are currently four supported option 'types':
   - string: a string
   - bool: a boolean
   - int: parsed as a signed integer (64-bit)
-  - flag: no-arg flag option. Is boolean under the hood. Defaults to false.
+  - flag: no-arg flag option. Presented as `true` if config specifies it.
 
 In addition, string and int types can specify `"multi": true` to indicate
 they can be specified multiple times.  These will always be represented in
-`init` as a (possibly empty) JSON array.
+`init` as a (possibly empty) JSON array.  "multi" flag types do not make 
+sense.
 
 Nota bene: if a `flag` type option is not set, it will not appear
 in the options set that is passed to the plugin.
@@ -229,7 +230,6 @@ Here's an example option set, as sent in response to `getmanifest`
     {
       "name": "run-hot",
       "type": "flag",
-      "default": None,  // defaults to false
       "description": "If set, overclocks plugin"
     },
     {

lightningd/plugin.c

@@ -973,8 +973,13 @@ static const char *plugin_opt_add(struct plugin *plugin, const char *buffer,
 				       "%s type \"%s\" cannot have multi",
 				       popt->name, popt->type);
 		/* We default flags to false, the default token is ignored */
-		if (json_tok_streq(buffer, typetok, "flag"))
+		if (json_tok_streq(buffer, typetok, "flag") && defaulttok) {
+			if (!deprecated_apis) {
+				return tal_fmt(plugin, "%s type flag cannot have default",
+					       popt->name);
+			}
 			defaulttok = NULL;
+		}
 	} else {
 		return tal_fmt(plugin,
 			       "Only \"string\", \"int\", \"bool\", and \"flag\" options are supported");