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feat: implement conditional headers for S3 CopyObject API#13554

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bentsku merged 5 commits intolocalstack:mainfrom
shubhiscoding:main
Feb 21, 2026
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feat: implement conditional headers for S3 CopyObject API#13554
bentsku merged 5 commits intolocalstack:mainfrom
shubhiscoding:main

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@shubhiscoding shubhiscoding commented Dec 20, 2025

Motivation

AWS S3 supports conditional headers (If-Match and If-None-Match) in the CopyObject API to enable atomic copy operations based on the destination object's ETag. LocalStack previously did not support these headers, making it difficult to test applications that rely on conditional copy operations. Fixes #13522

Changes

Implemented support for conditional headers in S3 CopyObject API:

  • Added If-Match header support: Allows copying only if the destination object's ETag matches the specified value
  • Added If-None-Match header support: Allows copying only if the destination object doesn't exist (using * wildcard)
  • Validation logic: Implemented proper error handling for invalid header combinations and unsupported values, matching AWS behavior
  • Precondition checks: Leveraged existing helper functions (object_exists_for_precondition_write and verify_object_equality_precondition_write) to validate conditions before copy operation
    The implementation follows the same pattern as the existing

PutObject conditional header logic.

Tests

Added tests in test_s3_api.py:

  • test_copy_object_if_none_match: Validates If-None-Match: * behavior
    • Successful copy when destination doesn't exist
    • PreconditionFailed error when destination already exists
  • test_copy_object_if_match: Validates If-Match behavior
  • Successful copy when destination ETag matches
  • PreconditionFailed error when ETag doesn't match
  • NoSuchKey error when destination doesn't exist

Related

#13522

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localstack-bot reviewed Dec 20, 2025
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Welcome to LocalStack! Thanks for raising your first Pull Request and landing in your contributions. Our team will reach out with any reviews or feedbacks that we have shortly. We recommend joining our Slack Community and share your PR on the #community channel to share your contributions with us. Please make sure you are following our contributing guidelines and our Code of Conduct.

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localstack-bot commented Dec 20, 2025 •
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shubhiscoding commented Dec 20, 2025

I have read the CLA Document and I hereby sign the CLA

localstack-bot added a commit that referenced this pull request Dec 20, 2025
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@bentsku bentsku added aws:s3 Amazon Simple Storage Service semver: patch Non-breaking changes which can be included in patch releases semver: minor Non-breaking changes which can be included in minor releases, but not in patch releases docs: skip Pull request does not require documentation changes notes: skip Pull request does not have to be mentioned in the release notes and removed semver: patch Non-breaking changes which can be included in patch releases labels Dec 22, 2025
@bentsku bentsku self-assigned this Dec 22, 2025
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Thanks a lot for your contribution!

Unfortunately I can see that the tests are not validated against AWS, and thus not following our contribution guidelines. It would be great if you could validate that.

There is also required changes in the CopyObject operation itself, as these conditions headers are there to prevent race conditions, and we do not have concurrency control in your implementation.

As written in the comments, our contribution expectations are pretty high regarding the S3 service as it is one of the most used and a lot of other services depends on it, we're very careful when modifying its behavior. Let me know if you'd need additional help for this one 👍

edit: forgot to add to both PR, it is holidays season here and we might have some delay in reviewing your changes, and might come back to it in January.

@bentsku bentsku mentioned this pull request Dec 22, 2025
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shubhiscoding commented Dec 24, 2025 •
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Thanks a lot for your contribution!

Unfortunately I can see that the tests are not validated against AWS, and thus not following our contribution guidelines. It would be great if you could validate that.

There is also required changes in the CopyObject operation itself, as these conditions headers are there to prevent race conditions, and we do not have concurrency control in your implementation.

As written in the comments, our contribution expectations are pretty high regarding the S3 service as it is one of the most used and a lot of other services depends on it, we're very careful when modifying its behavior. Let me know if you'd need additional help for this one 👍

edit: forgot to add to both PR, it is holidays season here and we might have some delay in reviewing your changes, and might come back to it in January.

Hii @bentsku I went through your suggestions and as per my understanding I've made the edits to keep it safe from race conditions (tried mimicking the same logic used in put_object) and also added tests for versioned bucket behavior and also I've ran all those tests against my aws account, I understand the importance of the S3 service, so feel free to suggest any further changes!
Happy Holidays!

@bentsku bentsku self-requested a review January 2, 2026 13:28
@bentsku bentsku added this to the 4.13 milestone Jan 15, 2026
@bentsku bentsku modified the milestones: 4.13, 4.14 Jan 26, 2026
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shubhiscoding commented Jan 29, 2026

Hii @bentsku are there anymore changes required here from my side?

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bentsku commented Jan 29, 2026

Hii @bentsku are there anymore changes required here from my side?

Hi @shubhiscoding, sorry for the delay here, I still need to do thorough testing and I've been a bit swamped, but will look at it tomorrow and do a review 👍 sorry for the delay again

@shubhiscoding shubhiscoding mentioned this pull request Feb 2, 2026
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@bentsku bentsku requested a review from aidehn as a code owner February 20, 2026 23:52
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LGTM, great work @shubhiscoding! Sorry I took so long to review this PR, but great work. I took the liberty to push some more changes (only tests) to your branch, in order to validate some scenarios I had in mind that could be problematic or have been in the past with precondition writes. Seems like everything is working perfectly, so I'll approve, and merge once green 👌

@bentsku bentsku merged commit 0f44763 into localstack:main Feb 21, 2026
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@bentsku bentsku mentioned this pull request Feb 23, 2026
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aws:s3 Amazon Simple Storage Service docs: skip Pull request does not require documentation changes notes: skip Pull request does not have to be mentioned in the release notes semver: minor Non-breaking changes which can be included in minor releases, but not in patch releases

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feature request: Support conditional copy operations in S3

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