[Pn7Hao]

Concurrent diff-cache eviction can unmap a cache after Chunker.Slice returns but before Chunker.ReadAt copies the bytes. The NBD fault guard then reports a MemoryFaultError, bypassing File.readAt's existing CacheClosedError retry and failing the guest read.

Separate waiting for data from acquiring the mmap slice. Protect mmap-header access and keep the cache read lock through ReadAt's copy. The lock is released before fetching, so a pending close cannot deadlock a fetch that needs its own read lock. A ready streaming prefix remains readable before the full chunk is marked cached; empty reads at EOF also retain their behavior. Slice still returns borrowed storage whose lifetime belongs to its caller.

The regression test closes the cache from the existing readiness metric callback, placing eviction deterministically between readiness and copying without adding a production test hook. On unmodified df41cd32, it produces MemoryFaultError; with this change it returns CacheClosedError and leaves the destination untouched. Two additional tests cover partial streaming reads and empty reads at EOF.

Validation:

• go test -race ./pkg/sandbox/block -run '^TestChunker_' -parallel 2 -count=1 -timeout 90s passes.
• The new eviction regression fails on unmodified df41cd32 with the expected invalid-memory fault.
• golangci-lint run --new-from-rev=HEAD --timeout=3m ./pkg/sandbox/block/... passes before committing.
• Full repository validation was attempted but not completed locally: the unrestricted block-package run exhausted the local machine's memory. The scoped checks above were rerun with a 2 GiB process-group memory limit.

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[Copilot]

🟢 Approval recommended

No active review issues are recorded for the change protecting chunk reads from concurrent cache unmapping.

No open issues 🎉

What changed in this PR

This PR protects orchestrator chunk reads from concurrent cache unmapping, allowing cache-closed errors to reach the existing retry logic instead of failing guest reads with memory faults.

Changes:

• Separates data readiness from mmap access and holds the cache read lock through copying.
• Preserves streamed-prefix reads and borrowed Slice behavior.
• Adds regression tests for eviction after readiness, partial streaming reads, and empty reads at EOF.

File	Description
packages/​orchestrator/​pkg/​sandbox/​block/​streaming_chunk.go	Guards mmap access and copying against concurrent cache closure.
packages/​orchestrator/​pkg/​sandbox/​block/​streaming_chunk_close_test.go	Tests eviction handling, streamed-prefix reads, and empty EOF reads.

_{Review effort: Balanced}

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[Pn7Hao]

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[Pn7Hao]

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