Takanawa .part Format

The .part file layout is:

[content bytes, length = content_len]
[metadata slot 0, length = aligned_slot_size]
[metadata slot 1, length = aligned_slot_size]

Each metadata slot is fixed-size for a given content_len and chunk_size. The active slot is the valid slot with the largest generation.

Metadata includes:

• magic and metadata version
• CRC32 over the whole slot with the CRC field zeroed
• generation
• URL SHA-256
• content length, chunk size, chunk count
• completion bitmap
• ETag and Last-Modified snapshots
• expected hash configuration

The writer may stream bytes into the content area before a chunk is complete. Those bytes are intentionally not durable resume state yet: a chunk is recoverable only after its bitmap bit is committed in metadata. If a process crashes while a chunk is partially written, the next run treats that chunk as incomplete and downloads it again, overwriting any partial content bytes.

The writer commits one completed chunk at a time:

1. write chunk bytes at the content offset
2. sync_data
3. mark the chunk complete in memory
4. write the next metadata slot generation
5. sync_all

This ordering means a crash can cause a recently written chunk to be downloaded again, but cannot mark missing data as complete.