Problem: TxoSpenderIndex values are never read: FindSpender() only uses the key, which already encodes the spender's disk position, but the values are written as single-byte \0(instead of empty values).
Fix: Write zero-byte markers for new txospenderindex entries instead.
Existing indexes stay readable because both formats use the same keys and the value is ignored.
Rebuilding is only needed to shrink old entries.
Related work: This complements #35568, which saves another ~4.5 GB by dropping bloom filters. A #35531-style rewrite could let existing indexes reclaim both savings without a full rebuild.
Reproducer: Automated tests weren't added, but the manual reproducer below builds both commits, checks old/new format compatibility through gettxspendingprevout, and prints rebuilt index sizes.
On my mainnet datadir this PR shrank txospenderindex by ~3.4 GB (from 89G to 86G).
<details><summary>Script + sample output</summary>
BEFORE="89b4000ae06c72c5a14ee05ad70d3aece3f1b382" AFTER="1be799ef858c33158a302857b084e169b5bf8c1f" DATA_DIR="/mnt/my_storage/BitcoinData" LOG="${DATA_DIR}/debug.log" OUT='[{"txid":"0437cd7f8525ceed2324359c2d0ba26006d92d856a9c20fa0241106ee5a597c9","vout":0}]' SPEND="f4184fc596403b9d638783cf57adfe4c75c605f6356fbc91338530e9831e9e16"; \
for b in before after; do [ -x ./build-$b/bin/bitcoin-cli ] && ./build-$b/bin/bitcoin-cli -datadir="${DATA_DIR}" stop >/dev/null 2>&1 || true; done; sleep 10; \
git reset --hard >/dev/null 2>&1 && git clean -fxd >/dev/null 2>&1 && (git fetch origin "$BEFORE" "$AFTER" >/dev/null 2>&1 || true) && \
for c in before:$BEFORE after:$AFTER; do git checkout ${c#*:} >/dev/null 2>&1 && cmake -B build-${c%:*} -DCMAKE_BUILD_TYPE=Release >/dev/null 2>&1 && cmake --build build-${c%:*} -j --target bitcoind bitcoin-cli >/dev/null 2>&1; done && \
for b in before after; do ./build-$b/bin/bitcoin-cli -datadir="${DATA_DIR}" stop >/dev/null 2>&1 || true; done; sleep 10; \
start_node() { [ "$2" = wipe ] && rm -rf "${DATA_DIR}/indexes/txospenderindex" "${LOG}"; : > "${LOG}"; ./build-"$1"/bin/bitcoind -datadir="${DATA_DIR}" -txospenderindex=1 -connect=0 -printtoconsole=0 & pid=$!; while ! grep -Fq 'txospenderindex is enabled at height' "${LOG}" 2>/dev/null; do kill -0 "$pid" 2>/dev/null || { tail -100 "${LOG}"; return 1; }; sleep 5; done; }; \
check_spend() { result="$(./build-"$1"/bin/bitcoin-cli -datadir="${DATA_DIR}" gettxspendingprevout "$OUT" '{"mempool_only":false}')"; echo "$2: $result"; echo "$result" | grep -q "$SPEND"; }; \
stop_node() { ./build-"$1"/bin/bitcoin-cli -datadir="${DATA_DIR}" stop >/dev/null; wait "$pid"; }; \
echo "prevout: 0437cd7f8525ceed2324359c2d0ba26006d92d856a9c20fa0241106ee5a597c9:0 -> ${SPEND}" && \
start_node before wipe && check_spend before "old binary reads old-format index" && du -sh "${DATA_DIR}/indexes/txospenderindex"; stop_node before && \
start_node after && check_spend after "new binary reads old-format index"; stop_node after && \
start_node after wipe && check_spend after "new binary reads new-format index" && du -sh "${DATA_DIR}/indexes/txospenderindex"; stop_node after && \
start_node before && check_spend before "old binary reads new-format index"; stop_node before
Expected result, trimmed to the relevant compatibility and size lines:
prevout: 0437cd7f8525ceed2324359c2d0ba26006d92d856a9c20fa0241106ee5a597c9:0 -> f4184fc596403b9d638783cf57adfe4c75c605f6356fbc91338530e9831e9e16
old binary reads old-format index: [ ... "spendingtxid": "f4184fc596403b9d638783cf57adfe4c75c605f6356fbc91338530e9831e9e16" ... ]
89G /mnt/my_storage/BitcoinData/indexes/txospenderindex
new binary reads old-format index: [ ... "spendingtxid": "f4184fc596403b9d638783cf57adfe4c75c605f6356fbc91338530e9831e9e16" ... ]
new binary reads new-format index: [ ... "spendingtxid": "f4184fc596403b9d638783cf57adfe4c75c605f6356fbc91338530e9831e9e16" ... ]
86G /mnt/my_storage/BitcoinData/indexes/txospenderindex
old binary reads new-format index: [ ... "spendingtxid": "f4184fc596403b9d638783cf57adfe4c75c605f6356fbc91338530e9831e9e16" ... ]
</details>
