google app engine - Configure GCS bucket to allow public write but not overwrite -


on google cloud storage, want public (allusers) able upload new files , download existing files, don't want public able overwrite existing file.

background: upload , download urls typically determined own app. under normal conditions there no problem because app guarantees urls unique when writing. malicious user hack app , potentially able upload files (bad) cloud storage , overwrite existing files (very bad).

i know solve problem proxying through app engine or using signed urls, trying avoid due timing constraints. timely processing essential app processes files (almost) in realtime , delay of 1,000 msec processing 2 consecutive requests long.

would possible configure cloud storage in way error returned in case existing file hit during upload, such example:

bucket: public has write access individual file: public has read access

would work? happens in gcs if bucket , file acls contradictory? in above example bucket allow write access, if upload hits existing file readonly access, such request honored gcs, or gcs consider file inexistent @ point , replace new content?

any other approach might work appreciated.


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