Support returning empty arrays from Lua (#1906)

src/lua_interp/test/lua_test.cpp

@@ -301,6 +301,17 @@ TEST_CASE("json")
     REQUIRE(j.empty());
   }
 
+  SUBCASE("empty array")
+  {
+    // With some work, it is possible to build a table that will become an empty
+    // JSON array
+    constexpr auto code(
+      "t = {}; setmetatable(t, {__was_object = false}); return t");
+    const auto j = Interpreter().invoke<nlohmann::json>(code);
+    REQUIRE(j.type() == nlohmann::json::value_t::array);
+    REQUIRE(j.empty());
+  }
+
   SUBCASE("roundtrip empty object")
   {
     const auto j1 = nlohmann::json::object();

src/runtime_config/gov.lua

@@ -83,7 +83,15 @@ return {
     table.insert(self, {func=_func, args=_args})
     return self
   end
-  Calls =  setmetatable({}, {__index = __Calls})
+  Calls = setmetatable({}, {__index = __Calls})
+
+  function empty_list()
+    return setmetatable({}, {__was_object=false})
+  end
+
+  function empty_object()
+    return setmetatable({}, {__was_object=true})
+  end
   ]],
 
   -- scripts that can be proposed to be called

src/runtime_config/gov_ca_cert_mismatch.lua

@@ -106,7 +106,15 @@ return {
     table.insert(self, {func=_func, args=_args})
     return self
   end
-  Calls =  setmetatable({}, {__index = __Calls})
+  Calls = setmetatable({}, {__index = __Calls})
+
+  function empty_list()
+    return setmetatable({}, {__was_object=false})
+  end
+
+  function empty_object()
+    return setmetatable({}, {__was_object=true})
+  end
   ]],
 
   -- scripts that can be proposed to be called

src/runtime_config/gov_ca_certs.lua

@@ -104,7 +104,15 @@ return {
     table.insert(self, {func=_func, args=_args})
     return self
   end
-  Calls =  setmetatable({}, {__index = __Calls})
+  Calls = setmetatable({}, {__index = __Calls})
+
+  function empty_list()
+    return setmetatable({}, {__was_object=false})
+  end
+
+  function empty_object()
+    return setmetatable({}, {__was_object=true})
+  end
   ]],
 
   -- scripts that can be proposed to be called

src/runtime_config/gov_veto.lua

@@ -87,7 +87,15 @@ return {
     table.insert(self, {func=_func, args=_args})
     return self
   end
-  Calls =  setmetatable({}, {__index = __Calls})
+  Calls = setmetatable({}, {__index = __Calls})
+
+  function empty_list()
+    return setmetatable({}, {__was_object=false})
+  end
+
+  function empty_object()
+    return setmetatable({}, {__was_object=true})
+  end
   ]],
 
   -- scripts that can be proposed to be called

src/runtime_config/operator_gov.lua

@@ -152,7 +152,15 @@ return {
     table.insert(self, {func=_func, args=_args})
     return self
   end
-  Calls =  setmetatable({}, {__index = __Calls})
+  Calls = setmetatable({}, {__index = __Calls})
+
+  function empty_list()
+    return setmetatable({}, {__was_object=false})
+  end
+
+  function empty_object()
+    return setmetatable({}, {__was_object=true})
+  end
   ]],
 
   -- scripts that can be proposed to be called

tests/sandbox/sandbox_gov.lua

@@ -46,7 +46,15 @@ return {
     table.insert(self, {func=_func, args=_args})
     return self
   end
-  Calls =  setmetatable({}, {__index = __Calls})
+  Calls = setmetatable({}, {__index = __Calls})
+
+  function empty_list()
+    return setmetatable({}, {__was_object=false})
+  end
+
+  function empty_object()
+    return setmetatable({}, {__was_object=true})
+  end
   ]],
 
   -- scripts that can be proposed to be called