Arrow: Fix NullabilityHolder.reset() to clear the isNull array

[jbbqqf]

Summary

NullabilityHolder.reset() previously zeroed numNulls but left the isNull marker array unchanged. Callers that reuse a holder across batches could see hasNulls() == false yet still get isNullAt(i) == 1 for indices set null in a previous batch.

This PR fixes reset() to also clear the isNull array, and adds focused unit tests covering the single-set, bulk-set, and idempotency paths.

Resolves #15808

Context

Today's vectorized read hot-path callers (e.g. the arrow vectorized readers) always overwrite all positions on the next batch via setNulls(start, num) / setNotNulls(start, num), so the latent stale state is never observed in production. The bug is therefore a contract trap rather than an active correctness bug — but the holder's contract documents reset() as restoring a clean initial state, and the original triage in #15808 already proposed the one-line fix.

This PR is a revival of #15809 (closed by stale-bot at 30 days of no review). The diff is unchanged in spirit; this version adds AI disclosure (which a reviewer asked for on the previous attempt) and a slightly broader test class to make the regression-test evidence explicit.

Changes

• arrow/src/main/java/org/apache/iceberg/arrow/vectorized/NullabilityHolder.java — reset() now also calls Arrays.fill(isNull, (byte) 0). A short comment explains the why (so a reviewer reading the diff cold doesn't have to re-derive it from the issue).
• arrow/src/test/java/org/apache/iceberg/arrow/vectorized/TestNullabilityHolder.java — new test class with three tests: resetClearsIsNullMarkers, resetClearsBulkSetNulls, resetIsIdempotent. All three fail on origin/main and pass with the fix.

The other two arrays in NullabilityHolder (nulls and nonNulls) are immutable arraycopy source buffers and intentionally not cleared — that's documented in the inline comment.

Reproduce BEFORE/AFTER yourself (copy-paste)

# --- one-time setup ---
git clone https://github.com/apache/iceberg.git /tmp/repro && cd /tmp/repro

# --- BEFORE (origin/main) ---
git checkout origin/main
# Pull in only the new test file from the fix branch so the regression
# is visible against unmodified production code:
git fetch https://github.com/jbbqqf/iceberg.git feat/15808-fix-nullability-holder-reset
git checkout FETCH_HEAD -- arrow/src/test/java/org/apache/iceberg/arrow/vectorized/TestNullabilityHolder.java
./gradlew :iceberg-arrow:test --tests "org.apache.iceberg.arrow.vectorized.TestNullabilityHolder"
# Expected: 3 tests, 3 failures — isNullAt() returns the stale 1 from
#           a setNull/setNulls call that preceded reset().

# --- AFTER (this PR) ---
git checkout FETCH_HEAD
./gradlew :iceberg-arrow:test --tests "org.apache.iceberg.arrow.vectorized.TestNullabilityHolder"
# Expected: 3 tests, 3 passes — reset() now zeros isNull as well as numNulls.

The only thing that changes between BEFORE and AFTER is the checked-out ref of the production class; the test file is the same in both runs.

What I ran locally

• ./gradlew :iceberg-arrow:test --tests "org.apache.iceberg.arrow.vectorized.TestNullabilityHolder" on origin/main with the new test file imported → 3 tests, 3 failures (regression visible).
• Same command on this branch → 3 tests, 3 passed.
• ./gradlew :iceberg-arrow:spotlessCheck → PASS.
• Full :iceberg-arrow:test build wrapping the change → BUILD SUCCESSFUL (existing tests untouched).

Edge cases tested

#	Scenario	Input	Expected	Verified by
1	Single setNull(idx) then reset()	setNull(3), reset()	isNullAt(3) == 0, all other indices == 0, numNulls() == 0	resetClearsIsNullMarkers
2	Bulk setNulls(start, num) then reset()	setNulls(2, 4), reset()	every index == 0, hasNulls() == false	resetClearsBulkSetNulls
3	Idempotency — call reset() twice	setNull(0), reset(), reset()	every index == 0, numNulls() == 0	resetIsIdempotent

Risk / blast radius

Internal: NullabilityHolder is package-private under org.apache.iceberg.arrow.vectorized and only used by the vectorized arrow read path. The fix adds a byte[] zero-fill in reset() (size = batch capacity, typically 4–8 KiB on hot paths). On the dominant hot path the holder is reused across batches and the next bulk setNulls/setNotNulls would have overwritten the array anyway, so the only observable effect is a single extra Arrays.fill per reset() — a few microseconds per batch on typical JVM heuristics.

Additive change; no API surface modified.

Release note

Fix NullabilityHolder.reset() to clear the isNull marker array, restoring the documented clean-state contract for callers that reuse a holder across batches.

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PR drafted with assistance from Claude Code. The change was reviewed manually against apache/iceberg's source on main. The reproducer block above was used during development; it is the same one a reviewer can paste verbatim.