Problem
CDBIterator::GetValue() cannot use SpanReader the same way as ::GetKey() because values are deobfuscated in place before deserialization, so it still needs an owning mutable buffer.
However, the current path allocates a fresh DataStream for every value read.
The same local-stream pattern also exists in CDBIterator::Seek(), while CDBBatch already owns reusable key/value buffers but still manually reserves and clears them on every Write() and Erase() call.
Fix
Add ScopedDataStreamUsage, a small RAII helper for caller-owned scratch streams.
It asserts that the stream is empty on entry (making accidental re-entry or concurrent use of the same scratch stream fail fast), and clears it on scope exit.
Use it to guard the reusable scratch streams in CDBBatch::Write(), ::Erase() and CDBIterator::Seek(), ::GetValue().
The const read-side CDBWrapper helpers stay unchanged, since they can be called concurrently on the same wrapper and should keep using local streams.
The production changes are preceded by tests covering repeated reuse on the same owning objects, including a failed iterator value decode followed by a successful read from the same iterator entry.
Context
Follow-up to #35128, #34483 and #35025.
Reproducer
gettxoutsetinfo gets an additional ~6% speedup on top of the previous iterator-key optimization:
<details><summary>2026-04-24 | gettxoutsetinfo | i9-ssd | x86_64 | Intel(R) Core(TM) i9-9900K CPU @ 3.60GHz | 16 cores | 62Gi RAM | xfs | SSD</summary>
COMMITS="2d5ab09f0dca4bfec0b365f5f431def2c0c9d70f 9e5fd595ae8ebeec74678c31a547fbc14f87bf89"; \
BASE_DIR="/mnt/my_storage"; DATA_DIR="$BASE_DIR/BitcoinData"; LOG_DIR="$BASE_DIR/logs"; \
mkdir -p "$LOG_DIR" && \
(echo ""; for c in $COMMITS; do git cat-file -e "$c^{commit}" 2>/dev/null || git fetch -q origin "$c" || exit 1; git log -1 --pretty='%h %s' "$c" || exit 1; done) && \
(echo "" && echo "$(date -I) | gettxoutsetinfo | $(hostname) | $(uname -m) | $(lscpu | grep 'Model name' | head -1 | cut -d: -f2 | xargs) | $(nproc) cores | $(free -h | awk '/^Mem:/{print $2}') RAM | $(df -T $BASE_DIR | awk 'NR==2{print $2}') | $(lsblk -no ROTA $(df --output=source $BASE_DIR | tail -1) | grep -q 1 && echo HDD || echo SSD)"; echo "") && \
hyperfine \
--sort command \
--runs 3 \
--export-json "$BASE_DIR/gettxoutsetinfo-$(sed -E 's/([a-f0-9]{8})[a-f0-9]* ?/\1-/g;s/-$//'<<<"$COMMITS")-$(date +%s).json" \
--parameter-list COMMIT ${COMMITS// /,} \
--prepare "killall -9 bitcoind 2>/dev/null || true; rm -f $DATA_DIR/debug.log; git clean -fxd && git reset --hard {COMMIT} && \
cmake -B build -G Ninja -DCMAKE_BUILD_TYPE=Release && ninja -C build bitcoind bitcoin-cli -j$(nproc) && \
./build/bin/bitcoind -datadir=$DATA_DIR -connect=0 -listen=0 -dnsseed=0 -coinstatsindex=0 -txindex=0 -blockfilterindex=0 -daemon -printtoconsole=0; \
./build/bin/bitcoin-cli -datadir=$DATA_DIR -rpcwait getblockcount >/dev/null" \
--conclude "./build/bin/bitcoin-cli -datadir=$DATA_DIR stop 2>/dev/null || true; killall bitcoind 2>/dev/null || true; sleep 10; \
grep -q 'Done loading' $DATA_DIR/debug.log && grep 'Bitcoin Core version' $DATA_DIR/debug.log | grep -q \"\$(git rev-parse --short=12 {COMMIT})\"; \
cp $DATA_DIR/debug.log $LOG_DIR/gettxoutsetinfo-{COMMIT}-$(date +%s).log" \
"./build/bin/bitcoin-cli -datadir=$DATA_DIR -rpcclienttimeout=0 -named gettxoutsetinfo hash_type='none' use_index='false' >/dev/null"
2d5ab09f0d Merge bitcoin/bitcoin#35124: bench: fix benchmark fixtures and setup checks
cb63e158d9 walletdb: reuse batch scratch streams
2026-04-24 | gettxoutsetinfo | i9-ssd | x86_64 | Intel(R) Core(TM) i9-9900K CPU @ 3.60GHz | 16 cores | 62Gi RAM | xfs | SSD
Benchmark 1: ./build/bin/bitcoin-cli -datadir=/mnt/my_storage/BitcoinData -rpcclienttimeout=0 -named gettxoutsetinfo hash_type='none' use_index='false' >/dev/null (COMMIT = 2d5ab09f0dca4bfec0b365f5f431def2c0c9d70f)
Time (mean ± σ): 60.063 s ± 1.623 s [User: 0.001 s, System: 0.002 s]
Range (min … max): 59.020 s … 61.933 s 3 runs
Benchmark 2: ./build/bin/bitcoin-cli -datadir=/mnt/my_storage/BitcoinData -rpcclienttimeout=0 -named gettxoutsetinfo hash_type='none' use_index='false' >/dev/null (COMMIT = cb63e158d9ae8276dbe54bba0b0cf8f35378ec71)
Time (mean ± σ): 56.853 s ± 0.179 s [User: 0.002 s, System: 0.001 s]
Range (min … max): 56.675 s … 57.033 s 3 runs
Relative speed comparison
1.06 ± 0.03 ./build/bin/bitcoin-cli -datadir=/mnt/my_storage/BitcoinData -rpcclienttimeout=0 -named gettxoutsetinfo hash_type='none' use_index='false' >/dev/null (COMMIT = 2d5ab09f0dca4bfec0b365f5f431def2c0c9d70f)
1.00 ./build/bin/bitcoin-cli -datadir=/mnt/my_storage/BitcoinData -rpcclienttimeout=0 -named gettxoutsetinfo hash_type='none' use_index='false' >/dev/null (COMMIT = cb63e158d9ae8276dbe54bba0b0cf8f35378ec71)
</details>
