assumeutxo: Add documentation on dumptxoutset serialization format

Author: fjahr

src/rpc/blockchain.cpp

@@ -2702,6 +2702,13 @@ UniValue CreateUTXOSnapshot(
     size_t written_coins_count{0};
     std::vector<std::pair<uint32_t, Coin>> coins;
 
+    // To reduce space the serialization format of the snapshot avoids
+    // duplication of tx hashes. The code takes advantage of the guarantee by
+    // leveldb that keys are lexicographically sorted.
+    // In the coins vector we collect all coins that belong to a certain tx hash
+    // (key.hash) and when we have them all (key.hash != last_hash) we write
+    // them to file using the below lambda function.
+    // See also https://github.com/bitcoin/bitcoin/issues/25675
     auto write_coins_to_file = [&](AutoFile& afile, const Txid& last_hash, const std::vector<std::pair<uint32_t, Coin>>& coins, size_t& written_coins_count) {
         afile << last_hash;
         WriteCompactSize(afile, coins.size());

src/validation.h

@@ -885,6 +885,12 @@ class ChainstateManager
     CBlockIndex* m_best_invalid GUARDED_BY(::cs_main){nullptr};
 
     //! Internal helper for ActivateSnapshot().
+    //!
+    //! De-serialization of a snapshot that is created with
+    //! CreateUTXOSnapshot() in rpc/blockchain.cpp.
+    //! To reduce space the serialization format of the snapshot avoids
+    //! duplication of tx hashes. The code takes advantage of the guarantee by
+    //! leveldb that keys are lexicographically sorted.
     [[nodiscard]] bool PopulateAndValidateSnapshot(
         Chainstate& snapshot_chainstate,
         AutoFile& coins_file,