Reduce arena allocations when materializing RocksDB range read results

[pchintar]

Closes issue #13272

Summary

Reduce arena allocations when materializing RocksDB range read results.

Currently, every KeyValueRef deep-copy performs two independent arena allocations:

• one for the key payload,
• one for the value payload.

This PR instead packs both payloads into a single contiguous arena allocation while preserving existing ownership and StringRef semantics.

Changes

Current implementation:

KeyValueRef(Arena& a, const KeyValueRef& copyFrom)
  : key(a, copyFrom.key), value(a, copyFrom.value) {}

This performs two separate StringRef(Arena&, ...) allocations per returned KV pair.

In contrast, my PR introduces a packed-copy path:

KeyValueRef(Arena& a, const KeyRef& key, const ValueRef& value)

which:

• allocates a single contiguous arena buffer for both payloads,
• copies key/value bytes into that shared allocation,
• constructs KeyRef / ValueRef views over the packed storage.

The RocksDB range-read paths now use:

result.emplace_back_deep(result.arena(), key, value);

instead of constructing a temporary KeyValueRef followed by generic deep-copy construction.

Overall this reduces arena allocation count from:

2 allocations per KV pair
    ->
1 allocation per KV pair

during range-result materialization.

No public APIs, wire formats, or ownership semantics are changed.

Testing

ninja -j2 fdbserver_kvstorelinktest

./bin/linktest/fdbserver_kvstorelinktest

Note: A full benchmark comparison and large-scale simulation workload validation couldn't be performed locally due to my hardware limitations

[saintstack]

Looking good. I think needs formatting?

[saintstack]

Good

[foundationdb-ci]

Result of foundationdb-pr-clang-arm on Linux CentOS 7

• Commit ID: deb2168
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• Error: Error while executing command: if [[ $(git diff --shortstat 2> /dev/null | tail -n1) == "" ]]; then echo "CODE FORMAT CLEAN"; else echo "CODE FORMAT NOT CLEAN"; echo; echo "THE FOLLOWING FILES NEED TO BE FORMATTED"; echo; git ls-files -m; echo; if [[ $FDB_VERSION =~ 7\.\3. ]]; then echo skip; else exit 1; fi; fi. Reason: exit status 1
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Result of foundationdb-pr-cluster-tests on Linux RHEL 9

• Commit ID: deb2168
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• Result: ❌ FAILED
• Error: Error while executing command: if [[ $(git diff --shortstat 2> /dev/null | tail -n1) == "" ]]; then echo "CODE FORMAT CLEAN"; else echo "CODE FORMAT NOT CLEAN"; echo; echo "THE FOLLOWING FILES NEED TO BE FORMATTED"; echo; git ls-files -m; echo; if [[ $FDB_VERSION =~ 7\.\3. ]]; then echo skip; else exit 1; fi; fi. Reason: exit status 1
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[foundationdb-ci]

Result of foundationdb-pr-clang on Linux RHEL 9

• Commit ID: deb2168
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• Error: Error while executing command: if [[ $(git diff --shortstat 2> /dev/null | tail -n1) == "" ]]; then echo "CODE FORMAT CLEAN"; else echo "CODE FORMAT NOT CLEAN"; echo; echo "THE FOLLOWING FILES NEED TO BE FORMATTED"; echo; git ls-files -m; echo; if [[ $FDB_VERSION =~ 7\.\3. ]]; then echo skip; else exit 1; fi; fi. Reason: exit status 1
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[foundationdb-ci]

Result of foundationdb-pr on Linux RHEL 9

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• Error: Error while executing command: if [[ $(git diff --shortstat 2> /dev/null | tail -n1) == "" ]]; then echo "CODE FORMAT CLEAN"; else echo "CODE FORMAT NOT CLEAN"; echo; echo "THE FOLLOWING FILES NEED TO BE FORMATTED"; echo; git ls-files -m; echo; if [[ $FDB_VERSION =~ 7\.\3. ]]; then echo skip; else exit 1; fi; fi. Reason: exit status 1
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Result of foundationdb-pr-macos-m1 on macOS Ventura 13.x

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Result of foundationdb-pr-macos on macOS Ventura 13.x

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[pchintar]

Looking good. I think needs formatting?

Hi @saintstack I just fixed all the formatting issues by using clang-format & so could you pls re-run the checks once more? thnx

[foundationdb-ci]

Result of foundationdb-pr-clang-ide on Linux RHEL 9

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[foundationdb-ci]

Result of foundationdb-pr-macos-m1 on macOS Ventura 13.x

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[foundationdb-ci]

Result of foundationdb-pr-macos on macOS Ventura 13.x

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[foundationdb-ci]

Result of foundationdb-pr-clang on Linux RHEL 9

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[foundationdb-ci]

Result of foundationdb-pr-cluster-tests on Linux RHEL 9

• Commit ID: 3d0286d
• Duration 1:01:20
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