Fix scan iterator [dev]

[badrishc]

Fix deferred DoReadPage drain callback accessing disposed ScanIteratorBase

BufferAndLoad registers DoReadPage as a deferred drain callback via BumpCurrentEpoch. This callback captures instance state including loadCompletionEvents. When the callback is for a read-ahead page and SafeToReclaimEpoch hasn't advanced yet, the callback remains in the drain list. If the scan completes and the iterator is disposed before the callback executes, loadCompletionEvents is set to null. When the callback later runs (during another thread's Drain), it accesses the null array, throwing NullReferenceException.

This exception from a drain callback leaves the epoch in a held state (SuspendDrain's Resume/Release is not exception-safe), causing the cascading 'Trying to acquire protected epoch' assertion failure seen in ObjectIterationPushLockTest(4,4,Iterate,False).

Fix: capture loadCompletionEvents into a local before accessing it in DoReadPage, and return early if the iterator has been disposed.

[Copilot]

Pull request overview

Fixes a concurrency issue in Tsavorite scan iterators where a deferred epoch drain callback could run after the iterator is disposed, leading to a NullReferenceException and potentially leaving the epoch in a held state.

Changes:

• In ScanIteratorBase.BufferAndLoad, updates the deferred DoReadPage callback to snapshot loadCompletionEvents into a local variable and return early if it has been nulled by Dispose().
• Adds explanatory comments documenting why the deferred callback can outlive the iterator instance state.

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