fix: make IAM role optional for AWS Batch job submission

[odncode]

Summary

Fixes #3208 — @batch steps fail with "No IAM role specified" on EC2 compute environments that use instance profiles.

Problem

BatchJob.execute() raises unconditionally when iam_role is None (L82-85 in batch_client.py). Additionally, _register_job_definition passes "jobRoleArn": None to boto3, which rejects None for string fields.
The AWS Batch ContainerProperties API documents jobRoleArn as optional. EC2 compute environments can use instance profiles for container credentials instead.

Changes Made

• Removed the unconditional IAM role check in BatchJob.execute()
• Made jobRoleArn conditional in _register_job_definition — only included when job_role is not None

Testing

Added test/unit/test_batch_optional_iam_role.py with three tests:

• test_execute_does_not_raise_when_iam_role_is_none — verifies the fix
• test_execute_still_requires_image — ensures docker image check remains
• test_job_definition_omits_job_role_arn_when_none — verifies jobRoleArn is conditional

Verified: tests PASS with fix, FAIL without it.

Closes #3208

[greptile-apps]

Greptile Summary

Makes iam_role optional for AWS Batch job submission by removing the unconditional BatchJobException guard in execute() and conditionally including jobRoleArn in the job-definition payload only when a role is provided. This aligns with the AWS Batch API, which documents jobRoleArn as optional for EC2 compute environments that rely on instance profiles.

• batch_client.py: Drops the four-line _iam_role is None guard from execute() and replaces the unconditional "jobRoleArn": job_role assignment with **({"jobRoleArn": job_role} if job_role else {}) so None is never forwarded to boto3.
• test/unit/test_batch_optional_iam_role.py: Adds three unit tests covering the removed guard, the still-required image check, and the conditional jobRoleArn inclusion.

Confidence Score: 5/5

Safe to merge — the production change is a small, targeted removal of a guard that was incorrectly mandatory, with no other code paths affected.

Both changed lines in batch_client.py are self-contained: removing the four-line IAM role check in execute() has no downstream side effects, and the conditional dict-unpacking in _register_job_definition correctly omits jobRoleArn from the boto3 payload when it is None or empty. job_role is not used anywhere else in the function. The Fargate path retains its own mandatory execution_role check, so EC2-specific relaxation does not weaken Fargate validation.

No files require special attention. The test file quality issues were flagged in prior review threads.

Important Files Changed

Filename	Overview
metaflow/plugins/aws/batch/batch_client.py	Removes the unconditional IAM role guard in execute() and makes jobRoleArn conditional in _register_job_definition using dict-unpacking; both changes are correct and minimal.
test/unit/test_batch_optional_iam_role.py	Adds three regression tests; the source-text inspection test and the try/except negative-assertion pattern are both noted in prior review threads.

_{Reviews (3): Last reviewed commit: "style: fix pre-commit formatting (black,..." | Re-trigger Greptile}

[greptile-apps]

test_job_definition_omits_job_role_arn_when_none validates behavior by parsing source text rather than exercising the code. The assert "if job_role" in source_text assertion is brittle: a semantically equivalent rewrite such as if job_role is not None or if bool(job_role) would make this test fail even though the behavior is correct — and a stray if job_role: elsewhere in the file would keep it green even if the fix were reverted. Consider mocking boto3 (or the Batch client) and directly calling _register_job_definition() with job_role=None, then asserting that "jobRoleArn" is absent from the payload passed to register_job_definition.

[LuisJG8]

Hi odncode, I think you are opening too many PRs. It would be better if you discuss the feature/bug first with the maintainers before opening a PR.

[odncode]

Hi @LuisJG8,

Thank you for the feedback. Completely Understood. My apologies for the noise.

I'll make sure to discuss on the issue first before opening PRs going forward. Happy to close any of the open ones you'd prefer not to review right now.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (master@6cc3431). Learn more about missing BASE report.

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #3211   +/-   ##
=========================================
  Coverage          ?   28.83%           
=========================================
  Files             ?      381           
  Lines             ?    52465           
  Branches          ?     9259           
=========================================
  Hits              ?    15129           
  Misses            ?    36323           
  Partials          ?     1013           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.