feat(ec2): improve subnet selection to support subnet group name and combined selection

[dwchiang]

Summary

This PR addresses issue #35422 by enhancing the SubnetSelection functionality to support more precise subnet selection when multiple public subnets with different purposes exist in a VPC.

Problem

Currently, when using SubnetSelection(subnet_type=SubnetType.PUBLIC), the CDK selects any public subnet without considering the specific subnet group name. This causes issues when users have multiple public subnet groups (e.g., "public" and "nat") and need to target a specific group.

Solution

This PR introduces the following enhancements:

1. Enhanced SubnetSelection: Improved support for combining subnet_type and subnet_group_name for more precise selection
2. Backward Compatibility: All existing subnet selection methods continue to work as before
3. Better Documentation: Updated README with examples showing the new selection capabilities

Changes Made

• packages/aws-cdk-lib/aws-ec2/lib/vpc.ts: Enhanced subnet selection logic
• packages/aws-cdk-lib/aws-ec2/lib/instance.ts: Updated to support new selection methods
• packages/aws-cdk-lib/aws-ec2/test/vpc.test.ts: Added comprehensive tests for new functionality
• packages/aws-cdk-lib/aws-ec2/test/instance.test.ts: Added tests for instance subnet selection
• packages/aws-cdk-lib/aws-ec2/test/integ.subnet-selection-combined.ts: New integration test
• packages/aws-cdk-lib/aws-ec2/README.md: Updated documentation with examples

Usage Examples

# Select by subnet group name
subnets = ec2.SubnetSelection(subnet_group_name="public")

# Combine type and group name for precise selection
subnets = ec2.SubnetSelection(
    subnet_type=ec2.SubnetType.PUBLIC,
    subnet_group_name="public"
)

Testing

• Unit tests added for new functionality
• Integration tests added
• Existing tests pass
• Manual testing in real AWS environment (to be done during review)

Breaking Changes

None. This change is fully backward compatible.

Fixes #35422

[aws-cdk-automation]

(This review is outdated)

[pahud]

Thank you for the PR.

packages/aws-cdk-lib/aws-ec2/test/integ.subnet-selection-combined.ts

let's move this integ test to packages/@aws-cdk-testing/framework-integ/test/aws-ec2/test and

$ cd framework-integ
$ yarn build --fix
$ yarn integ test/aws-ec2/test/integ.subnet-selection-combined.js --update-on-failed

to generate the new snapshot.

and you may want to

$ yarn integ --directory test/aws-ec2/test/ --update-on-failed

to make sure all existing aws-ec2 integ tests stay up-to-date and refreshed if necessary.

✅ Updated pull request passes all PRLinter validations. Dismissing previous PRLinter review.

Pull request has been modified.

[pahud]

Per CI error:

aws-cdk-lib: /codebuild/output/src1050340690/src/actions-runner/_work/aws-cdk/aws-cdk/packages/aws-cdk-lib/aws-ec2/lib/vpc.ts
aws-cdk-lib:   717:5  error  Trailing spaces not allowed  no-trailing-spaces
aws-cdk-lib:   722:5  error  Trailing spaces not allowed  no-trailing-spaces
aws-cdk-lib: ✖ 2 problems (2 errors, 0 warnings)
aws-cdk-lib:   2 errors and 0 warnings potentially fixable with the `--fix` option.
aws-cdk-lib: Error: /codebuild/output/src1050340690/src/actions-runner/_work/aws-cdk/aws-cdk/node_modules/eslint/bin/eslint.js . --ext=.ts --resolve-plugins-relative-to=/codebuild/output/src1050340690/src/actions-runner/_work/aws-cdk/aws-cdk/tools/@aws-cdk/cdk-build-tools/lib exited with error code 1

please run yarn lint --fix in aws-cdk-lib

Pull request has been modified.

[rix0rrr]

Looking at the original issue, it's not clear to me how the recommended:

    subnet_group_name="public"  # Specify your desired group name

solution is not sufficient.

Subnet groups of a given name are already of a defined type.

In other words in the following example:

# Combine type and group name for precise selection
subnets = ec2.SubnetSelection(
    subnet_group_name="public"
    subnet_type=ec2.SubnetType.PUBLIC,
)

Once we know that subnet_group_name="public", it must already follow that subnet_type=PUBLIC.

If you passed in any other value than PUBLIC here, you would always get an empty set.

It might be that the original reporter actually had an unfixable problem because there is a bug in the code, but if they do this is not the solution to it.

[aws-cdk-automation]

This PR has been in the CHANGES REQUESTED state for 3 weeks, and looks abandoned. Note that PRs with failing linting check or builds are not reviewed, please ensure your build is passing

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[aws-cdk-automation]

This PR has been deemed to be abandoned, and will be automatically closed. Please create a new PR for these changes if you think this decision has been made in error.

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