[FLINK-39951][python] Fix string reference equality bug in ArrayConstructor causing silent long value truncation

[jubins]

What is the purpose of the change

Fixes FLINK-39951 — ArrayConstructor.construct() uses reference equality (==) to compare the pickle typecode string "l" instead of value equality (.equals()). Since args[0] is a deserialized String object at runtime and not an interned literal, the == comparison always returns false, making the long[] code path effectively dead code.

As a result, any Python array with typecode 'l' (64-bit longs) silently falls through to super.construct(), which treats the values as int[] and truncates them to 32 bits. Users passing long values larger than Integer.MAX_VALUE receive silently corrupted data with no error or warning.

The fix replaces args[0] == "l" with "l".equals(args[0]) on line 30 of ArrayConstructor.java.

Brief change log

• Fixed ArrayConstructor.construct() to use "l".equals(args[0]) instead of args[0] == "l" for typecode comparison
• Added ArrayConstructorTest with four test cases covering: large long values above Integer.MAX_VALUE, single value arrays, empty arrays, and delegation to the parent class for non-'l' typecodes

Verifying this change

This change is covered by new unit tests in ArrayConstructorTest:

• testLongArrayPreservesValuesAboveIntMax() — the core regression test; uses new String("l") to guarantee a non-interned reference (proving the old == would have failed), then asserts that values like 3_000_000_000L and Long.MAX_VALUE are preserved correctly as long[]
• testLongArrayWithSingleValue() — verifies a single large long value is handled correctly
• testLongArrayWithEmptyList() — verifies empty input produces an empty long[]
• testNonLongTypecodesDelegateToSuper() — confirms other typecodes (e.g. 'i') still fall through to the parent super.construct() correctly

Does this pull request potentially affect one of the following parts

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery (JobManager, Checkpointing, Kubernetes/Yarn, ZooKeeper): no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no — it fixes a bug in existing functionality
• If yes, how is the feature documented? not applicable

Was generative AI tooling used to co-author this PR?

• Yes — Claude Code was used as a pair-programming assistant. All code was written, understood, and verified by the author.
  Generated-by: Claude Opus 4.8

[flinkbot]

CI report:

• 6948c3f Azure: SUCCESS

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[spuru9]

LGTM