tests: test upvalue debug functions

src/tests.zig

@@ -1054,16 +1054,59 @@ test "debug interface" {
     try lua.protectedCall(1, 1, 0);
 }
 
+test "debug upvalues" {
+    var lua = try Lua.init(testing.allocator);
+    defer lua.deinit();
+
+    try lua.doString(
+        \\f = function(x)
+        \\  return function(y)
+        \\    return x + y
+        \\  end
+        \\end
+        \\addone = f(1)
+    );
+    _ = lua.getGlobal("addone");
+
+    // index doesn't exist
+    try expectError(Error.Fail, lua.getUpvalue(1, 2));
+
+    // inspect the upvalue (should be x)
+    try expectEqualStrings("x", try lua.getUpvalue(-1, 1));
+    try expectEqual(@as(Number, 1), try lua.toNumber(-1));
+    lua.pop(1);
+
+    // now make the function an "add five" function
+    lua.pushNumber(5);
+    _ = try lua.setUpvalue(-2, 1);
+
+    // test a bad index (the valid one's result is unpredicable)
+    try expectError(Error.Fail, lua.upvalueId(-1, 2));
+
+    // call the new function (should return 7)
+    lua.pushNumber(2);
+    try lua.protectedCall(1, 1, 0);
+    try expectEqual(@as(Number, 7), try lua.toNumber(-1));
+    lua.pop(1);
+
+    try lua.doString(
+        \\addthree = f(3)
+    );
+
+    _ = lua.getGlobal("addone");
+    _ = lua.getGlobal("addthree");
+
+    // now addone and addthree share the same upvalue
+    lua.upvalueJoin(-2, 1, -1, 1);
+    try expect((try lua.upvalueId(-2, 1)) == try lua.upvalueId(-1, 1));
+}
+
 test "refs" {
     // temporary test that includes a reference to all functions so
     // they will be type-checked
 
     // debug
     _ = Lua.getStack;
-    _ = Lua.getUpvalue;
-    _ = Lua.setUpvalue;
-    _ = Lua.upvalueId;
-    _ = Lua.upvalueJoin;
 
     // auxlib
     _ = Lua.argCheck;

src/ziglua.zig

@@ -1229,8 +1229,9 @@ pub const Lua = struct {
     }
 
     /// Returns a unique identifier for the upvalue numbered `n` from the closure index `func_index`
-    pub fn upvalueId(lua: *Lua, func_index: i32, n: i32) ?*anyopaque {
-        return c.lua_upvalueid(lua.state, func_index, n);
+    pub fn upvalueId(lua: *Lua, func_index: i32, n: i32) !*anyopaque {
+        if (c.lua_upvalueid(lua.state, func_index, n)) |ptr| return ptr;
+        return Error.Fail;
     }
 
     /// Make the `n1`th upvalue of the Lua closure at index `func_index1` refer to the `n2`th upvalue