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Zhengchao An
3bc8d79fe5
fix(multipart): serialize the part commit per uploadId to prevent mixed-generation shards (backlog#853) Concurrently re-transmitting the same part could land two shard generations across different disks: each shard is individually bitrot-valid, so the corruption surfaces only as a silent mix at read time. The hazard is confined to rename_part, where two temp parts are moved cross-disk onto the SAME final part path — interleaving there can leave shards from two generations. Each concurrent stream already writes to its own unique temp dir, so the encode/stream phase never conflicts and must stay lock-free: holding a lock across it would serialize slow re-transmits of the same part, break the S3 'last finisher wins' semantics, and cause UploadPart lock-acquire timeouts (ServiceUnavailable). Scope a write lock to the uploadId namespace around only the rename_part commit, so each commit is atomic across disks and the last committer wins consistently. Mirrors MinIO's per-uploadID NS lock; disjoint from the object lock held by complete_multipart_upload (no lock-ordering cycle). Honors opts.no_lock. The pre-delete-before-rename half of backlog#853 was already fixed in #4316; this completes the issue. Adds an end-to-end regression test: six concurrent resends of the same part must all succeed (no lock-acquire timeout), exactly one generation stays visible, and the reassembled object equals one intact generation. Closes rustfs/backlog#853