[I] services/gcs: XML multipart writer cannot honor write_with_if_not_exists [opendal]

["Xuanwo (via GitHub)" <gi...@apache.org>]

Xuanwo opened a new issue, #8040:
URL: https://github.com/apache/opendal/issues/8040

   ## Summary
   
   GCS supports `write_with_if_not_exists` for single-shot uploads via `ifGenerationMatch=0`, but OpenDAL's multipart writer path uses the XML API multipart upload APIs, which do not support request preconditions.
   
   This was exposed by `test_writer_write_with_if_not_exists` after #8028 added chunked-writer precondition coverage. Against GCS the multipart write with `if_not_exists(true)` succeeds and overwrites an existing object instead of returning `ConditionNotMatch`.
   
   ## Evidence from GCS docs
   
   From [Request preconditions](https://docs.cloud.google.com/storage/docs/request-preconditions):
   
   > Preconditions cannot be used in XML API multipart uploads. Attempting to do so results in a `400 NotImplemented` error.
   
   `If-None-Match` is also not a substitute here: GCS documents it for retrieve/GET/HEAD style requests, and the Complete Multipart Upload request headers do not list precondition headers.
   
   ## Current OpenDAL behavior
   
   - Single-shot `write_with(...).if_not_exists(true)` works on GCS.
   - Capability currently reports `write_with_if_not_exists: true` and `write_can_multi: true`.
   - Chunked `writer_with(...).if_not_exists(true)` therefore looks supported, but the multipart path cannot enforce the condition.
   
   ## Possible directions
   
   1. Keep skipping `test_writer_write_with_if_not_exists` on GCS until there is a supported upload path.
   2. Move GCS multi-write onto an API that can enforce `ifGenerationMatch=0` (for example resumable upload), if that can preserve current writer semantics.
   3. Narrow the advertised capability so multipart conditional writes are not implied when only single-shot conditional writes are possible.
   
   ## References
   
   - Failing main Behavior Test: https://github.com/apache/opendal/actions/runs/31186355721
   - Related conditional-write tracking: #7889
   - Azblob multipart precondition fix that introduced the coverage: #8028


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Re: [I] services/gcs: XML multipart writer cannot honor write_with_if_not_exists [opendal]

["dgvj-work (via GitHub)" <gi...@apache.org>]

dgvj-work commented on issue #8040:
URL: https://github.com/apache/opendal/issues/8040#issuecomment-5247849851

   I'd like to take this. I'll look at how we can honor `write_with_if_not_exists` on the GCS multipart path (given XML multipart preconditions aren't supported) and open a PR.


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