[Iceberg] Fix manifest bounds being padded with trailing 0x00 bytes

[dejii]

Summary

SerializableDataFile.toByteArrayMap used ByteBuffer.array() to extract bound bytes. array() returns the full backing array, ignoring the buffer's [position, limit) content window.

Iceberg's Conversions.toByteBuffer delegates to JDK CharsetEncoder.encode(CharBuffer), which sizes its allocation by (int)(chars * averageBytesPerChar). For UTF-8 the average is 1.1, so a 15-char string lands in a 16-byte buffer with position=0, limit=15, capacity=16. The 16th byte is the zero-initialized tail of the backing array.

Reading via .array() leaks that tail into the byte[] form, and on the return trip ByteBuffer.wrap(byte[]) treats it as content. The manifest ends up with a lower bound that is strictly greater than the real minimum byte-lexicographically. Query engines using Iceberg metadata for predicate pushdown decide value < lower_bound for any equality filter on that minimum and prune every file, returning 0 rows.

Why the bug stays quiet for most queries

All three must hold:

1. Encoded length 10–N bytes (N = truncate(N) from metrics config, default 16). Below 10, the JDK encoder allocates exact-size buffers; above N, Iceberg trims the string first, so the bound holds a truncated form that a full-value query naturally sorts past.
2. Value is the file's actual lower bound on that column. Other values never compare against the padding byte.
3. Predicate is eligible for bound pruning (=, IN, etc.).

Fix

• Fix by copying the buffer's [position, limit) window. Added a regression test that builds a DataFile with bounds matching that shape, roundtrips through SerializableDataFile, and asserts the resulting byte[] match the encoded content rather than the inflated backing array.
• New writes will produce correct bounds. Tables already written by the buggy sink keep broken bounds until their manifests are rewritten

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[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a bug where manifest bounds in Iceberg tables were being incorrectly serialized due to the use of ByteBuffer.array(). This method returns the entire backing array, which often includes trailing zero-initialized bytes when the buffer is over-allocated by encoders. These leaked bytes corrupted the lower bounds, causing query engines to incorrectly prune files during predicate pushdown. The fix ensures that only the relevant content window is copied, and a regression test has been added to verify the fix.

Highlights

• Fixing manifest bound corruption: Updated SerializableDataFile to correctly extract byte arrays from ByteBuffers by respecting the [position, limit) window instead of using the full backing array.
• Regression testing: Added a new test case in SerializableDataFileTest that explicitly reproduces the over-allocated buffer scenario to ensure bounds are correctly serialized.

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[gemini-code-assist]

Code Review

This pull request fixes a bug in SerializableDataFile where ByteBuffer.array() was used to extract data, potentially leaking trailing bytes from the backing array into manifest bounds. A new toByteArray helper method now correctly copies only the active portion of the buffer. A corresponding unit test was added to verify that bounds are copied based on the buffer's limit rather than the backing array's capacity. I have no feedback to provide.

[github-actions]

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[dejii]

R: @ahmedabu98

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[dejii]

assign set of reviewers

[github-actions]

Assigning reviewers:

R: @kennknowles for label java.

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[ahmedabu98]

Great job finding this one. Left one comment

[dejii]

Seems like a transient CI failure:

> Could not GET 'https://repo.maven.apache.org/maven2/org/eclipse/platform/org.eclipse.text/3.9.0/org.eclipse.text-3.9.0.jar'. Received status code 403 from server: Forbidden

[ahmedabu98]

LGTM, thx for fixing this!