feat(batch): job dependencies (dependsOn) — gated launch (batch 4)

[vieiralucas]

Summary

AWS Batch batch 4 — job dependencies (dependsOn), the orchestration feature every rival is weak on (ministack waitForTaskToken no-ops #959).

• SubmitJob with dependsOn parks the job at PENDING and spawns a background waiter that launches it only once every dependency has SUCCEEDED; if any dependency FAILED the dependent job fails ("Dependent job failed"). The wait never blocks the SubmitJob call.
• Refactor: the launch path (launch / launch_ecs_task / spawn_status_sync) is now standalone free functions taking a LaunchCtx bundle, so the spawned waiter launches with no &self, exactly like the inline SubmitJob path. launch_job is a thin &self wrapper.

Test plan

• Unit (14): depends_on_parks_at_pending (no runtime → stays PENDING), plus existing.
• Docker-gated e2e: depends_on_job_waits_for_its_dependency — B dependsOn A: B is PENDING while A runs (3s), both SUCCEEDED after. The existing real-execution / array / fail e2e still pass against the refactored launch path (all 4 green locally).
• cargo clippy -p fakecloud-batch --bin fakecloud --all-targets -- -D warnings clean. Docs updated.

Next

Retry strategies + timeouts, then a CloudFormation AWS::Batch::* provisioner + tfacc.

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Summary by cubic

Adds Batch job dependencies via dependsOn. Dependent jobs park at PENDING and launch only after all parents succeed; they fail if any parent fails. The wait is non-blocking and works with real ECS-backed execution.

• New Features

  • SubmitJob with dependsOn sets job to PENDING and spawns a background waiter.
  • Launches when all dependencies are SUCCEEDED; fails with "Dependent job failed" if any is FAILED.
  • No blocking of the SubmitJob call; unchanged behavior when no runtime is present.
  • Tests: unit for PENDING gating; Docker e2e validates B waits for A. Docs updated.

• Refactors

  • Extracted launch flow into free functions: launch, launch_ecs_task, spawn_status_sync, driven by a LaunchCtx.
  • launch_job is now a thin wrapper. Enables launching from the dependency waiter without &self.

^{Written for commit 4819688. Summary will update on new commits.}