Fix copy endpoint for files >5 GB (PTFE-3441)

[matthiasL-scality]

Summary

• Root cause: copy_build.lua uses S3 CopyObject (x-amz-copy-source) for every file with no size check. Scaleway enforces a 5 GB hard limit (identical to AWS S3). Builds with files >5 GB (e.g. artesca promote jobs) fail with EntityTooLarge.
• Fix: On EntityTooLarge (HTTP 400), fall back to multipart copy using S3 UploadPartCopy. Parts are sized at max(5 MB, ⌈size/1000⌉) to stay within Scaleway's 1,000-part limit. Parts within a single file are copied in parallel batches of 16.
• Test coverage: COPY_OBJECT_SIZE_LIMIT=0 added to tests/.env forces all non-empty files through the multipart code path in CI, so the full initiate→part→complete cycle is exercised without actual >5 GB files.

Changes

File	Change
conf/nginx.conf.template	env COPY_OBJECT_SIZE_LIMIT; + 4 internal-only locations: copy-multipart-{initiate,part,complete,abort}
lua/compute_aws_s3_signature.lua	4 new signature modes: COPY_MULTIPART_{INITIATE,PART,COMPLETE,ABORT}
lua/copy_build.lua	multipart_copy() function; production path falls back on EntityTooLarge; test path uses size threshold
tests/.env	COPY_OBJECT_SIZE_LIMIT=0 to force multipart in CI
tests/end2end/test_copy.py	test_copy_via_multipart_copy verifies content integrity through the multipart path

Test plan

• CI passes — all existing test_copy.py tests now run through the multipart path (due to COPY_OBJECT_SIZE_LIMIT=0) and verify the implementation is correct end-to-end
• New test test_copy_via_multipart_copy specifically documents and asserts the multipart behavior
• Manual verification: deploy to a Scaleway-backed environment and promote a build containing a file >5 GB

Fixes: PTFE-3441

🤖 Generated with Claude Code

[claude]

• S3 CompleteMultipartUpload can return HTTP 200 with an error in the XML body, causing multipart_copy() to silently report success for an incomplete object
  - Add complete_res.body:find("<Error>", 1, true) to the status check on line 170, matching how initiate/part responses are already validated via body content
  
  Review by Claude Code

[claude]

• lua/copy_build.lua:172 — abort after complete failure is missing the /force_real_request/ prefix, so the subrequest inherits the external client's $remote_addr and the copy-multipart-abort location returns 400, leaving orphaned multipart parts on S3
  • Add the /force_real_request/ prefix to match all other subrequest calls in the function

Review by Claude Code

[claude]

LGTM

Multipart copy implementation is solid. Error handling properly aborts in-flight uploads on every failure path, preventing orphaned parts. Part sizing correctly respects both the 5 MB S3 minimum and Scaleway's 1,000-part cap. Signature construction follows the existing COPY mode patterns. Test coverage is good — the COPY_OBJECT_SIZE_LIMIT=0 trick exercises the full initiate/part/complete cycle, and per-test bucket isolation (managed_buckets fixture) keeps tests independent.

Review by Claude Code

[claude]

LGTM

Review by Claude Code