fix: handle missing dictionary batch for null-only columns in IPC reader#9623
Merged
alamb merged 4 commits intoapache:mainfrom Apr 8, 2026
Merged
fix: handle missing dictionary batch for null-only columns in IPC reader#9623alamb merged 4 commits intoapache:mainfrom
alamb merged 4 commits intoapache:mainfrom
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Per the IPC specification, dictionary batches may be omitted when all values in a dictionary-encoded column are null. The C++ implementation (Arrow C++ 17+) relies on this and does not emit a dictionary batch in such cases, which caused the Rust IPC reader to fail with: "Cannot find a dictionary batch with dict id: ..." When a dictionary batch is missing, synthesize an empty values array of the appropriate type so decoding can proceed.
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Looks like this was added to the spec 7 years ago: |
alamb
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Apr 6, 2026
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Thank you for the contribution @joaquinhuigomez -- this looks good to me
I double checked that this matches the spec and that thsi was added to the spec 7 years ago in apache/arrow@0ddc1f4 / apache/arrow#5585
cc @pierrebelzile as the filer of
I also took the liberty of merging up from main and fixing clippy and fmt and simplifying the test a bit
| // Each message is: [continuation (4 bytes)] [meta_len (4 bytes)] | ||
| // [metadata (meta_len bytes)] [body (bodyLength bytes)] | ||
| let mut header = [0u8; 4]; | ||
| if std::io::Read::read_exact(&mut cursor, &mut header).is_err() { |
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this is a strange way to write read_exact -- most other times I see it like
cursor.read_exact(&mut header).is_err() {
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I took the liberty of pushing a commit to simplify it
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Thanks again @joaquinhuigomez |
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Which issue does this PR close?
Rationale for this change
The IPC specification states:
Arrow C++ (v17+) relies on this and does not emit a dictionary batch when all values in a dictionary-encoded column are null. The Rust IPC reader currently fails with
"Cannot find a dictionary batch with dict id: ..."when reading such streams, making cross-language interop broken for this edge case.What changes are included in this PR?
When the IPC reader encounters a
Dictionary-typed column whosedict_idhas no corresponding entry indictionaries_by_id, it now synthesizes an empty values array of the appropriate type (vianew_empty_array) instead of returning an error. This matches the spec's allowance for omitted dictionary batches on null-only columns.Are these changes tested?
Yes. A new test (
test_read_null_dict_without_dictionary_batch) writes an IPC stream with an all-null dictionary column, strips the dictionary batch message from the raw bytes to simulate C++ behavior, then verifies the Rust reader successfully decodes the stream.Are there any user-facing changes?
IPC streams produced by C++ (or other implementations) that omit dictionary batches for null-only dictionary columns can now be read without error. Previously these streams caused a
ParseError.