fix: prevent concurrent access to non-thread-safe maps in SketchBufferAggregatorHelper

[zhang-arvin]

Description

Fixes #18040 - Runtime Exception when executing a query twice in a short duration.

Root Cause

The ParallelCombiner and StreamingMergeSortedGrouper are designed for concurrent read/write from different threads. However, SketchBufferAggregatorHelper used IdentityHashMap and Int2ObjectOpenHashMap, which are not thread-safe.

Concurrent access to these maps by:

• The writing thread (via aggregate() → getOrCreateUnion())
• The reading thread (via get())

could corrupt the internal map structure, causing ArrayIndexOutOfBoundsException during sketches aggregation (e.g., Index 180 out of bounds for length 129).

Fix

Replaced IdentityHashMap with ConcurrentHashMap and Int2ObjectOpenHashMap with ConcurrentHashMap to ensure thread safety when used with ParallelCombiner.

Changes

• extensions-core/datasketches/.../SketchBufferAggregatorHelper.java: Replace non-thread-safe maps with ConcurrentHashMap

Key Features/Changes

• Thread-safe access to Union cache and memory cache in SketchBufferAggregatorHelper
• Uses ConcurrentHashMap.computeIfAbsent for atomic map initialization

Verification

The existing tests for ParallelCombiner and StreamingMergeSortedGrouper cover the concurrent read/write pattern. The fix ensures that Datasketches-based aggregators work correctly under parallel combining.

[FrankChen021]

Severity	Findings
P0	0
P1	1
P2	0
P3	0
Total	1

Reviewed 1 of 1 changed files.

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This is an automated review by Codex GPT-5.6-Luna(max)

[FrankChen021]

[P1] Do not key caches by mutable ByteBuffers

ConcurrentHashMap uses ByteBuffer.equals/hashCode, which depend on the buffer's remaining contents. Union updates mutate those contents in place, changing the key hash after insertion. Subsequent unions.get(buf) calls can miss and get() returns SketchHolder.EMPTY; distinct equal-content buffers can also alias. Preserve identity/stable-key semantics while making access concurrent.