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Ensure sufficient stack size for Lua values #4482

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merged 4 commits into from Jul 19, 2022
Merged

Ensure sufficient stack size for Lua values #4482

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083187f
Ensure sufficient stack size
BMagnu Jul 9, 2022
1145217
Fix Warning
BMagnu Jul 10, 2022
234297c
Fix formatting
BMagnu Jul 18, 2022
8e49198
Rename variable and add docs
BMagnu Jul 19, 2022
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7 changes: 5 additions & 2 deletions code/scripting/lua/LuaFunction.cpp
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Expand Up @@ -172,12 +172,15 @@ LuaValueList LuaFunction::call(lua_State* L, const LuaValueList& args) const {
stackTop = lua_gettop(L);
}

if(!lua_checkstack(L, (int)args.size() + 1))
throw LuaException("Lua Stack Overflow!");

// Push the function onto the stack
this->pushValue(L);
this->pushValue(L, true);

// Push the arguments onto the stack
for (const auto& arg : args) {
arg.pushValue(L);
arg.pushValue(L, true);
}

// actually call the function now!
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5 changes: 4 additions & 1 deletion code/scripting/lua/LuaReference.cpp
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Expand Up @@ -80,10 +80,13 @@ bool UniqueLuaReference::isValid() const {
return true;
}

void UniqueLuaReference::pushValue(lua_State* thread) const
void UniqueLuaReference::pushValue(lua_State* thread, bool guaranteeStack) const
{
Assertion(thread != nullptr, "Valid thread state must be specified!");

if(!guaranteeStack)
lua_checkstack(thread, 1);
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if (this->isValid()) {
lua_rawgeti(thread, LUA_REGISTRYINDEX, this->getReference());
}
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2 changes: 1 addition & 1 deletion code/scripting/lua/LuaReference.h
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Expand Up @@ -101,7 +101,7 @@ class UniqueLuaReference {
* @brief Pushes the referenced value onto the stack.
* @param thread A specific thread state to push the value to. nullptr for the default state of this reference
*/
void pushValue(lua_State* thread) const;
void pushValue(lua_State* thread, bool guaranteeStack = false) const;
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Adding a "random" boolean variable to this call would, in my opinion, make this API confusing to use. Furthermore, since you do the relevant call before the Lua function anyway, I do not think adding this parameter (and also to LuaValue) is necessary.

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@BMagnu BMagnu Jul 18, 2022 •
edited

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While I make this call in this instance, it is necessary to do this for every case this function is called. Which is from quite a lot of places. You have to compromise at some point, either by asking the coder to always manually ensure the stack size before each call of pushValue (both for this and value), or eat the performance penalty of doing the allocation / check more than once if you push many values, or have the option to pass this as the API parameter.
Since I would really not want to have to rely on the programmer to always remember manually ensuring the stack size, it's basically a question of "do we want to waste performance in a low level function" vs "do we want one more parameter in the low level function".
IMO, with this case we should pick performance over pretty API.

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Ok, I can understand that reasoning.
One thing I noticed though is that the current name suggests that a true value will trigger the lua_checkstack call but that is not the case. Maybe a parameter name like checkStack would be more appropriate here.

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ok, clarified variable name and added documentation

};
}

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4 changes: 2 additions & 2 deletions code/scripting/lua/LuaValue.cpp
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Expand Up @@ -88,10 +88,10 @@ bool LuaValue::isValid() const {
return _reference && _reference->isValid();
}

bool LuaValue::pushValue(lua_State* thread) const
bool LuaValue::pushValue(lua_State* thread, bool guaranteeStack) const
{
if (this->_reference->isValid()) {
this->_reference->pushValue(thread);
this->_reference->pushValue(thread, guaranteeStack);
return true;
} else {
return false;
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2 changes: 1 addition & 1 deletion code/scripting/lua/LuaValue.h
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Expand Up @@ -186,7 +186,7 @@ class LuaValue {
* @param thread The thread stack onto which this value should be pushed. May be nullptr for the default state of
* this value
*/
bool pushValue(lua_State* thread) const;
bool pushValue(lua_State* thread, bool guaranteeStack = false) const;

lua_State* getLuaState() const;

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