fix(streaming): don't recycle in-flight pooled buffers mid-batch

[ReubenBond]

Problem

Fixes #10263.

Pooled stream caches (EventHub, Memory, Generator) can silently corrupt message payloads, which later surfaces as an ArgumentOutOfRangeException in SegmentBuilder.ReadNextString.

ChronologicalEvictionStrategy.OnBlockAllocated eagerly freed the previous pooled buffer whenever the cache reported empty (PurgeObservable.IsEmpty). That was safe when the strategy was written, but callers now encode an entire batch into pooled buffers before committing it to the cache. During that encode IsEmpty stays true, so the buffer being freed can still hold in-flight, uncommitted segments for the current batch. The freed buffer is returned to the (LIFO) object pool, and the very next allocation in the same batch pops it straight back and overwrites the earlier segments from offset 0 — corrupting those messages. The three pooled caches all share this eviction strategy, so all of them are affected, not just EventHub.

Solution

Remove the eager-free branch in OnBlockAllocated. In-use buffers are already reclaimed correctly at purge time by FreePurgedBuffers, which only releases buffers once their segments are no longer referenced by committed messages.

Removing the eager-free would, on its own, expose a pre-existing buffer leak: for a message too large to ever fit in a pooled buffer, the eager-free was the only thing that reclaimed the freshly allocated (and never-committed) block. To close that, the EventHub, Memory and Generator caches now dispose the freshly allocated block back to the pool and throw before registering it with the eviction strategy, so repeated oversized (poison) messages reuse a single block instead of growing the retained set without bound.

Adds regression tests for multi-buffer batch payload integrity from an empty cache and for the oversized-message no-leak behavior. Both fail without the fix and pass with it.

Microsoft Reviewers: Open in CodeFlow

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[Copilot]

Pull request overview

This PR fixes a streaming buffer-corruption bug in Orleans’ pooled queue cache infrastructure by preventing in-flight pooled buffers from being recycled mid-batch, and adds regression tests to cover the corruption scenario plus an oversized-message buffer leak.

Changes:

• Removed eager buffer recycling in ChronologicalEvictionStrategy.OnBlockAllocated to avoid returning still-in-use buffers to the pool mid-batch.
• Updated EventHub/Memory/Generator pooled caches (and the test converter) to validate segment fit before registering a new buffer with the eviction strategy, disposing the unused buffer on failure to avoid leaks.
• Added regression tests validating multi-buffer batch payload integrity from an empty cache and ensuring repeated oversized messages do not leak buffers.

Show a summary per file

File	Description
test/Orleans.Streaming.Tests/OrleansRuntime/Streams/PooledQueueCacheTests.cs	Adds regression tests for mid-batch corruption and oversized-message non-leak; updates test converter allocation order accordingly.
src/Orleans.Streaming/MemoryStreams/MemoryPooledCache.cs	Disposes newly-allocated buffers on oversized-message failure before eviction registration to prevent leaks.
src/Orleans.Streaming/Generator/GeneratorPooledCache.cs	Same oversized-message no-leak allocation ordering fix for generator cache.
src/Orleans.Streaming/Common/PooledCache/ChronologicalEvictionStrategy.cs	Removes eager-free logic from OnBlockAllocated to prevent recycling in-flight buffers while cache is “empty”.
src/Azure/Orleans.Streaming.EventHubs/Providers/Streams/EventHub/EventHubQueueCache.cs	Same oversized-message no-leak allocation ordering fix for EventHub queue cache.

Copilot's findings

• Files reviewed: 5/5 changed files
• Comments generated: 3