feat(runners): support OnDemandOptions.AllocationStrategy in EC2 Fleet

[piotrj]

Summary

Currently, createInstances always sets SpotOptions.AllocationStrategy regardless of the fleet's targetCapacityType. For on-demand fleets, AWS silently ignores SpotOptions and defaults to lowest-price, making the instance_allocation_strategy variable and instance_types ordering meaningless for on-demand users who want prioritized.

This PR fixes that by conditionally setting SpotOptions or OnDemandOptions based on targetCapacityType, so the allocation strategy is correctly applied for both spot and on-demand fleets.

Why instance_type_priorities?

The priority-aware allocation strategies — prioritized (on-demand) and capacity-optimized-prioritized (spot) — don't simply use the array order of launch template overrides. They rely on an explicit Priority field on each override entry. Without setting Priority, all overrides have equal (lowest) priority and these strategies effectively ignore the intended instance-type ordering.

We introduced the optional instance_type_priorities variable (map(number), e.g. { "m5.large" = 1, "c5.large" = 2 }) as a non-breaking addition to give users explicit control over the priority assigned to each instance type. When not provided, priorities automatically fall back to the index position in instance_types, so existing configurations continue to work unchanged — the first instance type in the list gets the highest priority (0), the second gets 1, and so on. This makes the priority-aware strategies work correctly out of the box while still allowing fine-grained control when needed.

Note: The Priority field is only set on fleet overrides when instance_allocation_strategy is "prioritized" (on-demand) or "capacity-optimized-prioritized" (spot). For all other allocation strategies, overrides remain unchanged — no Priority is added. This ensures that existing setups using strategies like capacity-optimized or lowest-price are completely unaffected by this change.

What changed

Conditional SpotOptions / OnDemandOptions

• The createInstances function now checks targetCapacityType and sets SpotOptions (with MaxTotalPrice and AllocationStrategy) for spot fleets, or OnDemandOptions (with AllocationStrategy) for on-demand fleets.
• "prioritized" is added to the instance_allocation_strategy validation list in all Terraform variable definitions.

Allocation strategy sanitization

• The instance_allocation_strategy variable accepts the union of spot and on-demand strategies, so a value valid for one capacity type can be invalid for the other. Before building the CreateFleet request, the strategy is now sanitized against the target capacity type and falls back to lowest-price (the AWS default) when invalid — preventing invalid values from reaching SpotOptions/OnDemandOptions and preserving backwards compatibility for on-demand users whose spot-only strategy was previously ignored.
• The same sanitization is reused when falling back from spot to on-demand.

Override priorities

• generateFleetOverrides sets Priority on each override for the priority-aware strategies — prioritized (on-demand) and capacity-optimized-prioritized (spot). It uses instance_type_priorities if provided, or the index in instance_types otherwise. All other strategies leave overrides unchanged.
• The new instance_type_priorities variable is threaded through the full chain: root Terraform variables → runners module → Lambda environment variables (INSTANCE_TYPE_PRIORITIES as JSON) → ec2instanceCriteria → generateFleetOverrides.

[piotrj]

Rebased the PR to resolve conflicts.
For what it's worth, we've been running this implementation in our organization without issues.

[Brend-Smits]

Hey @piotrj

This looks like a great change! I apologize for the delay in getting this reviewed/merged. We're not cleaning up the backlog of PR's and will make sure to get faster review cycles in.

Could you please make sure the conflicts are resolved in your branch? I will review afterwards!
Thank you for taking the time to contribute this. Great work!

[Copilot]

Pull request overview

This PR updates the runner scale-up implementation to correctly apply EC2 Fleet allocation strategy settings for both Spot and On-Demand fleets, and introduces optional per–instance-type priority controls to make the prioritized strategy behave as intended.

Changes:

• Conditionally set SpotOptions vs OnDemandOptions in the EC2 CreateFleet request based on targetCapacityType.
• Add and plumb through instance_type_priorities (Terraform → Lambda env var → control-plane criteria → fleet overrides).
• Extend control-plane tests to cover on-demand allocation strategies and override priorities.

Reviewed changes

Copilot reviewed 13 out of 14 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
variables.tf	Adds prioritized to allocation strategy validation and introduces instance_type_priorities at root.
main.tf	Wires instance_type_priorities into the runners module.
modules/runners/variables.tf	Adds module-level instance_type_priorities and expands allocation strategy validation.
modules/runners/scale-up.tf	Exposes INSTANCE_TYPE_PRIORITIES env var to the scale-up Lambda.
modules/runners/pool.tf	Threads instance_type_priorities into the pool submodule config.
modules/runners/pool/variables.tf	Extends pool config schema to include optional instance_type_priorities.
modules/runners/pool/main.tf	Exposes INSTANCE_TYPE_PRIORITIES env var to the pool Lambda.
modules/multi-runner/variables.tf	Adds instance_type_priorities option to multi-runner configuration and documents it.
modules/multi-runner/runners.tf	Passes instance_type_priorities from multi-runner config into the runners module.
lambdas/functions/control-plane/src/scale-runners/scale-up.ts	Parses INSTANCE_TYPE_PRIORITIES and forwards it into runner creation criteria.
lambdas/functions/control-plane/src/pool/pool.ts	Parses INSTANCE_TYPE_PRIORITIES and forwards it into runner creation criteria.
lambdas/functions/control-plane/src/aws/runners.ts	Implements override priority injection and conditional Spot vs On-Demand fleet options.
lambdas/functions/control-plane/src/aws/runners.test.ts	Adds/updates tests for on-demand strategies and per-type priority handling.
lambdas/functions/control-plane/src/aws/runners.d.ts	Updates runner input typings to allow on-demand allocation strategies and priorities.

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[piotrj]

@Brend-Smits rebased the PR so the conflicts are gone.

I let the ec2OverrideConfig from #5003 be the last item in the creation of CreateFleetCommand so that in case there is Priority set there then it will be the one that stands. But I still think that priority setting through dynamic labels is not very useful.

Also, I expanded the priorities to spot's capacity-optimized-prioritized allocation strategy. Before, even if someone used that allocation strategy it wouldn't really work cause there were no priorities set.