4 files changed, 60 insertions, 3 deletions

diff --git a/Server/Plugins/Debuggers/Debuggers.lua b/Server/Plugins/Debuggers/Debuggers.lua
index 6c47a8a17..0965a17c2 100644
--- a/Server/Plugins/Debuggers/Debuggers.lua
+++ b/Server/Plugins/Debuggers/Debuggers.lua
@@ -2279,6 +2279,18 @@ end
 
 
 
+function HandleConsoleTestErr(a_Split, a_EntireCmd)
+	cRoot:Get():GetDefaultWorld():ForEachEntity(
+		function (a_CBEntity)
+			error("This error should not abort the server")
+		end
+	)
+end
+
+
+
+
+
 function HandleConsoleTestJson(a_Split, a_EntireCmd)
 	LOG("Testing Json parsing...")
 	local t1 = cJson:Parse([[{"a": 1, "b": "2", "c": [3, "4", 5], "d": true }]])
diff --git a/Server/Plugins/Debuggers/Info.lua b/Server/Plugins/Debuggers/Info.lua
index e3b0beee1..b9eb0eeda 100644
--- a/Server/Plugins/Debuggers/Info.lua
+++ b/Server/Plugins/Debuggers/Info.lua
@@ -350,6 +350,12 @@ g_PluginInfo =
 			HelpString = "Tests inter-plugin calls with various values"
 		},
 
+		["testerr"] =
+		{
+			Handler = HandleConsoleTestErr,
+			HelpString = "Tests the server's ability to recover from errors in callbacks (GH #3733)",
+		},
+
 		["testjson"] =
 		{
 			Handler = HandleConsoleTestJson,
diff --git a/src/Bindings/LuaState.cpp b/src/Bindings/LuaState.cpp
index ca0a258d9..947e337fc 100644
--- a/src/Bindings/LuaState.cpp
+++ b/src/Bindings/LuaState.cpp
@@ -1454,8 +1454,12 @@ bool cLuaState::CallFunction(int a_NumResults)
 		LOGWARNING("Error in %s calling function %s()", m_SubsystemName.c_str(), CurrentFunctionName.c_str());
 
 		// Remove the error handler and error message from the stack:
-		ASSERT(lua_gettop(m_LuaState) == 2);
-		lua_pop(m_LuaState, 2);
+		auto top = lua_gettop(m_LuaState);
+		if (top < 2)
+		{
+			LogStackValues(Printf("The Lua stack is in an unexpected state, expected at least two values there, but got %d", top).c_str());
+		}
+		lua_pop(m_LuaState, std::min(2, top));
 		return false;
 	}
 
diff --git a/src/Bindings/LuaState.h b/src/Bindings/LuaState.h
index ac911557d..9cbfbf69d 100644
--- a/src/Bindings/LuaState.h
+++ b/src/Bindings/LuaState.h
@@ -105,6 +105,41 @@ public:
 		#define ASSERT_LUA_STACK_BALANCE(...)
 	#endif
 
+
+	/** Makes sure that the Lua state's stack has the same number of elements on destruction as it had on construction of this object (RAII).
+	Can only remove elements, if there are less than expected, throws. */
+	class cStackBalancePopper
+	{
+	public:
+		cStackBalancePopper(cLuaState & a_LuaState):
+			m_LuaState(a_LuaState),
+			m_Count(lua_gettop(a_LuaState))
+		{
+		}
+
+		~cStackBalancePopper()
+		{
+			auto curTop = lua_gettop(m_LuaState);
+			if (curTop > m_Count)
+			{
+				// There are some leftover elements, adjust the stack:
+				m_LuaState.LogStackValues(Printf("Re-balancing Lua stack, expected %d values, got %d:", m_Count, curTop).c_str());
+				lua_pop(m_LuaState, curTop - m_Count);
+			}
+			else if (curTop < m_Count)
+			{
+				// This is an irrecoverable error, rather than letting the Lua engine crash undefinedly later on, abort now:
+				LOGERROR("Unable to re-balance Lua stack, there are elements missing. Expected at least %d elements, got %d.", m_Count, curTop);
+				throw std::runtime_error(Printf("Unable to re-balance Lua stack, there are elements missing. Expected at least %d elements, got %d.", m_Count, curTop));
+			}
+		}
+
+	protected:
+		cLuaState & m_LuaState;
+		int m_Count;
+	};
+
+
 	/** Provides a RAII-style locking for the LuaState.
 	Used mainly by the cPluginLua internals to provide the actual locking for interface operations, such as callbacks. */
 	class cLock
@@ -704,7 +739,7 @@ public:
 	template <typename FnT, typename... Args>
 	bool Call(const FnT & a_Function, Args &&... args)
 	{
-		ASSERT_LUA_STACK_BALANCE(m_LuaState);
+		cStackBalancePopper balancer(*this);
 		m_NumCurrentFunctionArgs = -1;
 		if (!PushFunction(std::forward<const FnT &>(a_Function)))
 		{