Add `IUserDataApi.Keys`/`userdata.get_keys`

src/BizHawk.Client.Common/Api/Classes/UserDataApi.cs

@@ -1,4 +1,6 @@
 using System;
+using System.Collections.Generic;
+using System.Linq;
 
 namespace BizHawk.Client.Common
 {
@@ -6,6 +8,14 @@ namespace BizHawk.Client.Common
 	{
 		private readonly IMovieSession _movieSession;
 
+#if NET6_0
+		public IReadOnlySet<string> Keys
+			=> throw new NotImplementedException();
+#else
+		public IReadOnlyCollection<string> Keys
+			=> _movieSession.UserBag.Keys.ToList();
+#endif
+
 		public UserDataApi(IMovieSession movieSession) => _movieSession = movieSession;
 
 		/// <exception cref="InvalidOperationException">type of <paramref name="value"/> cannot be used in userdata</exception>

src/BizHawk.Client.Common/Api/Interfaces/IUserDataApi.cs

@@ -1,7 +1,15 @@
-namespace BizHawk.Client.Common
+using System.Collections.Generic;
+
+namespace BizHawk.Client.Common
 {
 	public interface IUserDataApi : IExternalApi
 	{
+#if NET6_0
+		IReadOnlySet<string> Keys { get; }
+#else
+		IReadOnlyCollection<string> Keys { get; }
+#endif
+
 		void Set(string name, object value);
 		object Get(string key);
 		void Clear();

src/BizHawk.Client.Common/lua/CommonLibs/UserDataLuaLibrary.cs

@@ -1,6 +1,9 @@
 using System;
+using System.Collections.Generic;
 using System.ComponentModel;
 
+using NLua;
+
 // ReSharper disable UnusedMember.Global
 namespace BizHawk.Client.Common
 {
@@ -37,5 +40,11 @@ namespace BizHawk.Client.Common
 		[LuaMethod("containskey", "returns whether or not there is an entry for the given key")]
 		public bool ContainsKey([LuaArbitraryStringParam] string key)
 			=> APIs.UserData.ContainsKey(key);
+
+		[LuaMethodExample("console.writeline(#userdata.get_keys());")]
+		[LuaMethod("get_keys", "returns a list-like table of valid keys")]
+		[return: LuaArbitraryStringParam]
+		public LuaTable GetKeys()
+			=> _th.ListToTable((List<string>) APIs.UserData.Keys); //HACK cast will succeed as long as impl. returns Dictionary<K, V>.Keys.ToList() as IROC<K>
 	}
 }