ircd:Ⓜ️:user: Add a query for user to user visibility.

include/ircd/m/user.h

@@ -160,9 +160,13 @@ struct ircd::m::user::mitsein
 	bool for_each(const closure_bool &) const;
 	void for_each(const closure &) const;
 
+	// Counting convenience
 	size_t count(const m::user &, const string_view &membership = {}) const;
 	size_t count(const string_view &membership = {}) const;
 
+	// Existential convenience (does `user` and `other` share any common room).
+	bool has(const m::user &other, const string_view &membership = {}) const;
+
 	mitsein(const m::user &user);
 };
 

ircd/m/m.cc

@@ -2711,6 +2711,20 @@ ircd::m::user::mitsein::mitsein(const m::user &user)
 {
 }
 
+bool
+ircd::m::user::mitsein::has(const m::user &other,
+                            const string_view &membership)
+const
+{
+	// Return true if broken out of loop.
+	return !for_each(other, membership, rooms::closure_bool{[]
+	(const m::room &, const string_view &)
+	{
+		// Break out of loop at first shared room
+		return false;
+	}});
+}
+
 size_t
 ircd::m::user::mitsein::count(const string_view &membership)
 const

modules/console.cc

@@ -8312,6 +8312,45 @@ console_cmd__user__events__count(opt &out, const string_view &line)
 	return true;
 }
 
+bool
+console_cmd__user__sees(opt &out, const string_view &line)
+{
+	const params param{line, " ",
+	{
+		"user_id_a", "user_id_b", "membership"
+	}};
+
+	const m::user user_a
+	{
+		m::user(param.at("user_id_a"))
+	};
+
+	const m::user user_b
+	{
+		m::user(param.at("user_id_b"))
+	};
+
+	const string_view membership
+	{
+		param.at("membership", "join"_sv)
+	};
+
+	const m::user::mitsein mitsein
+	{
+		user_a
+	};
+
+	const bool result
+	{
+		mitsein.has(user_b, membership)
+	};
+
+	out << std::boolalpha << result
+	    << std::endl;
+
+	return true;
+}
+
 //
 // users
 //