Merge bitcoin/bitcoin#35634: txospenderindex: use zero-byte entry values

113402286e doc: add txospenderindex release note (Lőrinc)
ce06878288 index: shrink txospenderindex value markers (Lőrinc)

Pull request description:

  **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>

  ```bash
  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:

  ```text
  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>

ACKs for top commit:
  davidgumberg:
    crACK 113402286e
  sedited:
    ACK 113402286e
  ekzyis:
    utACK 113402286e
  andrewtoth:
    ACK 113402286e

Tree-SHA512: 07945b9470670115e27a1ef51e64164da7b91e550fdcb59952bfc98d862264b405e8fd8c3267c42718a39823f853057390d031d936d6e0b2172ec07e36cfdd67

doc/release-notes-35634.md

@@ -0,0 +1,9 @@
+Indexes
+-------
+
+- The transaction output spender index (`-txospenderindex`) now stores one byte
+  less per spender entry for newly indexed blocks. Existing indexes remain
+  compatible and no action is required. To apply the same saving to entries
+  indexed before upgrading, stop the node, delete the
+  `<datadir>/indexes/txospenderindex/` directory, and restart with
+  `-txospenderindex` enabled to rebuild it.

src/index/txospenderindex.cpp

@@ -23,15 +23,17 @@
 #include <util/fs.h>
 #include <validation.h>
 
+#include <cstddef>
 #include <cstdio>
 #include <exception>
 #include <ios>
+#include <span>
 #include <string>
 #include <utility>
 #include <vector>
 
 /* The database is used to find the spending transaction of a given utxo.
- * For every input of every transaction it stores a key that is a pair(siphash(input outpoint), transaction location on disk) and an empty value.
+ * For every input of every transaction it stores a key that is a pair(siphash(input outpoint), transaction location on disk) and a zero-byte value.
  * To find the spending transaction of an outpoint, we perform a range query on siphash(outpoint), and for each returned key load the transaction
  * and return it if it does spend the provided outpoint.
  */
@@ -91,8 +93,9 @@ void TxoSpenderIndex::WriteSpenderInfos(const std::vector<std::pair<COutPoint, C
     CDBBatch batch(*m_db);
     for (const auto& [outpoint, pos] : items) {
         DBKey key(CreateKey(m_siphash_key, outpoint, pos));
-        // key is hash(spent outpoint) | disk pos, value is empty
-        batch.Write(key, "");
+        // The key encodes the spent outpoint hash and disk position. The value is only a marker.
+        // Older entries may contain serialized empty strings; FindSpender() reads only keys.
+        batch.Write(key, std::span<const std::byte>{});
     }
     m_db->WriteBatch(batch);
 }