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Posted to reviews@spark.apache.org by "liuzqt (via GitHub)" <gi...@apache.org> on 2026/09/29 04:02:48 UTC
[PR] [SPARK-59845][CORE] Register an accumulator with the TaskContext only after it is fully deserialized [spark]
liuzqt opened a new pull request, #59124:
URL: https://github.com/apache/spark/pull/59124
### What changes were proposed in this pull request?
Move the `TaskContext` registration out of `readObject` and into `readResolve`.
`AccumulatorV2` currently registers from `readObject`, which runs at AccumulatorV2's own slot. Java serialization reads hierarchy data **superclass-first**, so no subclass field is assigned yet — while `registerAccumulator` publishes `this` into `TaskMetrics` where other threads can read it. The PR drops `readObject` (beyond registering, all it did was in.defaultReadObject(), which the framework does anyway) and registers from `final protected def readResolve()` instead, which runs only after every class in the hierarchy has been read.
It's `final` for the same reason `writeReplace` already is: `readObject` must be `private` so it's invoked per class, whereas `readResolve` is inherited — a subclass defining one would silently stop registration.
### Why are the changes needed?
Subclasses are published before their state exists, so a subclass holding state in a `val` exposes a `null`. The reader that hits it is the executor heartbeater, calling `isZero` on every registered accumulator. The NPE propagates out of a `scheduleAtFixedRate` task, so `ScheduledThreadPoolExecutor` stops rescheduling — the executor never heartbeats again, the driver removes it on timeout.
Plus the JLS 17.5.3 aspect: a val is a final field written reflectively after publication, so a reader has no guarantee of seeing it and the JIT may fold the read.
SPARK-20977 fixed this locally in CollectionAccumulator; every subclass since has re-derived the workaround and not all have — SetAccumulator in sql/execution/debug still dereferences private val _set from isZero, and EventTimeStatsAccum escapes the NPE only incidentally via Scala's null-safe ==, reporting the wrong answer instead.
### Does this PR introduce _any_ user-facing change?
NO
### How was this patch tested?
New test case
### Was this patch authored or co-authored using generative AI tooling?
Yes. Claude Opus 5.0
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Re: [PR] [SPARK-59845][CORE] Register an accumulator with the TaskContext only after it is fully deserialized [spark]
Posted by "cloud-fan (via GitHub)" <gi...@apache.org>.
cloud-fan commented on PR #59124:
URL: https://github.com/apache/spark/pull/59124#issuecomment-5886874109
**Merge Summary:**
- merged into master https://github.com/apache/spark/commit/5b7d3fe1d74733926ad60c108cd3f2f502e85498
- merged into branch-4.x https://github.com/apache/spark/commit/ba033ef70ab71a5607dd508e5d241afc5d72a54b
- merged into branch-4.3 https://github.com/apache/spark/commit/c0b4f9f8a0cd21cb83ee05f686def5272d776d57
*Posted by `merge_spark_pr.py`*
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Re: [PR] [SPARK-59845][CORE] Register an accumulator with the TaskContext only after it is fully deserialized [spark]
Posted by "cloud-fan (via GitHub)" <gi...@apache.org>.
cloud-fan closed pull request #59124: [SPARK-59845][CORE] Register an accumulator with the TaskContext only after it is fully deserialized
URL: https://github.com/apache/spark/pull/59124
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