Fix NPE in StandardJanusGraphTx.getVertices() on concurrent graph close (#4907)

[batrived]

Summary

Fixes #4907.

StandardJanusGraphTx.getVertices(Object...) builds its result by calling vertexCache.contains(id) and then vertexCache.get(id, ...). If the transaction is released concurrently — e.g. graph.close() on another thread — between those two calls, releaseTransaction() swaps vertexCache to an EmptyVertexCache whose get() returns null, after getVertices() has already passed verifyOpen(). That null was added to the result list and then dereferenced by:

result.removeIf(JanusGraphElement::isRemoved);

throwing a NullPointerException in ArrayList.removeIf.

The singular getVertex() method already guards against this (return (null == v || v.isRemoved()) ? null : v;). This change aligns getVertices() with that behavior:

result.removeIf(v -> v == null || v.isRemoved());

Test

Adds StandardJanusGraphTxGetVerticesTest, a regression test that swaps in a vertex cache reproducing the contains()/get() race window (contains() returns true, get() returns null) and asserts getVertices() no longer throws and filters out the null entry. The test fails with the exact NPE (ArrayList.removeIf → StandardJanusGraphTx.getVertices) before the fix and passes after it.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

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Fixes a concurrency-related NullPointerException in StandardJanusGraphTx.getVertices(Object...) when a concurrent transaction release swaps the vertexCache between contains() and get(), causing null entries to be added and later dereferenced during filtering.

Changes:

• Update getVertices() to filter out null entries in addition to removed vertices.
• Add a regression test that simulates the contains()/get() race (contains true, get null) and asserts null-safe behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
janusgraph-core/src/main/java/org/janusgraph/graphdb/transaction/StandardJanusGraphTx.java	Makes getVertices() null-safe by filtering out null results before checking isRemoved().
janusgraph-test/src/test/java/org/janusgraph/graphdb/transaction/StandardJanusGraphTxGetVerticesTest.java	Adds a regression test that reproduces the cache race window and verifies getVertices() does not throw and excludes nulls.

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[porunov]

LGTM. Thank you @batrived !