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Support zstd blob decompression in Puffin#3575
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Rationale for this change
Prepares PuffinFile for
apache-datasketches-theta-v1blob support by removing the hard constraint that all blobs aredeletion-vector-v1and by adding zstd blob decompression.Two fixes were required to make this work correctly:
Fix: blob offsets were off by 8
_payload stored puffin[8:], but blob offset values in the Puffin footer are measured from byte 0 of the file. For the existing deletion-vector-v1 case this was harmless — the 8-byte shift accidentally cancelled the 8-byte blob framing ([length:4][magic:4]) that was never stripped. With arbitrary blob types and real offsets the cancellation breaks, so _payload now holds the full file bytes.
Widen PuffinBlobMetadata.type from Literal["deletion-vector-v1"] to str
PuffinFile is a generic Puffin format parser, not a deletion-vector-specific reader. Locking type to a single literal means parsing any Puffin file that contains a non-DV blob (e.g., a theta sketch) raises a Pydantic validation error before we can even read the footer. Widening to str keeps the parser general; blob-type-specific logic belongs in callers (deletion_vectors_from_puffin_file, and the forthcoming theta sketch reader).
Are these changes tested?
Yes. Copied Puffin files from https://github.com/apache/iceberg/tree/main/core/src/test/resources/org/apache/iceberg/puffin/v1
Are there any user-facing changes?
No. PuffinFile is internal. The type: str widening is backwards-compatible.