fix: Use separate header object for each upload in Transfer Manager MPU

[MattIrv]

I didn't see test failures due to this (are the transfer manager tests running continuously?) but in our own repo (GoogleCloudPlatform/gcs-connector-for-pytorch#209) a copied implementation of this caused upload failures (with errors like google.cloud.storage.exceptions.DataCorruption: Checksum mismatch: checksum calculated by client and server did not match. Error code: BadDigest, Error message: The MD5 you specified in Content-MD5 or x-goog-hash did not match what we computed., Error details: The specified CRC (-1631273713) does not match what we computed (1851139595) ) because the headers dict gets updated in upload.py but passing the headers dict to executor.submit here causes all threads to share the same object.

[gemini-code-assist]

Summary of Changes

Hello @MattIrv, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical bug in the Transfer Manager's multipart upload functionality. Previously, concurrent upload tasks were inadvertently sharing the same HTTP headers dictionary, leading to data corruption errors such as checksum mismatches. The change ensures thread safety and reliable multipart uploads by passing a distinct copy of the headers to each individual upload task, thereby preventing unintended modifications across concurrent operations.

Highlights

• Concurrent Uploads: Ensures each concurrent upload in the Transfer Manager's MPU (Multipart Upload) uses a separate copy of the HTTP headers, preventing shared state issues that could lead to checksum mismatches and upload failures.

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[gemini-code-assist]

Code Review

This pull request correctly addresses a critical concurrency issue within the Transfer Manager's multipart upload functionality. By ensuring each worker thread receives a separate copy of the headers dictionary, the change effectively prevents race conditions that were causing data corruption errors. The fix is simple, targeted, and essential for the stability of concurrent uploads. I approve of this change.

[chandra-siri]

Thanks for your fix!

[MattIrv]

@chandra-siri I don't see a way to re-run the approvers check so that I can submit this. Are you able to do so? Thanks!

[chandra-siri]

@chandra-siri I don't see a way to re-run the approvers check so that I can submit this. Are you able to do so? Thanks!

done!