add guard against OnDefeat failures for AI by relent0r · Pull Request #7256 · FAForever/fa

relent0r · 2026-08-28T01:40:12Z

Description of the proposed changes

There are certain niche scenarios when an AIs platoon will not have cplatoon functions available. After analyzing a replay it was found that when an AI ACU was destroyed units around it that were destroyed in the blast had a chance of being part way through the destroy state when the ondefeat function is called.

Testing done on the proposed changes

Tested based on a replay with the issue present. The following log watch generated.

warning: Error running lua script: ...forever\replaydata\gamedata\lua.nx2\lua\simutils.lua(1611): attempt to call method `Stop' (a nil value)
         stack traceback:
             ...forever\replaydata\gamedata\lua.nx2\lua\simutils.lua(1611): in function <...forever\replaydata\gamedata\lua.nx2\lua\simutils.lua:1598>
             ...aforever\replaydata\gamedata\lua.nx2\lua\aibrain.lua(445): in function `SetDefeatStatus'
             ...aforever\replaydata\gamedata\lua.nx2\lua\aibrain.lua(465): in function `OnDefeat'
             ...ua\sim\victorycondition\abstractvictorycondition.lua(406): in function `DefeatForArmy'
             ...a\lua.nx2\lua\sim\victorycondition\unitcondition.lua(62): in function `EvaluateVictoryCondition'
             ...ua\sim\victorycondition\abstractvictorycondition.lua(212): in function <...ua\sim\victorycondition\abstractvictorycondition.lua:210>

After adding guards the issue no longer happened and the ondefeat function could complete successfully.

Checklist

Changes are annotated, including comments where useful
Changes are documented in a changelog snippet according to the guidelines.
Request 2-3 reviewers from the list of reviewers and their areas of knowledge.

Summary by CodeRabbit

Bug Fixes
- Improved AI disable handling during defeat events by safely checking platoon functions before calling them.
- Prevented errors when optional platoon commands are unavailable.
- Corrected platoon disband handling so the command runs when supported.
- Improved stability when AI-controlled armies are defeated.

…g platoon death

coderabbitai · 2026-08-28T01:41:43Z

No actionable comments were generated in the recent review. 🎉

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changelog/snippets/ai.7256.md

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changelog/snippets/ai.7256.md

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📝 Walkthrough

Walkthrough

DisableAI now checks for platoon cleanup methods before calling them. The disband guard uses the correct method name. A changelog snippet records the fix.

Changes

Platoon cleanup

Layer / File(s)	Summary
Guard cleanup method calls `lua/SimUtils.lua`, `changelog/snippets/ai.7256.md`	`DisableAI` conditionally calls `Stop()` and `PlatoonDisbandNoAssign()` when the corresponding handle fields exist. The changelog records the fix.

Estimated code review effort: 2 (Simple) | ~5 minutes

Merge Risk: ⚪ Minimal · up to d689d

This localized change guards AI defeat handling against a known nil-method failure and is merge-ready after normal checks and review; no actionable merge-blocking risk remains.

Suggested reviewers: speed2cz, blackyps, 4z0t

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the main change: adding guards to prevent AI OnDefeat failures. It is concise and related to the changeset.
Description check	✅ Passed	The description explains the failure scenario, documents replay-based testing, includes the required checklist, and confirms that the error no longer occurs. The optional Additional context section is…
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 1 files. (1 skipped: 1 …
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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The description explains the failure scenario, documents replay-based testing, includes the required checklist, and confirms that the error no longer occurs. The optional Additional context section is omitted, but the description remains sufficiently complete.

Full details: Docstring Coverage

Explanation

Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 1 files. (1 skipped: 1 unsupported.)

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Actionable comments posted: 1

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Inline comments:
In `@lua/SimUtils.lua`:
- Around line 1443-1445: Update the guard in the DisableAI platoon-disband block
to check handle.PlatoonDisbandNoAssign, matching the method invoked by
handle:PlatoonDisbandNoAssign().

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changelog/snippets/ai.7256.md
lua/SimUtils.lua

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lL1l1

Technically this works but it doesn't seem like it should be fixed this way.

lL1l1 · 2026-08-28T10:56:36Z

+                    if handle.Stop then
+                        handle:Stop()
+                    end
+                    if handle.PlatoonDisbandNoAssign then
+                        handle:PlatoonDisbandNoAssign()
+                    end


I read a bit of the platoon code, and it seems like there's some manual setmetatable use that could be leaving platoons with an incomplete/wrong platoon class?
For example, AIPlatoonAdaptiveSilo has a typo using AIPlatoonAdaptiveSiloBehavior instead of AIPlatoonAdaptiveSilo, but it's not being used anywhere so nobody caught the issue.

fa/lua/aibrains/platoons/platoon-adaptive-silo.lua

Line 249 in 4179780

setmetatable(platoon, AIPlatoonAdaptiveSiloBehavior)

Could you post the replay and maybe investigate the metatable of the platoon that fails at being destroyed (for example if it is a mod using the adaptive silo)? These functions should never be missing because they're part of the base class. Even if the platoon is from a mod.

This was based off a replay for my own AI.
https://replay.faforever.com/27584457

I don't disagree that it may have something to do with the state machine platoon implementation. The platoons in question were otherwise functioning fine.

I thought this would be the safest change to make that would be low risk of unforeseen consequences.

I did a dump of the platoon table when troubleshooting as I thought maybe I'd accidentally set it to an empty table, but the resulting dump looked the same as any other platoon and had the c object attached so I was thinking that the platoon was mid was through its own destruction.

In my testing I only had the issue happen once in say 10 games but a player pointed out that it happened alot more often for them with the difference being that they had a much higher player count.

If you have suggestions of I could better troubleshoot it then I'm happy to try find the core reason.

I did some metatable trickery in this branch to track down tracebacks of what changes were done to the platoon that failed:
https://github.com/FAForever/fa/tree/investigate-ai-ondefeat-failures

It's useful so I'll make a PR for it on a more organized branch.

I have included the output log in the branch files. What stands out to me is this final assignment of the failing platoon's metatable to nil, which would cause it to not have the necessary commands.

"[283.745270] Assign \"<metatable>\" as nil from stack traceback: ...\lua\globalinit.lua:18> ...\lua\system\class.lua(802): in function `ChangeState' ...\mods\rngai\lua\ai\statemachines\platoon-base-rng.lua(284): in function `ChangeStateExt' ...\mods\rngai\lua\ai\statemachineutilities.lua(1777): in function `cb' ...\lua\sim\unit.lua(4177): in function `DoUnitCallbacks' ...\lua\sim\unit.lua(4321): in function `DoOnFailedToBuildCallbacks' ...\lua\sim\unit.lua:2936> ...\lua\sim\units\constructionunit.lua:117> [C]: in function `IssueClearCommands' ...\mods\rngai\lua\ai\statemachines\platoon-engineer-resource.lua(726): in function <...\mods\rngai\lua\ai\statemachines\platoon-engineer-resource.lua:416>"

After investigating RNGAI, I saw:

unit.PlatoonHandle:ChangeStateExt(unit.PlatoonHandle.PerformBuildTask)

Where PerformBuildTask is a State.
But that State only exists in lua\AI\StateMachines\platoon-engineer-utility.lua AIPlatoonEngineerBehavior
Our engineer is clearly using lua\AI\StateMachines\platoon-engineer-resource.lua AIPlatoonEngineerBehavior,
Despite having the same name, that file's class does not have PerformBuildTask and does not inherit from the utility engineer class.
Therefore we change the unit to a nil State, which loses the Stop and PlatoonDisbandNoAssign methods.
ChangeState has no nil check, so your nil does go directly into the metatable:

fa/lua/system/class.lua

Lines 784 to 818 in 653be9b

--- Switches up the sate of a class instance by inserting the new state between the instance and its class

---@param instance table The current instance we want to switch states for

---@param newState State the state we want to insert between the instance and its base class

function ChangeState(instance, newState)

-- call on-exit function

if instance.OnExitState then

instance:OnExitState()

end

-- keep track of the original thread and forget about it inside the object

local old_main_thread = instance.__mainthread

instance.__mainthread = nil

-- change the state accordingly by switching up the meta tables:

-- - entity

-- - state <-- introduced as an intermediate, prevents a lot of duplicated values and tables

-- - class

setmetatable(instance, newState)

-- call on-enter function

if instance.OnEnterState then

instance:OnEnterState()

end

-- start the new main thread if it wasn't already created during an OnEnterState

if instance.Main and not instance.__mainthread then

instance.__mainthread = ForkThread(instance.Main, instance)

end

-- remove the old main thread, threads are de-allocated when they've completed their computation chain

if old_main_thread then

old_main_thread:Destroy()

end

end

Co-authored-by: lL1l1 <82986251+lL1l1@users.noreply.github.com>

relent0r · 2026-09-09T03:36:58Z

Thanks @lL1l1 I hadn't considered that getting one state machine to perform an adhoc build command would then fire a callback thats designed for a different state machine. Makes sense. In that case I guess I can fix this within my AI rather than in this PR.

I guess to avoid this for myself and other people that use the platoon states performing a nil check would also be useful in the base class. Then I could at least make it warn.

relent0r added 2 commits August 28, 2026 13:33

add guard against cases of platoon functions not being availble durin…

5ad2ab6

…g platoon death

add snippet

bb66e1c

relent0r requested review from 4z0t and BlackYps August 28, 2026 01:42

fix snippet typo as I can't spell

b1ad7c0

coderabbitai Bot reviewed Aug 28, 2026

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Comment thread lua/SimUtils.lua Outdated

typo that the bot caught (good bot)

6e3601d

BlackYps requested a review from speed2CZ August 28, 2026 09:42

lL1l1 reviewed Aug 28, 2026

View reviewed changes

Update changelog/snippets/ai.7256.md

d689d10

Co-authored-by: lL1l1 <82986251+lL1l1@users.noreply.github.com>

This was referenced Sep 2, 2026

Update replay debugging documentation #7274

Draft

Debug utility to track operations on a specific table #7275

Draft

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add guard against OnDefeat failures for AI - #7256

add guard against OnDefeat failures for AI#7256
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	--- Switches up the sate of a class instance by inserting the new state between the instance and its class
	---@param instance table The current instance we want to switch states for
	---@param newState State the state we want to insert between the instance and its base class
	function ChangeState(instance, newState)

	-- call on-exit function
	if instance.OnExitState then
	instance:OnExitState()
	end

	-- keep track of the original thread and forget about it inside the object
	local old_main_thread = instance.__mainthread
	instance.__mainthread = nil

	-- change the state accordingly by switching up the meta tables:
	-- - entity
	-- - state <-- introduced as an intermediate, prevents a lot of duplicated values and tables
	-- - class
	setmetatable(instance, newState)

	-- call on-enter function
	if instance.OnEnterState then
	instance:OnEnterState()
	end

	-- start the new main thread if it wasn't already created during an OnEnterState
	if instance.Main and not instance.__mainthread then
	instance.__mainthread = ForkThread(instance.Main, instance)
	end

	-- remove the old main thread, threads are de-allocated when they've completed their computation chain
	if old_main_thread then
	old_main_thread:Destroy()
	end
	end

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Description of the proposed changes

Testing done on the proposed changes

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Summary by CodeRabbit

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