zipsync is a focused tool for packing and unpacking build cache entries using a constrained subset of the ZIP format for high performance. It optimizes the common scenario where most files already exist in the target location and are unchanged.

• Optimize partial unpack: Most builds reuse the majority of previously produced outputs. Skipping rewrites preserves filesystem and page cache state.
• Only write when needed: Fewer syscalls.
• Integrated cleanup: Removes the need for a separate rm -rf pass; extra files and empty directories are pruned automatically.
• ZIP subset: Compatibility with malware scanners.
• Fast inspection: The central directory can be enumerated without inflating the entire archive (unlike tar+gzip).

for each file F
  write LocalFileHeader(F)
  stream chunks:
    read -> hash + crc + maybe compress -> write
  finalize compressor
  write DataDescriptor(F)
add metadata entry (same pattern)
write central directory records

load archive -> parse central dir -> read metadata
scan filesystem & delete extraneous entries
for each entry (except metadata):
  if unchanged (sha1 matches) => skip
  else extract (decompress if needed)

Pros for this scenario:

• Central directory enables cheap listing without decompressing entire payload.
• Widely understood / tooling-friendly (system explorers, scanners, CI tooling).
• Per-file compression keeps selective unpack simple (no need to inflate all bytes).

Trade-offs:

• Tar+gzip can exploit cross-file redundancy for better compressed size in datasets with many similar files.